Study leader microbiologist Taia Wang of the Mount Sinai School of Medicine in New York said in a review of about 600 cases, the World Health Organization estimated the virus strain kills more than half its victims, but the WHO study does not account for a majority of infections, USA Today reported.
The team compiled evidence of antibodies to the virus collected from 12,677 Asians, Africans and Europeans in 27 studies from 1997 to 2009 and estimated about 1.2 percent of them had survived mild bird flu cases.
Many might have become infected and recover without being very ill, the researchers said.
For example, the death rate for the 1918 pandemic that killed more than 675,000 in the United States was about 0.5 percent, while the seasonal flu has a lower death rate of about 0.1 percent, mostly among the elderly, the researchers said.
The findings were published in the journal Science.