NEJM - Update on Avian Influenza A (H5N1) Virus Infection in Humans

[sharon sanders]

Update on Avian Influenza A (H5N1) Virus Infection in Humans

<!-- AUTHOR_DISPLAY --> <center> Writing Committee of the Second World Health Organization Consultation on Clinical Aspects of Human Infection with Avian Influenza A (H5N1) Virus

The unprecedented epizootic of avian influenza A (H5N1) virusesamong birds continues to cause human disease with high mortalityand to pose the threat of a pandemic. This review updates a2005 report¹ and incorporates information recently publishedor presented at the Second World Health Organization (WHO) Consultationon Clinical Aspects of Human Infection with Avian InfluenzaA (H5N1) Virus.² Viral Ecology
Highly pathogenic avian influenza A (H5N1) viruses are entrenchedamong poultry in parts of Asia, Africa, and perhaps the MiddleEast. The highly pathogenic avian influenza H5 hemagglutininhas evolved into many phylogenetically distinct clades and subclades(Figure 1)⁴^,⁵ that generally correlate with antigenic differencesthat must be considered in the selection of candidates for H5N1vaccines.⁶^,⁷ These diverse lineages have been largely separategeographically since 2005 (Figure 1),⁵ although clade 2.3 virusesfrom China have recently circulated in other Southeast Asiancountries.⁸
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</nobr> </td><td align="left" valign="top"> Figure 1. Evolution of the Hemagglutinin and Other Key Mutations Associated with Virulence or Drug Resistance in Avian Influenza A (H5N1) Virus. The phylogenetic tree is for the hemagglutinin gene of highly pathogenic avian influenza A (H5N1) viruses. The geographic distributions refer to avian isolates, and the tree is based on publicly available sequences. Clade 0 includes viruses that were first recognized to cause human infections in Hong Kong Special Administrative Region in 1997. Viruses from clades and subclades 0, 1, 2.1, 2.2, 2.3, and 7 have caused human disease. Clade 1 viruses predominated in Vietnam, Thailand, and Cambodia in the early phase of the outbreak (2004?2005), and clade 2.1 viruses are endemic in Indonesia. Clade 2.2 viruses were associated with a major outbreak of H5N1 disease in migratory birds in Qinghai Lake, China, and have since spread, causing avian disease in Central and South Asia, the Middle East, Europe, and Africa and human disease in western Asia, the Middle East, and Africa. Clade 2.3 has become dominant in southern China and has also been detected in adjacent countries. (Modified from the WHO Web site: www.who.int/csr/disease/avian_influenza/guidelines/nomenclature/en/index.html.) The influenza genome contains eight individual segments of RNA, several of which encode two proteins. Within clade 1 or clade 2.1 viruses, polymerase basic protein 2 (PB2) Glu627Lys is observed in some isolates of human viruses but not in avian viruses.³ Some human clade 1 viruses without PB2 627Lys have PB2 701Asn; clade 2.2 viruses of both human and avian origin have PB2 Glu627Lys.⁴ The importance of sequence variations in NS1, in which most influenza A (H5N1) viruses contain a carboxyl-terminus?sequence motif that mediates binding to various cellular proteins bearing a PDZ domain, remains to be determined.

