Influenza is an acute respiratory disease that occurs both in yearly outbreaks, or epidemics, and in much larger, global outbreaks called pandemics. Predicting the next pandemic strain is an important goal because advance preparation of a vaccine could save many lives.
The outbreaks of avian influenza in east and southeast Asia in 2003-2004 has lead to speculation that the next influenza pandemic is at hand. These H5N1 viruses cause disease in millions of domestic birds such as chickens, and furthermore have been shown to infect humans. Time magazine recently ran a cover story on this issue, entitled “Bird Flu Is Asia hatching the next human pandemic?”.
Do the avian H5N1 strains pose a threat to humans? There is no doubt that these viruses can infect and cause disease in humans. The real question is can they be efficiently transmitted from humans to humans, a requirement for a pandemic strain. Some scientists feel that it is only a matter of time before the viruses acquire this property. Or is it?
Avian H5 influenza viruses have apparently been infecting humans for years. In a 1992 study published in Seminars in Respiratory Infections, KF Shortridge and colleagues report significant levels of antibodies against the H5 virus in Asian populations. For example, 7% of sera sampled in Jiangsu Province of China were positive for antibodies against H5 virus. In a region with millions of inhabitants, this number represents a high level of infection. The conclusion is that H5N1 viruses have been infecting humans for years with little consequence. It therefore seems unlikely that these viruses pose any significant threat to humans.
There have been some reports that H5N1 viruses can be transmitted from human to human. All of these reports originate in Asia where it is difficult to determine if the virus that infected multiple household members came from a human, or a bird living in close proximity. To date there are no reliable data which suggest human to human transmission of H5N1 viruses.
The cycling of influenza virus strains in the recent past suggests a different candidate for the next pandemic strain. The H1N1 strains circulated from 1918 until 1957. From that year until 1968, H2N2 strains were prevalent, followed by H3N2 strains. The H1N1 strains re-emerged in 1977. Today, both H3N2 and H1N1 strains cause yearly epidemics of influenza. If influenza strains re-emerge when existing immunity wanes, then it would seem that the next pandemic strain would be an H2N2 strain. Since these strains stopped circulating in humans in 1968, anyone born after that year would be susceptible to infection with an H2N2 virus, because they would have no previous immunity.
In my opinion, we should stop worrying about avian influenza and concentrate on H2N2 strains as the next pandemic influenza virus.