H5N1 fatality rate questioned

The current controversy over publishing data concerning transmissible strains of the H5N1 avian flu has revived a debate over how lethal the virus is.

The fatality rate for officially confirmed human cases of H5N1 avian influenza infection is considered a remarkably high 59 percent. There have been 345 deaths in 584 cases, according to CIDRAP News.

Proponents of publishing the full details of two independent studies that created H5N1 strains that can be spread through the air and are transmissible in humans say the true fatality rate is probably much lower because of unreported, mild or asymptomatic cases of the illness.

Several seroprevalence studies, where researchers look for the presence of H5N1 antibodies in those who are likely to have been exposed to the virus, have been conducted since 2003.

A 2011 analysis of 17 seroprevalence studies showed the proportion of seropositive subjects to range from zero to 2.8 percent, with most reporting no seropositive subjects, CIDRAP News reports.

Experts note that the data is clouded by uncertainty, including the range of differences in methodology and the antibody titer level used to determine a positive antibody finding. It also remains unclear how long H5N1 antibodies remain in the blood.

"There are many, many uncertainties," Dr. Tim Uyeki of the U.S. Centers for Disease Control and Prevention said, according to CIDRAP News. Uyeki is the deputy chief for science in the CDC's Epidemiology Branch of the Influenza Division and has been involved in H5N1 research around the world.

"There are a number of H5N1 antibody studies that have been conducted in several countries, and they've been conducted in different populations with different sampling methods and different lab methodologies," Uyeki said, CIDRAP News reports. "At the same time, fatal cases reported are probably an underestimate of those that have occurred," he said. "So the case-fatality proportion is likely to be lower, but it's not really clear that it's orders of magnitude lower."