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The influenza A (H5N1) viruses that have infected humans havebeen entirely avian in origin, and they reflect strains circulatinglocally among poultry and wild birds. Avian influenza virusescan be maintained, amplified, and disseminated in live-poultrymarkets. Migratory birds may spread A (H5N1) viruses to newgeographic regions, but their importance as an ecologic reservoiris uncertain. The spread of influenza A (H5N1) viruses appearsto be principally related to the movement of poultry and poultryproducts,⁹^,¹⁰ although recent outbreaks of clade 2.2 virus infectionin sub-Saharan Africa,¹¹ Egypt, and Europe may indicate introductionof the virus by wild birds. The risk of the introduction ofinfluenza A (H5N1) viruses into North America by birds migratingthrough Alaska appears to be low.¹²
Epidemiology of Human Infections
Incidence and Demographic Characteristics
Despite widespread exposures to poultry infected with avianinfluenza A (H5N1) viruses,¹³^,¹⁴ influenza A (H5N1) diseasein humans remains very rare. Since May 2005, the numbers ofboth affected countries¹³ and confirmed cases of influenza A(H5N1) virus infection (340 cases as of December 14, 2007) haveincreased, in part because of the spread of clade 2.2 virusesacross Eurasia and to Africa⁵^,¹⁵ (Fig. 1 of the Supplementary Appendix,available with the full text of this article at www.nejm.org).
The median age of patients with influenza A (H5N1) virus infectionis approximately 18 years, with 90% of patients 40 years ofage or younger and older adults underrepresented.¹⁶ The overallcase fatality proportion is 61%; it is highest among persons10 to 19 years of age and lowest among persons 50 years of ageor older.¹⁶ Whether preexisting immunity, differences in exposure,or other factors might contribute to the apparently lower frequencyof infection and lethal illness among older adults is uncertain.Most patients with influenza A (H5N1) virus infection were previouslyhealthy. Of six affected pregnant women, four have died, andthe two survivors had a spontaneous abortion.¹⁷
Increases in human cases of influenza A (H5N1) have been observedduring cooler months in association with increases in outbreaksamong poultry (see Fig. 1 of the Supplementary Appendix).¹⁸However, because cases have occurred year-round, cliniciansmust be alert to possible human infection at any time, especiallyin countries with outbreaks of influenza A (H5N1) among birds.To date, no cases of influenza A (H5N1) illness have been identifiedamong short-term travelers visiting countries affected by outbreaksamong poultry or wild birds,¹⁹ although clinicians in unaffectedcountries should consider this possibility in travelers withexposures to poultry.
Surveillance for cases of influenza A (H5N1) has focused onpatients with severe illness, but milder illnesses in children,which are not pneumonic,²⁰^,²¹ occur. Limited seroepidemiologicstudies conducted since 2003 involving villagers living withbackyard poultry, workers in live-poultry markets, and healthcare workers suggest that asymptomatic or mild human influenzaA (H5N1) virus infection is rare (Table 1 of the Supplementary Appendix).¹⁴
Transmission
Direct avian-to-human H5N1 virus transmission is the predominantmeans of human infection, although the exact mode and sitesof influenza A (H5N1) virus acquisition in the respiratory tractare incompletely understood. Handling of sick or dead poultryduring the week before the onset of illness is the most commonlyrecognized risk factor.²²^,²³ Most patients have acquired A (H5N1)infection from poultry raised inside or outside their houses.Slaughtering, defeathering, or preparing sick poultry for cooking;playing with or holding diseased or dead poultry; handling fightingcocks or ducks that appear to be well; and consuming raw orundercooked poultry or poultry products have all been implicatedas potential risk factors.²¹^,²²^,²³^,²⁴ The defeathering of deadwild swans was implicated in one case cluster.²⁵
The influenza A (H5N1) virus can also infect multiple mammalianhosts,²⁶^,²⁷ including domestic cats²⁸ and dogs.²⁹ None havebeen implicated in influenza A (H5N1) virus transmission tohumans yet, but any animal infected with the virus theoreticallyposes a risk of transmission and of being a host for viral adaptationto mammals.²⁶
Clusters of human influenza A (H5N1) illness with at least twoepidemiologically linked cases have been identified in 10 countriesand have accounted for approximately one quarter of cases.²⁰^,²¹^,²⁴^,³⁰^,³¹^,³²Most clusters have involved two or three persons; the largestaffected eight. More than 90% of case clusters have occurredamong blood-related family members, suggesting possible geneticsusceptibility, although one statistical model indicated thatthese clusters might have occurred because of chance alone.³³Most persons in case clusters probably acquired infection fromcommon-source exposures to poultry, but limited, nonsustainedhuman-to-human transmission has probably occurred during veryclose, unprotected contact with a severely ill patient.²⁰^,³⁰^,³²In the largest cluster, transmission probably occurred fromthe index case to six blood-related family members and subsequentlyto another family member.³² Respiratory secretions and all bodilyfluids, including feces, should be considered potentially infectious.
In one quarter or more of patients with influenza A (H5N1) virusinfection, the source of exposure is unclear, and environment-to-humantransmission remains possible.²⁰^,²⁴ For some patients, the onlyidentified risk factor was visiting a live-poultry market.³⁴^,³⁵Plausible transmission routes include contact with virus-contaminatedfomites or with fertilizer containing poultry feces, followedby self-inoculation of the respiratory tract or inhalation ofaerosolized infectious excreta. It is unknown whether influenzaA (H5N1) virus infection can begin in the human gastrointestinaltract. In several patients, diarrheal disease preceded respiratorysymptoms,³⁶ and virus has been detected in feces.³^,³⁷ Acquisitionof influenza A (H5N1) virus infection in the gastrointestinaltract has been implicated in other mammals.²⁶ Drinking potablewater and eating properly cooked foods are not considered tobe risk factors, but ingestion of virus-contaminated productsor swimming or bathing in virus-contaminated water might posea risk.
Incubation Period
After exposure to infected poultry, the incubation period generallyappears to be 7 days or less, and in many cases this periodis 2 to 5 days. In clusters in which limited, human-to-humantransmission has probably occurred, the incubation period appearsto be approximately 3 to 5 days, although in one cluster itwas estimated to be 8 to 9 days.²⁰^,³⁰
Pathogenesis
Viral Factors
