EuroFlu - Weekly Electronic Bulletin – Week 5 : 28/01/2013-03/02/2013 - 08 February 2013, Issue No. 474: H1N1pdm09 remains the prevalent strain; flu-related SARI up

[Giuseppe Michieli]

[Source: EuroFlu, full page: (LINK). Extract, edited.]


EuroFlu - Weekly Electronic Bulletin – Week 5 : 28/01/2013-03/02/2013 - 08 February 2013, Issue No. 474

Influenza activity continues to rise across Europe



Summary, week 5/2013

• Clinical activity rates continue to increase in most countries in the WHO European Region, with predominance of influenza A(H1N1)pdm09 and co-circulation of influenza A(H3N2) and type B viruses.
• The proportion of samples from sentinel and non-sentinel sources testing positive for influenza reached its highest level so far this season.
• In line with the increasing activity in outpatient surveillance, the proportion of hospitalized cases of severe acute respiratory infection (SARI) that were positive for influenza continued to increase, and was mainly due to influenza A(H1N1)pdm09 viruses.

The EuroFlu bulletin describes and comments on influenza activity in the 53 Member States in the WHO European Region to provide information to public health specialists, clinicians and the public on the timing of the influenza season, the spread of influenza, the prevalence and characteristics of circulating viruses (type, subtype and lineage) and severity.

For a description of influenza surveillance in the WHO European Region see below.



Virological surveillance for influenza

The overall number of specimens testing positive for influenza in the Region and the influenza positivity rate continued to rise slowly. A total of 6542 specimens tested positive for influenza in week 05/2013, with 4627 (71%) being influenza A. This distribution is similar to the previous two weeks (Fig. 1).

In week 05/2013, influenza A(H1N1)pdm09 viruses remained dominant: of 2976 influenza A viruses subtyped, 2314 (78%) were A(H1N1)pdm09 while only 662 (22%) were A(H3N2) (Fig. 2a).

Since the beginning of the season (week 40/2012), 30 119 influenza viruses from sentinel and non-sentinel sources have been typed.

A fairly even distribution of influenza virus types has been observed consistently since week 47/2012: 21 097 (70%) were influenza A and 9 022 (30%) influenza B (Fig. 2b).

Of the influenza A viruses, 13 107 were subtyped: 9 355 (71%) as A(H1N1)pdm09 and 3 752 (29%) as A(H3N2).

In addition, the lineage for 1260 influenza B viruses has been determined since week 40/2012: 1129 (90%) belonged to the B/Yamagata lineage and 131 (10%) to B/Victoria.

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Circulation of influenza viruses in week 05/2013 remained diverse, and similar to the previous two weeks, across the Region.

Influenza A, mainly A(H1N1)pdm09, was reported as the dominant virus in countries in northern, eastern and central Europe, and Turkey, while influenza B was reported as the dominant virus in some countries in the southern and western parts of the Region, as well as the United Kingdom (Northern Ireland) (Map 1).

Between these areas, co-circulation of A(H1N1)pdm09, A(H3N2) and influenza B was reported.



Virus strain characterizations

For the 2012/2013 northern hemisphere influenza season, WHO recommends inclusion of A/California/7/2009 (H1N1)pdm09-like, A/Victoria/361/2011 (H3N2)-like and B/Wisconsin/1/2010-like (from the B/Yamagata lineage) viruses in vaccines (see more at WHO web site).

Since week 40/2012, 1269 influenza viruses characterized antigenically by 11 countries (Denmark, Germany, Greece, Latvia, Portugal, Romania, the Russian Federation, Slovakia, Slovenia, Switzerland and the United Kingdom (England and Scotland)) corresponded with the viruses recommended by WHO for inclusion in the current northern hemisphere seasonal influenza vaccine (Fig. 3).

The United Kingdom characterized the most viruses – 862 (68%) – with 485 (71%) of the 682 influenza (H3N2) viruses being characterized by Scotland as A(H3) A/Victoria/361/2011 (H3N2)-like.

