The only thing we have to fear is fear itself – Franklin D. Roosevelt
Why is there such widespread fear of avian H5N1 influenza virus?
Why did Paul Keim, chair of theÂ National Science Advisory Board for BiosecurityÂ (NSABB) say “I can’t think of another pathogenic organism that is as scary as this one”. Â What lead Donald McNeil, writing about H5N1 in the New York Times, to conclude that “In its natural form, it is known to have infected only about 600 people since its discovery in 1997, but it killed more than half of them.”
McNeil’sÂ statement is incorrect. Yet it summarizes why Paul Keim, the NSABB, and many others fear the virus.
The problem is that we cannot say with any certainty that the virus has infected only about 600 people. What we do know is that among the 600 seriously ill individuals infected with influenza H5N1 who are admitted to hospital, over half of them die.
To know the fatality rate of avian H5N1 influenza virus in humans, we need to divide the number of fatalities by the number of infections. We do not know that last number – but there are hints that it could be quite large. In a recent study of rural Thai villagers, sera from 800 individuals were collected and analyzed for antibodies against several avian influenza viruses, including H5N1, by hemagglutination-inhibition and neutralization assays. The results indicate that 73 participants (9.1%) had antibody titers against one of two different H5N1 strains. The authors conclude that ‘people in rural central Thailand may have experienced subclinical avian influenza virus infections’. A subclinical infection is one without apparent signs of illness.
If 9% of the rural Asian population has been subclinically infected with avian H5N1 influenza virus strains, it would dramatically change our view of the pathogenicity of the virus.Â Extensive serological studies must be done to determine the extent of human infection with avian H5N1 influenza viruses.
Until we know how many individuals are infected with avian influenza H5N1, we must refrain from making dire conclusions about the pathogenicity of the virus. Doing so has only lead us down a dangerous path of fearing that H5N1 influenza virus might be used as a weapon of bioterrorism, and restricting the publication of scientific papers on the virus.
Update. A meta-analysis reveals that about 1.3% of over 8,500 study participants had serological evidence of infection with influenza H5N1 (Palese, personal communication).
Khuntirat, B., Yoon, I., Blair, P., Krueger, W., Chittaganpitch, M., Putnam, S., Supawat, K., Gibbons, R., Pattamadilok, S., Sawanpanyalert, P., Heil, G., Friary, J., Capuano, A., & Gray, G. (2011). Evidence for Subclinical Avian Influenza Virus Infections Among Rural Thai Villagers Clinical Infectious Diseases, 53 (8) DOI: 10.1093/cid/cir525