The viral and host factors that determine host-restriction anddisease manifestations are incompletely understood.³⁸ Preferentialbinding of the influenza A (H5N1) virus to 2,3-linked sialicacid receptors on avian cells³⁹ is thought to be key in preventinginfluenza A (H5N1) and other avian influenza viruses from readilyinfecting humans. Some influenza A (H5N1) viruses isolated fromhumans have acquired mutations that permit binding to both 2,3-linkedsialic acid receptors and 2,6-linked sialic acid receptors,⁴⁰but these mutations appear to be insufficient for efficienthuman-to-human transmission. To date, influenza A (H5N1) viruseshave shown no transmissibility or poor transmissibility betweenferrets and between swine, and reassortment between an influenzaA (H5N1) virus and an influenza A (H3N2) virus did not confertransmissibility in ferrets.⁴¹ Changes in multiple viral genesare probably required to generate a potentially pandemic influenzaA (H5N1) virus.
All recent influenza A (H5N1) viruses retain a polybasic aminoacid motif at the HA1?HA2 connecting peptide that is characteristicof highly pathogenic avian influenza viruses. Geographic variationsin this motif have not been associated with obvious changesin the virulence of infection in humans. Amino acid substitutionsin the polymerase basic protein 2 (PB2) gene are associatedwith mammalian adaptation, virulence in mice, and replicationat temperatures present in the upper respiratory tract (Figure 1).⁴²However, these mutations do not correlate with obvious differencesin mortality among humans with this viral infection.³^,²¹
Viral Replication
The primary pathologic process that causes death is fulminantviral pneumonia. The target cells for replication of the influenzaA (H5N1) virus include type 2 alveolar pneumocytes and macrophages.¹⁷^,⁴³^,⁴⁴Bronchiolar and alveolar cells, but not epithelia from the tracheaor upper respiratory tract, express detectable 2,3-linked sialicacid receptors.⁴³^,⁴⁴^,⁴⁵ However, influenza A (H5N1) virusesreplicate in ex vivo organ cultures of the upper respiratorytract,⁴⁴ postmortem studies show virus in tracheal epithelia,¹⁷^,⁴⁶and high titers of virus are detectable in specimens of throatand tracheal aspirates from humans infected with influenza A(H5N1) virus.³ These findings suggest that the initial infectionmay occur in either the upper or lower respiratory tract, althoughthe latter may support more efficient replication.
Limited data show that patients with influenza A (H5N1) diseasemay have detectable viral RNA in the respiratory tract for upto 3 weeks, presumably because of negligible preexisting immunityand possibly viral evasion of immune responses.³ One patientwith fatal infection treated with both antiviral agents andcorticosteroids had viral antigen and RNA in tracheal sampleson day 27 after the onset of illness.¹⁷ Viral loads in the pharynxare higher and plasma viral RNA is detected more often in patientswith fatal disease than in those with nonfatal disease, indicatingthat levels of viral replication influence the outcome.³ Thereported presence of infectious virus in the blood, cerebrospinalfluid, or viscera of several patients with fatal disease indicatesthat, as in birds and several mammalian species, disseminatedinfection occurs in some humans.³^,¹⁷^,³⁶^,³⁷^,⁴⁶ A fatal influenzaA (H5N1) infection in one pregnant woman who received corticosteroidsfor treatment of the disease was associated with virus infectionof the brain, placenta, and fetus.¹⁷ Infectious virus and viralRNA have been detected in feces and intestines, suggesting thatthe virus sometimes replicates in the gastrointestinal tract.¹^,³^,³⁶^,³⁷^,⁴⁶
Pathological Findings
The few reported autopsies of patients with influenza A (H5N1)virus infection have shown diffuse alveolar damage with hyalinemembrane formation, patchy interstitial lymphoplasmacytic infiltrates,bronchiolitis with squamous metaplasia, and pulmonary congestionwith varying degrees of hemorrhage.¹⁷^,⁴⁶^,⁴⁷ Acute exudative,diffuse alveolar damage with macrophages, neutrophils, and activatedlymphocytes has been detected in patients who died within 2weeks after the onset of illness. Apoptosis in alveolar cellsand infiltrating leukocytes are prominent findings.⁴⁶ Lymphocytedepletion occurs in the spleen, lymph nodes, and tonsils; histiocytichyperplasia and reactive hemophagocytosis presumably resultfrom host cytokine responses and viral infection. Edema anddegeneration of myocytes in the heart and extensive acute tubularnecrosis in the kidney have been observed.
Host Responses
Higher plasma levels of macrophage and neutrophil-attractantchemokines and both proinflammatory and antiinflammatory cytokines(interleukin-6, interleukin-10, and interferon-) have been observedin patients with influenza A (H5N1) virus infection ?particularly in patients with fatal infection ? than inpatients with conventional influenza.³ Plasma levels of cytokinesand chemokines correlate positively with pharyngeal viral loads,³suggesting that these responses are driven by high-level viralreplication. In vitro experiments involving primary human macrophagesand lung pneumocytes show differential up-regulation of multiplecytokines by influenza A (H5N1) virus as compared with humaninfluenza viruses,⁴⁸ indicating that viral hyperinduction probablycontributes to hypercytokinemia.
In mouse models of influenza A (H5N1) virus infection, micewith deficient induction of interleukin-6, macrophage inflammatoryprotein 1, or tumor necrosis factor or its receptors⁴⁹^,⁵⁰ andmice treated with glucocorticoids,⁵⁰ had similar mortality ascompared with wild-type animals; mice without interleukin-1receptors had increased mortality.⁴⁹ Tissue damage in humaninfluenza A (H5N1) disease probably results from the combinedeffects of unrestrained viral infection and inflammatory responsesinduced by influenza A (H5N1) infection. Knowledge of the mechanismsof hypercytokinemia is insufficient to guide safe, rationalimmunomodulatory treatment at present.
Clinical Features
Currently, illness due to influenza A (H5N1) viruses typicallymanifests as severe pneumonia that often progresses rapidlyto the acute respiratory distress syndrome. The time from theonset of illness to presentation (median, 4 days) or to death(median, 9 to 10 days) has remained unchanged from 2003 through2006 (Table 1).¹⁶ Observed differences in mortality among patientswith presumed clade 1 and clade 2 virus infections (Table 1and Table 2)¹^,²¹^,²⁴^,³⁵^,⁵¹ are difficult to interpret becauseof variations in medical practices and the time from the onsetof illness to treatment among affected countries.
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</nobr> </td><td align="left" valign="top"> Table 1. Case Fatality Proportion According to Clade or Subclade and Median Time from Onset of Illness to Hospitalization or Death in Patients with Confirmed Influenza A (H5N1) Illness.
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</nobr> </td><td align="left" valign="top"> Table 2. Clinical and Common Laboratory Features of Influenza A (H5N1) Disease at Hospital Admission.
