(HealthDay News) – Based on cases in hospitalized patients confirmed by the World Health Organization (WHO), the current estimated overall case fatality rate for H5N1 infection in humans is greater than 50%, but the stringent criteria for confirmation of the infection mean the actual fatality rate may be much lower, according to a meta-analysis published online Feb. 23 in Science.

Taia T. Wang, from the Mount Sinai School of Medicine in New York City, and colleagues conducted a meta-analysis of studies to assess the serological evidence of H5N1 infections in humans. In a primary analysis, 19 studies were included, with 7,304 participants. A secondary analysis was conducted on 10 studies that could not be interpreted by WHO guidelines, and which included 6,774 participants. A sub-analysis was conducted of 2,729 participants who were employed as poultry workers.

The researchers found that, for studies in the primary analysis, rates of seropositivity ranged from 0%–5.3%, except for one study that reported 11.7% positivity. The overall seropositivity rate was 1.2% in the meta-analysis. The seropositivity rate was 1.9% for the 10 studies that could not be interpreted by the WHO guidelines. In the poultry worker sub-analysis, seropositivity rates were approximately 1.4%. Based on data from 20 studies comprising 12,677 participants, avian H5N1 was found to cause mild or subclinical infections not currently accounted for under the WHO criteria, and, therefore, the fatality rate may be much lower than previously thought.

“We suggest that further investigation, on a large scale and by a standardized approach, is warranted to better estimate the total number of H5N1 infections that have occurred in humans,” the authors write.

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