Discussion - When is an influenza pandemic over?

[Laidback Al]

When is an influenza pandemic over?
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A simple, true, and useless answer is ?when everyone in the world has been infected by the virus?.
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But of course, it is not that simple. First of all, not everyone is sickened by a novel pandemic virus, estimates I have seen range from 30-50% of any given population. It is assumed that either cross-protective immunity or some other kind of hidden genetic trait makes the remainder of the population immune from infection.
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Yet, my understanding again, the 1918 novel strain continued circulating at lower levels of lethality for a decade and by all accounts eventually supplanted prior circulating seasonal flu strain(s). So it would seem that the 1918 pandemic virus took years to work its way through the worldwide population, eventually infecting everyone who was susceptible. When was the 1918 pandemic over?
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What about the H1N1 pandemic? We have already seen news reports and commentaries that it is over. How will we know when this pandemic is over?
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The general dismissal of this pandemic seems to be based on a lack of virulence for the current circulating strain. Is this pandemic over when the number of fatalities declines? Influenza viruses change. If the H1N1 virus changes and returns with a higher virulence in the southern hemisphere in the next few months, is it a new pandemic?

[Missouriwatcher]

Re: Discussion - When is an influenza pandemic over?

Before a pandemic is no longer a pandemic, it would need to no longer match the description: "Phase 6, the pandemic phase, is characterized by community level outbreaks in at least one other country in a different WHO region in addition to the criteria defined in Phase 5. (Phase 5 is characterized by human-to-human spread of the virus into at least two countries in one WHO region)

http://www.who.int/csr/disease/avian_influenza/phase/en/index.html

Tuberculosis (TB) still remains a global pandemic, "one-third of the world's population is currently infected with the TB bacillus" and "Someone in the world is newly infected with TB bacilli every second."

http://www.disabled-world.com/health/tuberculosis.php

[sbrooks]

Re: Discussion - When is an influenza pandemic over?

Its convenentional wisdom that the virus has to hit herd immunity based on the previous 4 pandemics that we know something about, then it will
quietly supplant the seasonal strain. the longest period was over 2 years
from the moment it was observed in the population and that was in 1889.

Based on previous pandemics this one will get better at infecting people.

I would judge it over if we could test antibodies in populations and it showed
most have built up immunity.

[gsgs]

Re: Discussion - When is an influenza pandemic over?

2 years

[Missouriwatcher]

Re: Discussion - When is an influenza pandemic over?

Why wouldn't it continue for years?
Why isn't any influenza, by definition,
a pandemic because yearly it spreads throughout the world?

[gsgs]

Re: Discussion - When is an influenza pandemic over?

> Why wouldn't it continue for years?

immunity

> Why isn't any influenza, by definition,
> a pandemic because yearly it spreads throughout the world?

the same strain worldwide, not different strains in different locations (and infecting only ~10%)

[Laidback Al]

Re: Discussion - When is an influenza pandemic over?

I am still interested in knowing how we decide when a pandemic is over. It seems to me that the start of a pandemic is now determined by the identification of a novel influenza virus for which there is no immunity. It also seems to me that the "end" if a pandemic is assumed once mortality and morbidity rates fall below average annual levels. If so, then there is clearly a a "virulence component" to any definition of a pandemic.

[gsgs]

Re: Discussion - When is an influenza pandemic over?

you could just use infection rates, if these go to normal then it's over.

Ore seroprevalence. If ~50% of the population have antibodies, then it's no longer "new"

[AlaskaDenise]

Re: Discussion - When is an influenza pandemic over?

I am still interested in knowing how we decide when a pandemic is over. It seems to me that the start of a pandemic is now determined by the identification of a novel influenza virus for which there is no immunity. It also seems to me that the "end" if a pandemic is assumed once mortality and morbidity rates fall below average annual levels. If so, then there is clearly a a "virulence component" to any definition of a pandemic.

If we accept the definition of a pandemic to be the global spread of a novel pathogen, then once that pathogen has completed a thorough "cycle" (not truncated by seasonal impacts) globally, I'd say the pandemic is over.