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Febrile upper respiratory illnesses without pneumonia in childrenhave been reported more frequently since 2005.²⁰^,²¹ Early consultationand antiviral therapy may have altered the clinical course ofthese illnesses. Less frequent gastrointestinal symptoms havebeen reported since 2005 (Table 2), suggesting that some manifestationsof clade 1 and 2 virus infections may differ from each other.Leukopenia, lymphopenia, mild-to-moderate thrombocytopenia,and elevated levels of aminotransferases are common but notuniversal (Table 2). Lymphopenia and increased levels of lactatedehydrogenase at presentation have been associated with a poorprognosis.¹^,³^,²¹^,³⁷ Other reported abnormalities include elevatedlevels of creatine phosphokinase, hypoalbuminemia, and increasedd-dimer levels and changes indicative of disseminated intravascularcoagulopathy.²⁰^,²¹
The nonspecific clinical presentation of influenza A (H5N1)disease has often resulted in misdiagnosis of subsequently confirmedcases (Table 3); influenza A (H5N1) virus infection has beensuspected in only a small number of patients. Health care staffshould include influenza A (H5N1) virus infection in the differentialdiagnosis for patients who present with epidemiologic risk factorsand unusual courses of illness, especially rapidly progressingpneumonia (see Fig. 2 of the Supplementary Appendix).
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</nobr> </td><td align="left" valign="top"> Table 3. Initial Diagnosis in Patients with Confirmed Influenza A (H5N1) Virus Infection.
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Laboratory Diagnosis
Detection of viral RNA by means of conventional or real-timereverse-transcriptase polymerase chain reaction remains thebest method for the initial diagnosis of influenza A (H5N1).⁵²These assays can provide results within 4 to 6 hours and canbe performed under biosafety level 2 conditions. The geneticvariability of influenza A (H5N1) viruses⁷^,⁸ calls for frequentupdating of primers and probes. Consequently, access to sequencesfrom recent influenza A (H5N1) viral isolates is essential.To detect other influenza A virus infections and reduce falsenegative results due to mutations in the H5 hemagglutinin gene,a conserved influenza A gene (e.g., matrix or nucleoprotein)should also be targeted.
Diagnostic yields are higher with throat specimens than withnasal swabs because of higher viral loads of influenza A (H5N1)in the throat.¹^,³ However, nasal swabs are useful for detectinghuman influenza viruses, so collection of both specimens isrecommended. If they are available, tracheal aspirates havehigher viral titers and yields than specimens from the upperrespiratory tract.³ Negative results in single respiratory specimensdo not rule out influenza A (H5N1) virus infection,²¹ and repeatedcollection of multiple specimen types is recommended.⁵² Previousantiviral treatment may reduce the diagnostic yield. Detectionof influenza A (H5N1) viral RNA in feces or blood may provideprognostic information,³ but it has lower diagnostic sensitivitythan influenza A (H5N1) viral RNA in respiratory specimens.
Commercially available rapid assays for influenza-antigen detectionhave poor clinical sensitivity for the detection of influenzaA (H5N1) virus (Table 2 of the Supplementary Appendix),¹^,²⁰^,²¹and they do not differentiate between human and avian subtypesof influenza A viruses. Although rapid antigen tests have similaranalytic sensitivity for detecting human and avian influenzaA (H5N1) viruses, they require 1000 times higher levels of virusthan viral cultures to be positive.⁵³
The detection of anti-H5 antibodies is essential for epidemiologicinvestigations and may provide retrospective diagnostic confirmationin patients. Seroconversion generally occurs 2 to 3 weeks afterinfection. Microneutralization assays are the most reliablemethods for detecting antibodies to avian viruses, but theyare labor-intensive and require biosafety level 3 facilitiesand appropriate strains of influenza A (H5N1) viruses. As comparedwith initial samples, elevations of four times or more or singletiters of 1:80 or more in convalescent-phase samples are consideredto be diagnostic.⁵² Modified nonpathogenic influenza A (H5N1)virus generated by reverse genetics or lentivirus pseudotypedwith H5 hemagglutinin⁵⁴ may provide alternatives for performingneutralization tests in biosafety level 2 facilities. Hemagglutination-inhibitionassays with the use of horse erythrocytes show promising resultsbut require further validation.
Treatment
Antiviral Agents
Susceptibility to current antiviral agents varies among circulatingstrains of influenza A (H5N1) viruses. Clade 1 viruses and mostclade 2 viruses from Indonesia are fully resistant to M2 inhibitors,whereas clade 2 viruses from the lineages in other parts ofEurasia and Africa are usually susceptible (Klimov A: personalcommunication). As compared with influenza A (H5N1) virusesfrom 1997 or influenza A (H1N1) viruses in vitro,⁵⁵ clade 1viruses generally show enhanced susceptibility to oseltamivircarboxylate, but the high-level replication of some oseltamivir-susceptiblestrains requires higher doses or more prolonged administration,or both, in animal models.⁵⁵^,⁵⁶ Clade 1 viruses appear to be15 to 30 times more sensitive to oseltamivir than clade 2 isolatesfrom Indonesia and Turkey,⁵⁶^,⁵⁷ although the possible clinicalrelevance of such differences in oseltamivir susceptibilityremains to be determined. During oseltamivir therapy, the emergenceof highly resistant variants with an H274Y neuraminidase mutationmay be associated with a fatal outcome.⁵⁸ Infection by influenzaA (H5N1) viruses containing an N294S mutation that causes areduction in oseltamivir susceptibility by a factor of 12 to15 times was reported to be present in two Egyptian patientswith fatal disease before therapy,⁵⁹ and avian influenza A (H5N1)viruses with reduced susceptibility to neuraminidase inhibitorsare occasionally detected.⁶⁰
Early treatment with oseltamivir is recommended,⁶¹^,⁶² and datafrom uncontrolled clinical trials suggest that it improves survival(Table 4), although the optimal dose and duration of therapyare uncertain. Mortality remains high despite administrationof oseltamivir; late initiation of therapy appears to be a majorfactor. Uncontrolled viral replication, as reflected in thedetection of persistent pharyngeal RNA after completion of standardtherapy, is associated with a poor prognosis.⁵⁸ Higher levelsof viral replication and slower clearance of infection probablyoccur in the lower respiratory tract.³ The oral bioavailabilityof oseltamivir in patients with severe diarrhea or gastrointestinaldysfunction related to influenza A (H5N1) virus infection orthose in whom the drug has been administered extemporaneously(e.g., by means of a nasogastric tube) is uncertain.
