Human infection with H5N1 influenza may not be nearly as deadly as supposed up to now, based on a meta-analysis published online on Feb. 23 that reviewed 20 reports since 1999 that included a total of more than 12,000 people from areas of H5N1 outbreaks.
Until now, estimates of the risk posed by avian H5N1 flu infection came from reports on 573 documented infections in people, which had a 59% fatality rate. But these cases may have only represented the most severe infections that required hospitalizations, while other, milder or subclinical infections may have been overlooked, suggested Dr. Peter Palese and his co-workers at Mount Sinai School of Medicine in New York.
If their hypothesis is correct, if could mean that the intense concerns raised about the safety of recent H5N1 research may be overwrought.
To get a better sense of how many people may have been infected by H5N1, they searched the medical literature since 1999 and found 20 published studies that included assessment of 12,677 people. These studies primarily occurred in East Asia — China, Thailand, Cambodia, Vietnam, and Indonesia — as well as in Turkey and Nigeria, and mostly included people who worked with poultry, were health-care workers, or had other sources of exposure. When tallied together, the reports collectively showed a seropositivity rate that averaged 1%-2%.
“If one assumes a 1%-2% infection rate in exposed populations, this would likely translate into millions of people who have been infected, worldwide,” the researchers concluded in their study. Although their data do not allow calculation of a revised fatality rate, the numbers they found suggested that “the true fatality rate for H5N1 influenza viruses is likely to be less than the frequently reported rate of more than 50%.”
—Mitchel Zoler (on Twitter @mitchelzoler)