Director-General of the World Health Organization Margaret Chan, MD, MPH, has declared an end to the H1N1 influenza pandemic, following the advice of an emergency committee convened to address the issue.
"The new H1N1 virus has largely run its course," she said from Hong Kong in a conference call with reporters.
Based on previous pandemics, Chan said the virus is expected to behave like seasonal influenza viruses and continue to circulate "for some years to come." It also will likely continue to cause serious disease in younger individuals, at least in the immediate post-pandemic period, she said.
Chan noted that significant localized outbreaks of H1N1 were still possible, as is currently occurring in New Zealand.
Because of the continuing threat, the WHO continues to recommend immunization with both the monovalent H1N1 vaccine and the trivalent seasonal vaccine, of which the new H1N1 virus is a component.
The decision to declare an end to the pandemic was made based on several factors, according to Chan: the lack of out-of-season -- or summer -- outbreaks in either the Northern or Southern Hemisphere, the fact that current H1N1 outbreaks are similar in intensity to those of seasonal influenza, and the increasing mix of circulating influenza viruses.
During the pandemic, H1N1 largely crowded out the seasonal viruses, accounting for nearly all tested samples. Now, Chan noted, countries are observing a mix of H1N1, H3N2, and B viruses.
An additional factor that went into the decision to end the pandemic designation is evidence of community-level immunity. Studies have indicated that 20% to 40% of the population in some areas have been infected with H1N1 and, thus, have some level of immunity.
Good vaccine coverage in some areas has also contributed to community-wide immunity, Chan added.
Keiji Fukuda, MD, special adviser to Chan on pandemic influenza, urged countries to continue surveillance and remain alert for changes in the H1N1 virus. Public health officials should maintain control efforts, especially in the countries that continue to have intense activity, he added.
Chan echoed the need for continued vigilance.
"Pandemics are unpredictable and prone to deliver surprises," she said.
She noted that the mild nature of the pandemic was aided by good luck -- the virus did not mutate into a more lethal form, widespread resistance to oseltamivir did not develop, and the vaccine was a good match with the circulating virus and had an excellent safety profile.
"Had things gone wrong in any of these areas," Chan said, "we would be in a very different situation today".