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</nobr> </td><td align="left" valign="top"> Table 4. Effects of Treatment and Time to Treatment with Oseltamivir on Survival among Patients with Influenza A (H5N1) Infection.
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A higher dose of oseltamivir (e.g., 150 mg twice daily in adults)and an increased duration of therapy, for a total of 10 days,may be reasonable, given the high levels of replication of theinfluenza A (H5N1) virus, observations of progressive diseasedespite early administration of standard-dose oseltamivir (75mg twice daily for 5 days in adults) within 1 to 3 days afterthe onset of the illness, and the proven safety of higher dosesin adults with seasonal influenza, especially if there is pneumonicdisease at presentation or evidence of clinical progression.⁶²In mouse models of amantadine-sensitive influenza A (H5N1) virusinfection, as compared with monotherapy, the combination ofoseltamivir and amantadine significantly increased survivalrates and inhibited viral replication in the internal organs.⁶⁴No adverse pharmacologic interactions have been shown in humans.⁶⁵In areas where influenza A (H5N1) viruses are likely to be susceptibleto amantadine, combination treatment with oseltamivir wouldbe reasonable, especially in seriously ill patients.
Although zanamivir is active against oseltamivir-resistant variantswith N1 neuraminidase mutations at H274Y⁶⁶ or N294S, the valueof inhaled zanamivir has not been studied in human influenzaA (H5N1) disease. Suboptimal delivery to sites of infectionin patients with pneumonic or extrapulmonary disease is a concern.Parenteral delivery of zanamivir or the neuraminidase inhibitorperamivir results in antiviral activity in animal models ofinfluenza A (H5N1) virus infection; these agents and othersare under clinical development (Table 3 of the Supplementary Appendix).
Other Treatments
Supportive care with correction of hypoxemia and treatment ofnosocomial complications remains fundamental in the managementof influenza A (H5N1) disease.²^,⁶² Corticosteroids should notbe used routinely.⁶² Corticosteroid therapy has thus far notbeen shown to be effective in patients with influenza A (H5N1)virus infection,¹ and prolonged or high-dose corticosteroidtherapy can result in serious adverse events, including opportunisticinfections such as central nervous system toxoplasmosis (SoerosoS: unpublished data). In northern Vietnam, mortality was 59%among 29 recipients of corticosteroids, as compared with 24%among 38 persons who did not receive corticosteroids (P=0.004)(Cao T, Thanh Liem N: personal communication). The possiblevalue of other immunomodulators remains to be determined.
Prevention
Avian influenza A viruses are readily inactivated by a varietyof chemical agents and physical conditions, including soaps,detergents, alcohols, and chlorination.⁶⁷^,⁶⁸ Guidelines forthe prevention of infection with influenza A (H5N1) virus invarious risk groups, including poultry workers, travelers, andhealth care workers, are available from the U.S. Centers forDisease Control and Prevention and the WHO.
Antiviral Chemoprophylaxis
WHO guidelines for the use of antiviral agents for prophylaxisin persons who have been exposed to influenza A (H5N1) virusesin the current pandemic-alert period have been published.⁶¹Mathematical models of an emerging outbreak of influenza A (H5N1)in rural Asia predict that a strategy of mass, targeted antiviralchemoprophylaxis and social-distancing measures might extinguishor delay pandemic spread of the virus. The WHO has a stockpileof oseltamivir for this purpose and is working with partnersfor implementation of its distribution in the event of an outbreak.⁶⁹
Immunization
Safe and immunogenic inactivated H5 vaccines have been developed.⁶Reverse genetics permits the rapid generation of seed viruseswith attenuated virulence, but the changing antigenicity ofcirculating strains of influenza A (H5N1) viruses calls fornew candidate vaccines from different lineages⁶ and the developmentof vaccines that elicit cross-clade immunogenicity. H5 hemagglutininappears to be a weak human immunogen. For subvirion vaccineswithout adjuvants, persons who have not received a priming doserequire two doses with a high hemagglutinin antigen content(Table 4 of the Supplementary Appendix). As compared with conventionalsubunit vaccines, certain oil-in-water adjuvant agents⁶^,⁷⁰^,⁷¹or the use of whole-virus H5N1 vaccines⁶^,⁷²^,⁷³ can substantiallyreduce the amount of vaccine antigen required to induce immuneresponses in persons who have not received a priming dose, andthey can induce immune responses to antigenically drifted viruses.However, the specific adjuvant, formulation, dose, stability,and ratio with the antigen are important variables that requireclinical testing for each candidate vaccine. Alum adjuvantshave not consistently improved the responses to H5 vaccines,⁶^,⁷³^,⁷⁴whereas certain proprietary adjuvants (e.g., MF59 and AS03)appear to be highly effective and allow for considerable antigen-sparingand cross-reactive antibody responses.⁶^,⁷⁰^,⁷¹ These adjuvantshave also been associated with increased rates of local andsometimes systemic reactogenicity.
The antibody levels required for protection against human influenzaA (H5N1) illness are unclear. The durability of antibody responsesis limited, but boosting with a homologous vaccine⁷⁰ or virusvaccine with viral antigen from another clade⁷⁵ appears to beeffective in persons who have received two priming doses. Preprimingmight allow single doses of a homologous vaccine to be sufficientfor an antigenically drifted pandemic virus. However, decisionsregarding the use of vaccine before a pandemic and stockpilingrequire complex risk?benefit and cost?benefit analysesthat include effects on the seasonal capacity of vaccine production,because the timing and cause of the next influenza pandemicare unknown, and it is unclear whether immunization of largepopulations could have adverse consequences.
Initial studies in children and elderly persons suggest thatantibody responses to subvirion vaccines at high doses (45 or90 ?g) are similar to those in young adults. Approximately15 to 20% of older adults have some baseline neutralizing antibodiesto H5N1 virus and may have a response to a single dose.⁶ Themechanisms leading to these antibodies are uncertain. Otherstudies to date have shown that intradermal H5 vaccines at lowdoses are poorly immunogenic and may be associated with injection-sitereactions.⁶ Intranasal live attenuated H5 vaccines are highlyeffective in animal models,⁷⁶ but they show a variable abilityto replicate in humans and to induce immune responses. Variousinvestigational approaches, including conserved antigen vaccines,vectored H5 vaccines, and other adjuvants, are being explored.



