The influenza virus behind the global pandemic of 2009, often referred to as “swine flu”, faces two probable fates: it will either continue to cause low or moderate mortality or it will go extinct. That’s the judgment of the authors of a new Perspectives piece in mBio, which points out that the impact of the virus this flu season will depend largely on the degree of immunity in the population, and since the virus was quick to spread last year and H1N1 vaccines have been effective, population immunity is high.
The 2009 pandemic H1N1 virus caused widespread alarm last flu season after it emerged in Mexico and spread quickly to the United States and around the globe. Today, population immunity to the virus is around 59% or greater, making explosive recurrences of H1N1 unlikely, according to the paper. And the history of influenza epidemics in the past offers still more optimism: it is likely that H1N1 will eventually establish stable circulation in the population, since every pandemic flu virus since the Spanish influenza outbreak of 1918 became less severe after successive antigenic shifts.
“I think that this review is well-placed, hopefully, to allay concerns,” says Keith Klugman, the William H. Foege Chair of Global Health in the Hubert Department of Global Health at Emory University, and a member of mBio’s board of editors.
Not only are the population immunity statistics promising, Klugman says, medicine is also better prepared to deal with complications of flu. “Up to 90% of deaths in the 1918 outbreak were due to the virus triggering bacterial infections,” says Klugman. We now give patients with severe influenza antibiotics specifically to prevent secondary bacterial infection, he says. “In the context of a modern world, this has led to reduced mortality,” says Klugman.
But the evolution of influenza viruses still holds many mysteries, and although the outlook for the coming season is positive, the authors caution that antigenic changes in the pandemic H1N1 virus could yet turn the tables. The virus may mutate using mechanisms such as reassortment with seasonal H3N2 or H1N1 viruses, which could conceivably produce an oseltamivir-resistant virus, seriously complicating treatment and prophylaxis for at-risk patients, or antigenic drift in the virus could result in unforeseen changes. “Flu is very difficult to predict,” agrees Klugman. “You have to be very cautious in predicting influenza behavior.”
Regardless of the optimism and uncertainty, the authors recommend vaccination programs continue apace this fall, with aggressive targeting of the elderly, who are at greatest risk of complications from flu, and the under-50 age group, who are least likely to have immunity from exposure to prior flu strains.