12 countries (Austria, Belgium, Denmark, Finland, Germany, Greece, Norway, Portugal, Scotland, Spain, Sweden, Switzerland) have characterized 382 influenza viruses genetically (Fig. 4).

# Included in the WHO-recommended composition of influenza virus vaccines for use in the 2012/2013 northern hemisphere influenza season.

* Included in the WHO-recommended composition of influenza virus vaccines for use in the 2013 southern hemisphere influenza season.

Both A(H1N1)pdm09 and A(H3N2) viruses have evolved to fall into a number of different genetic groups, which are all antigenically similar to their vaccine viruses, A/California/7/2009 and A/Victoria/361/2011, respectively.
Influenza B viruses of the B/Victoria/2/87 and the B/Yamagata/16/88 lineages are co-circulating with dominance of the B/Yamagata lineage viruses this season (90%).

Influenza B viruses of the B/Victoria lineage all fall within the B/Brisbane/60/2008 clade and are antigenically indistinguishable.

B/Yamagata lineage viruses fall into two distinct genetic clades, represented by B/Estonia/55669/2011 (Clade 2) and B/Wisconsin/1/2010 (Clade 3) respectively.

Viruses in these clades can be distinguished antigenically from each other, but remain antigenically similar to the current vaccine virus, B/Wisconsin/1/2010.



Monitoring of susceptibility to antiviral drugs

Since week 40/2012, 8 countries (Denmark, Germany, Greece, the Netherlands, Norway, Spain, Sweden and the United Kingdom) have screened 278 viruses for susceptibility to the neuraminidase inhibitors oseltamivir and zanamivir.

The 96 influenza A(H3N2) and 74 influenza B viruses showed susceptibility to both drugs.

Of the 108 A(H1N1)pdm09 viruses tested, 107 showed susceptibility to both drugs and 1 virus carrying the neuraminidase H275Y amino acid substitution, causing resistance to oseltamivir, was detected in the Netherlands in a hospitalized immunocompromised patient exposed to oseltamivir through treatment.

The 14 influenza A(H3N2) and 10 influenza A(H1N1)pdm09 viruses screened for susceptibility to adamantanes were found to be resistant.



Outpatient surveillance for influenza-like illness (ILI) and/or acute respiratory infection (ARI)

ILI and ARI consultation rates continued to increase in most of the Region (Map 4), with most countries reporting medium intensity levels (Map 2) although some north-western countries (such as Norway, Poland and the United Kingdom) seem to have experienced peaks in clinical activity earlier in the season (see Country data and graphsf or more details).

Most of the countries across the Region reported regional or widespread circulation of influenza for week 05/2013 (Map 3). Of the 20 countries that have established epidemic thresholds, only 2 (Kazakhstan and Ukraine) reported ILI/ARI consultation rates remaining below their national threshold levels.

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The number of specimens testing positive for influenza from sentinel outpatient clinics reached the highest level so far this season. Overall, 2949 specimens were tested, of which 1541 (52%) were positive for influenza in week 05/2013 (Fig. 5).

In the 28 countries testing 20 or more sentinel specimens, influenza positivity ranged from 0% to 89.3%, with a median of 45.5% (mean: 45.2%).

Similar to the previous week, of the 1541 influenza-positive specimens from sentinel sources, 66% were influenza A (Fig. 6a). Fig. 6b gives a detailed overview of cumulative influenza virus detections by type and subtype since week 40/2012. Click here for a detailed overview in a table format.

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Hospital surveillance for SARI

The number of hospitalizations, as well as the proportion of respiratory specimens from patients with SARI testing positive for influenza, has continued to increase since week 01/2013 (Fig. 7), in line with increasing influenza activity in countries reporting on SARI hospitalizations. Overall, the majority of countries reported cases mainly in the group aged 0–4 years, (see Country data and graphsfor individual country data).