Dr. Chotpitayasunondh reports receiving grant support from SanofiPasteur and lecture fees from Sanofi Pasteur, GlaxoSmithKline,and Merck; and Dr. Peiris, consulting fees from GlaxoSmithKlineand Novartis and travel expenses and lecture fees from Novartis,Roche, and Sanofi Pasteur. No other potential conflict of interestrelevant to this article was reported.
Two authors (Drs. Hayden and Shindo) are staff members of theWHO. The authors alone are responsible for the views expressedin this article, and they do not necessarily represent the decisionsor the stated policy of the WHO. The views expressed in thisarticle do not necessarily reflect those of the other organizationswhose staff participated in the WHO consultation.
? World Health Organization 2008. All rights reserved.Published with permission from the World Health Organization.
We thank Drs. Christoph Steffen and Kaat Vandermaele of theWHO for their help with access to data and development of themanagement algorithm, Diane Ramm of the University of Virginiafor her assistance in the preparation of an earlier versionof the manuscript, and our colleagues in countries affectedby A (H5N1) virus for their willingness to share unpublishedclinical data for this article.
<!-- null --> ^* Affiliations of the writing committee are listed in the Appendix.The participants in the meeting of the Second World Health OrganizationConsultation on Clinical Aspects of Human Infection with AvianInfluenza A (H5N1) Virus, Antalya, Turkey, March 19?21,2007, are listed in the Supplementary Appendix, which is availablewith the full text of this article at www.nejm.org.