But to mix virulence into the equation may be comparing apples and oranges, since a new pathogen could have spread globally yet remain highly virulent to humans for years (e.g., H5N1), or a new mild pathogen that had been around for a few years could acquire virulence genes (e.g., if novel H1N1 were to pick up part of H5N1 extra cleavage genes), just like later "epidemics" of old-old H1N1 did back in the 40s.

I think gsgs's estimate of 2 years is good. If you look back at H2N2 in 1957, there were some early cases, then it spread, yet there were still some old H1N1 cases hanging in there for over a year.

.

[SusanC]

Re: Discussion - When is an influenza pandemic over?

when is a pandemic over? I'd say, when the level of herd immunity is such that the morbidity (AR) and/or mortality falls within the range of seasonal flu. Which as we know vary over a wide range. For 2009, in many countries, mortality did not rise above seasonal flu, but the morbidity did.

[Laidback Al]

Re: Discussion - When is an influenza pandemic over?

Comments from WHO . . .

What is post-pandemic?

10 August 2010

What does post-pandemic mean? How do we know that the pandemic is over?

The H1N1 pandemic was characterized by the emergence of a new influenza virus to which many people had no pre-existing immunity. It caused unusual and extensive outbreaks of disease in the summer months in many countries and very high levels of disease in winter months. It was also characterized by an almost complete dominance of the pandemic virus over other seasonal influenza viruses, and by unusual clinical patterns where the most severe cases occurred most often in younger age groups.
We are now moving into a situation where the virus has spread to all countries, when many people in all age groups in many countries have some immunity to the new virus, and where no large and unusual summer outbreaks have occurred in either Northern or Southern Hemispheres, and where seasonal influenza A (H3N2) and influenza B viruses are being reported in many countries. Based on this overall picture, the evidence is strong that the recent influenza pandemic patterns are transitioning towards seasonal patterns of influenza. However, some countries, such as India and New Zealand, are continuing to experience significant levels of influenza H1N1 (2009), and national authorities in these countries will need to maintain implementation of outbreak response measures for some time to come. It is important to realize that the H1N1 (2009) virus can be expected to remain for many years and that individual influenza seasons themselves can be highly variable. In some years, the impact can be mild while in other years it can be quite severe.
In the post-pandemic period, cases and outbreaks due to the H1N1 (2009) virus are expected to continue to occur. In addition, it is most likely that for some period of time, younger age groups, including pregnant women, will continue to be affected disproportionately by severe disease from H1N1 (2009), including viral pneumonia. It is impossible to predict if younger people will remain at higher risk over the long term for severe disease or whether and when this will change. Given this picture, it will remain important for people to continue to take prudent steps to protect themselves. Actions to generally reduce risks of influenza infection, such as through use of vaccines and hand and respiratory hygiene, will also reduce the risks from H1N1 infection specifically.
WHO guidance: pandemic influenza preparedness and response

Does the end of the influenza pandemic mean that fewer people will become sick and die of the H1N1 (2009) virus?

The future impact of the H1N1 (2009) virus is not possible to predict. On the one hand, the H1N1 (2009) virus is expected to continue to circulate as a seasonal influenza strain for the foreseeable future and because of this, more people will develop immunity to this current version of the virus. On the other hand, the virus is also expected to change over time as a result of antigenic drift and these changes may mean that the immunity that has developed to this strain of the virus may become less protective against future strains of this virus. In addition, many people were not infected by the H1N1 (2009) virus during the pandemic, and there may be areas of some countries that were less severely affected during the pandemic that can still experience significant outbreaks.

Why did WHO take so long to declare the end of the pandemic?

WHO has been monitoring epidemiological and virological evidence from all parts of the world, including both Northern and Southern Hemispheres, to determine when the pandemic is over. WHO has waited until the winter in the Southern Hemisphere in order to determine whether the H1N1 (2009) virus is beginning to behave like seasonal influenza.

What does the end of the pandemic mean for individuals?

Regardless of whether the world is in a pandemic situation or not, influenza viruses pose a risk of disease to many individuals and therefore, individuals should take prudent steps to reduce their risk of infection. Such steps include the use of influenza vaccine when available and especially if they are in a group recognised to be at higher risk for severe illness. Most individuals who become ill can be treated safely at home, but caretakers and family members should be aware that if an individual develops symptoms suggesting more severe disease, such as prolonged high fever, trouble breathing, or confusion, then professional medical care should be sought.

