SUMMARY: The World Health Organization (WHO) announced this week that 2009 H1N1 influenza A, popularly known as swine flu, has now entered a post-pandemic period. Although localized outbreaks are likely to continue, H1N1 is no longer being widely transmitted worldwide, as it was last fall and winter. Nevertheless, the agency stressed that the course of influenza pandemics is unpredictable and continued vigilance is warranted.

By Liz Highleyman

The decision to declare the post-pandemic status was made by an Emergency Committee based on assessment of the global situation, as well as reports from several countries currently experiencing influenza outbreaks. WHO waited until winter in the Southern hemisphere to determine whether the H1N1 virus is now starting to behave like normal seasonal influenza.

"As we enter the post-pandemic period, this does not mean that the H1N1 virus has gone away," said WHO Director-General Margaret Chan at a virtual press conference announcing the change. "Based on experience with past pandemics, we expect the H1N1 virus to take on the behavior of a seasonal influenza virus and continue to circulate for some years to come."

Globally, the current levels and patterns of H1N1 transmission differ significantly from what was observed during the pandemic, Chan explained. Out-of-season outbreaks are no longer being reported in either the northern or southern hemisphere, and flu outbreaks -- including those primarily caused by H1N1 -- are of an intensity similar to that of normal seasonal epidemics.

During the pandemic period, the H1N1 virus "crowded out" other influenza strains and became the dominant virus, but this is no longer the case, she said. Many countries are now reporting a mix of influenza viruses, as is typically seen during normal seasonal outbreaks.

The pandemic, which was first identified in Mexico in the Spring of 2009, spread quickly among a global population with minimal immunity, though evidence emerged to suggest that healthy adults might have residual immunity from prior exposure to a similar flu virus.

"Recently published studies indicate that 20%-40% of populations in some areas have been infected by the H1N1 virus and thus have some level of protective immunity," Chan reported. "Many countries report good vaccination coverage, especially in high-risk groups, and this coverage further increases community-wide immunity."

"Based on available evidence and experience from past pandemics, it is likely that the virus will continue to cause serious disease in younger age groups, at least in the immediate post-pandemic period," she said. "Groups identified during the pandemic as at higher risk of severe or fatal illness will probably remain at heightened risk, though hopefully the number of such cases will diminish."

Despite the official end of the pandemic, WHO continues to recommend influenza vaccination, especially for healthcare workers and groups at high risk for severe disease, such as pregnant women. Vaccines now being produced protect against both expected seasonal flu strains and H1N1.

The H1N1 pandemic turned out to be less problematic than public health authorities had feared. The H1N1 virus did not mutate into a more lethal form, develop widespread resistance to oseltamivir (Tamiflu, the most widely used flu drug), and the vaccine rushed into production was a good match with circulating viruses and had a good safety profile.

But pandemics -- like the viruses that cause them -- are unpredictable, and continued vigilance is extremely important, Chan emphasized.

"No two pandemics are ever alike," she concluded. "This pandemic has turned out to be much more fortunate than what we feared a little over a year ago...This time around, we have been aided by pure good luck."

For further information:

8/13/10

Sources

M Chan. H1N1 in post-pandemic period. Media statement. August 10, 2010.

World Health Organization. WHO recommendations for the post-pandemic period. Pandemic (H1N1) 2009 Briefing Note 23. August 10, 2001.