More people may have been infected by the virus that causes bird flu than previously thought, and the number of deaths from H5N1 infection may also have been "overestimated," New York scientists conclude in a study of 12,500 people worldwide.

The study, published Thursday in the journal Science, notes that the World Health Organization (WHO) has documented fewer than 600 cases of H5N1 influenza A infection — also called avian infection — in humans.

However, the WHO's stringent criteria for confirming bird flu in humans — often based on whether the symptoms are severe enough to result in hospitalization or even death — doesn't account for the majority of infections, according to Dr. Taia T. Wang of the Mount Sinai School of Medicine in New York City and his colleagues.

Confirmed cases, deaths from bird flu in humans:

  • Azerbaijan: 8 cases, 5 deaths.
  • Bangladesh: 3, 0.
  • Cambodia: 18, 16.
  • China: 40, 26.
  • Djibouti: 1, 0.
  • Egypt: 155, 53.
  • Indonesia: 182, 150.
  • Iraq: 3, 2.
  • Lao's People's Democratic Republic: 2, 2.
  • Myanmar (Burma): 1, 0.
  • Nigeria: 1, 1.
  • Pakistan: 3, 1.
  • Thailand: 25, 17.
  • Turkey: 12, 4.
  • Vietnam: 119, 59.

Source: World Health Organization

As well, the research suggests, the current fatality rate based on WHO criteria is probably "overestimated."

The WHO says the primary risk factor for human infection with the avian virus appears to be direct or indirect exposure to infected live or dead animals, or contaminated environments. But experts fear the bird flu virus could mutate into a form easily transmissible between humans, with the potential to kill millions in a pandemic.

People in Indonesia, Vietnam hardest hit

The study published Thursday is based on an analysis of 12,500 participants in 20 studies around the world. The data included people who didn't work with poultry, as well as those who did.

Fact: Most avian influenza viruses do not cause disease in humans. However, some are zoonotic, meaning they can infect humans and cause disease. The most common example is the avian influenza subtype H5N1 viruses in poultry in parts of Asia and northeast Africa that have caused human disease and deaths. Other avian influenza subtypes, including H7N7 and H9N2, have also infected people and caused some deaths, but many of those infections have been mild or caused no symptoms in humans. (World Health Organization)

The researchers found one to two per cent of the subjects in the analysis had evidence of prior H5N1 infection, but because their symptoms were so mild, reporting no respiratory illness, for instance, they did not meet the WHO's criteria for confirming infection.

N5N1 can cause a rate of "mild or subclinical infections" in humans that is not currently documented. As a result, the true fatality rate from the infection is likely to be less than the frequently reported rate of more than 50 per cent, the study also concludes.

The researchers added that further study, using a large-scale, standardized approach, is needed to determine the true rate of infection and deaths from the virus.

Since 2003, the H5N1 strain has infected 573 people and killed 336, notably in Indonesia, Egypt, China, Cambodia and Indonesia, according to WHO figures from 2003-2011.

Indonesia, which has been hardest hit by bird flu with two human deaths, recorded two fatalities in the capital of Jakarta in January.