Will the H1N1 (2009) influenza virus continue to cause severe illness among the same higher risk groups?

Based on available evidence, the H1N1 (2009) virus currently continues to pose a higher risk for severe illness in some groups, including young children, pregnant women and those with respiratory or chronic health conditions.
Who is more at risk of severe illness? What about other risks?

What can individuals do to protect themselves from the H1N1 (2009) influenza virus?

Individuals can continue to protect themselves and others from the H1N1 (2009) virus, and other influenza viruses, through preventive measures such as practicing respiratory etiquette, including covering your mouth when you sneeze or cough, and good hand hygiene. WHO also continues to recommend vaccination against the virus as the best protection, when available. Vaccination can be with a monovalent (single virus) pandemic vaccine, or a trivalent seasonal influenza vaccine (which includes the H1N1 (2009) strain, as well as other seasonal strains H3 and B), depending on whichever is available.
What can I do?

What major public health measures are recommended to countries now?

As we move out of the pandemic period, it is important for countries to remain vigilant and alert for infections and outbreaks and to continue to take steps to protect their populations against influenza.
WHO recommends that countries:

• maintain monitoring for influenza, through routine respiratory disease surveillance and reporting as well as monitoring and investigating unusual disease patterns suggestive of potential changes in the severity, and monitoring of the H1N1 virus for important changes;
• continue prevention and control efforts to reduce the impact of influenza, including vaccination against influenza. The pandemic monovalent H1N1 vaccine and the seasonal influenza vaccines both now cover the H1N1 virus, either may be used depending on which is available and according to national guidelines.

The H1N1 pandemic provided a significant challenge to all countries. WHO encourages countries to assess their experience and identify lessons that can be applied to strengthen their preparedness and response to future pandemics.

Does WHO still recommend oseltamivir use now that the pandemic is over?

WHO's guidelines for use of antiviral medicines cover both seasonal and pandemic influenza and should continue to be followed. Although we are now in the post pandemic period, the H1N1 (2009) virus is still circulating as one of the seasonal flu strains. It is likely that we will still see severe illness in higher risk individuals as well as otherwise healthy persons. Early recognition and appropriate treatment remain important.
WHO guidelines for use of antiviral medicines

Should people continue to be vaccinated against the H1N1 (2009) influenza virus? If so, how?

WHO recommends the use of influenza vaccine to protect people as a safe and effective countermeasure to reduce the chances of developing severe illness. The H1N1 (2009) influenza virus is expected to continue to circulate worldwide for many years, and many people are still susceptible to infection. The H1N1 (2009) virus caused most of its severe or fatal disease in younger people, both those with chronic conditions as well as healthy persons, and caused many more cases of viral pneumonia than are normally seen with seasonal influenza. WHO particularly recommends vaccination for health care workers and groups at high risk for severe disease.
The current seasonal trivalent vaccine includes the H1N1 (2009) strain, as well as other seasonal strains (H3, B), and will therefore protect against all the expected seasonal influenza viruses. The monovalent (single virus) pandemic vaccine will only protect against the H1N1 (2009) virus.
Since no one can accurately predict which, or how many, of the circulating influenza viruses will infect them, the trivalent seasonal influenza vaccine will provide the broadest protection. However, in some places the trivalent vaccine is not available and in such circumstances, it is still prudent to be vaccinated against the H1N1 (2009) virus using the monovalent H1N1 vaccine to prevent severe illness.
Vaccines for pandemic (H1N1) 2009

[gsgs]

Re: Discussion - When is an influenza pandemic over?

wait until it peaks in the Southern Hemisphere.
~5 weeks, I estimate

when it starts to go down, you can best estimate how bad
(contagious) it was.

But it starts late in NZ,AUS this year, pandemics usually start early

Europe didn't get much last season, except some East-Europe
countries. They may get this season, what USA,Canada had last season.
Except maybe Sweden who is vaccinated.

It was similar here in 2004, which was bad in USA(Fujian-H3N2-flu)
but mild in Germany. The next year it was vice versa.

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