Source Information
The members of the writing committee (Abdel-Nasser Abdel-Ghafar, M.D., Tawee Chotpitayasunondh, M.D., Zhancheng Gao, M.D., Ph.D., Frederick G. Hayden, M.D., Nguyen Duc Hien, M.D., Ph.D., Menno D. de Jong, M.D., Ph.D., Azim Naghdaliyev, M.D., J.S. Malik Peiris, M.D., Nahoko Shindo, M.D., Santoso Soeroso, M.D., and Timothy M. Uyeki, M.D.) assume responsibility for the overall content and integrity of the article.
Address reprint requests to Dr. Hayden at the Global Influenza Program, Department of Epidemic and Pandemic Alert and Response, World Health Organization, 20 Ave. Appia, Ch-1211, Geneva 27, Switzerland, or at haydenf@who.int<script type="text/javascript"><!-- var u = "haydenf", d = "who.int"; document.getElementById("em0").innerHTML = '<a href="mailto:' + u + '@' + d + '">' + u + '@' + d + '<\/a>'//--></script>.
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Appendix The writing committee's affiliations are as follows: the Ministryof Health and Population, Cairo (A.-N.A.-G.); the Queen SirikitNational Institute of Child Health, Bangkok, Thailand (T.C.);the Peking University People's Hospital, Beijing (Z.G.); theGlobal Influenza Program, World Health Organization, Geneva(F.G.H., N.S.); the University of Virginia, Charlottesville(F.G.H.); the National Institute for Infectious and TropicalDiseases, Hanoi (N.D.H.); the Oxford University Clinical ResearchUnit, Hospital for Tropical Diseases, Ho Chi Minh City, Vietnam(M.D.J.); the Abulfaz Karayev Children's Hospital No. 2, Baku,Azerbaijan (A.N.); the University of Hong Kong, Hong Kong (J.S.M.P.);the National Infectious Diseases Hospital, Jakarta, Indonesia(S.S.); and the Centers for Disease Control and Prevention,Atlanta (T.M.U.).