For week 05/2013, 11 countries (Armenia, Belarus, Belgium, Georgia, Kazakhstan, Kyrgyzstan, the Republic of Moldova, the Russian Federation, Serbia, Slovakia and Ukraine) reported 87 influenza detections among 282 specimens tested, 78% of which (68) were influenza A (Fig. 8a).

Since week 40/2012, 2898 SARI specimens have been collected and tested for influenza with the relative distribution of influenza types and subtypes in hospitalized SARI patients similar to that observed from other sentinel data sources. Click here for a detailed overview in table format.

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Among the countries reporting on hospitalization of severe influenza cases to the European Centre for Disease Prevention and Control (ECDC), 71 case was reported for week 05/2013. To date, A(H1N1)pdm09, A(H3N2) and influenza B have been detected among hospitalized patients.

For more information on surveillance of confirmed hospitalized influenza, please see ECDC’s Weekly Influenza Surveillance Overview (WISO) at European Centre for Disease Prevention and Control web site.



EuroMOMO (European Mortality Monitoring Project)

EuroMOMO is a project set up to develop and operate a routine public health mortality monitoring system to detect and measure, on a real-time basis, excess deaths related to influenza and other possible public health threats across 20 European Union (EU) countries.

Pooled analysis of week 05/2013 data from 12 countries, or regions within countries, shows no substantial excess all-cause mortality as yet this season. For more information about the EUROMOMO mortality monitoring system please click here)



Respiratory syncytial virus (RSV)

Based on the data presented by countries reporting on RSV, the positivity rate peaked in week 52/2012, after which the number of detections has gradually decreased (see Country data and graphsfor individual country data).



Description of influenza surveillance

Most of the 53 Member States of the WHO European Region monitor influenza activity through surveillance of ILI and/or ARI in primary care clinics, with some countries also conducting hospital-based surveillance for severe disease. Surveillance data in the Region are collected from sentinel and non-sentinel systems. Sentinel data come from a network of designated clinicians who routinely and systematically collect respiratory specimens from ILI, ARI or SARI cases according to standard case definitions. Non-sentinel data come from a variety of other sources, including community outbreaks, general practitioners and hospitals that are not part of the sentinel surveillance system for influenza and may not use a standard case definition for ILI, ARI or SARI. The EuroFlu bulletin collates and interprets epidemiological and virological data from the different surveillance systems in the Region, to provide information on the timing of the influenza season, the spread of influenza, the prevalence and characteristics of circulating influenza viruses according to influenza type and subtype (A(H3N2) and A(H1N1)pdm09) or lineage (B/Victoria of B/Yamagata), and severity. In addition, influenza viruses are assessed each season for their antigenic and genetic characteristics, to determine the extent of their antigenic and genetic similarity to the viruses included in the seasonal influenza vaccine and the prevalence of mutations that affect pathogenicity or are associated with reduced susceptibility to antiviral drugs.

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Country comments (where available)

• Denmark: In Denmark we had a peak in week 1, however; for week 5 we again experience an increase in diagnostic samples tested for influenza and in the amount of influenza A and B positives. Influenza A(H3N2) is still the dominant subtype when looking at all samples, but influenza B (Yamagata) is increasing. In sentinel specimens the distribution between influenza A and B positives is 50/50 and the amount of influenza B positives are increasing.
• Republic of Moldova: This week were registered 2 cases of death from seasonal influenza A (H1N1) pdm (non-sentinel): a person of 62 years, female, Floreşti district; and a person of 27 years, male, Ungheni district. In both cases patients asked late for medical care, antiviral treatment was initiated late. Both people have not been vaccinated against influenza. Totally, this 5th sentinel week, 38 samples were tested for Influenza viruses, from which 11 samples were positive for RNA Influenza virus A(H1N1)pdm and 3 samples were positive for RNA Influenza virus type B.

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