http://content.nejm.org/cgi/content/full/358/3/261

</center>

[sharon sanders]

Figure 1


Figure 1. Evolution of the Hemagglutinin and Other Key Mutations Associated with Virulence or Drug Resistance in Avian Influenza A (H5N1) Virus.
The phylogenetic tree is for the hemagglutinin gene of highly pathogenic avian influenza A (H5N1) viruses. The geographic distributions refer to avian isolates, and the tree is based on publicly available sequences. Clade 0 includes viruses that were first recognized to cause human infections in Hong Kong Special Administrative Region in 1997. Viruses from clades and subclades 0, 1, 2.1, 2.2, 2.3, and 7 have caused human disease. Clade 1 viruses predominated in Vietnam, Thailand, and Cambodia in the early phase of the outbreak (2004?2005), and clade 2.1 viruses are endemic in Indonesia. Clade 2.2 viruses were associated with a major outbreak of H5N1 disease in migratory birds in Qinghai Lake, China, and have since spread, causing avian disease in Central and South Asia, the Middle East, Europe, and Africa and human disease in western Asia, the Middle East, and Africa. Clade 2.3 has become dominant in southern China and has also been detected in adjacent countries. (Modified from the WHO Web site: www.who.int/csr/disease/avian_influenza/guidelines/nomenclature/en/index.html.) The influenza genome contains eight individual segments of RNA, several of which encode two proteins. Within clade 1 or clade 2.1 viruses, polymerase basic protein 2 (PB2) Glu627Lys is observed in some isolates of human viruses but not in avian viruses.³ Some human clade 1 viruses without PB2 627Lys have PB2 701Asn; clade 2.2 viruses of both human and avian origin have PB2 Glu627Lys.⁴ The importance of sequence variations in NS1, in which most influenza A (H5N1) viruses contain a carboxyl-terminus?sequence motif that mediates binding to various cellular proteins bearing a PDZ domain, remains to be determined.

[sharon sanders]

Table 1


Table 1. Case Fatality Proportion According to Clade or Subclade and Median Time from Onset of Illness to Hospitalization or Death in Patients with Confirmed Influenza A (H5N1) Illness.

[sharon sanders]

Table 2


Table 2. Clinical and Common Laboratory Features of Influenza A (H5N1) Disease at Hospital Admission.

[sharon sanders]

Table 3


Table 3. Initial Diagnosis in Patients with Confirmed Influenza A (H5N1) Virus Infection.

[sharon sanders]

Table 4


Table 4. Effects of Treatment and Time to Treatment with Oseltamivir on Survival among Patients with Influenza A (H5N1) Infection.

[Johnny Yuma]

Re: Table 2

Table 2. Clinical and Common Laboratory Features of Influenza A (H5N1) Disease at Hospital Admission.

I see glaring omissions in these clinical features of H5N1.

There is no mention of meningitis/encephalitis symptoms or a rash. Is "depressed consciousness" a clinical evaluation? There have been confirmed H5N1 cases with neurological symptoms, including an inability to walk.

These neurological symptoms should be noted.

The 1918 virus had major neurological symptoms and these lingered for years if patients survived. See "encephalitis lethargica" at this supposed professional neurological association which does not even mention the 1918 flu pandemic has been repeatedly implicated in the disease:

"The cause of encephalitis lethargica is unknown. Between 1917 to 1928, an epidemic of encephalitis lethargica spread throughout the world, but no recurrence of the epidemic has since been reported"

http://www.ninds.nih.gov/disorders/encephalitis_lethargica/encephalitis_lethargica.htm

I also believe the records of human clusters and sources of infection need further analysis.

Can anyone analyze this article and summarize what it says is definitely known?

There seem to be too many "unknown" statements.