In a new perspective article today, experts from the World Health Organization (WHO) acknowledged the puzzling preponderance of older men among H7N9 influenza patients in China and suggested that studying the habits of elderly Chinese men may help provide some answers.

Experts from the WHO´s Western Pacific Region Office (WPRO) wrote that among the 63 H7N9 cases reported from Mar 31 through Apr 16, the median age was 64, and 45 patients (71%) were male. Thirty-nine of the 63 patients (62%) were at least 60 years old.

This contrasts with the profile of the general Chinese population, "which has a large proportion of young and middle-aged adults and a greater number of women among the elderly," says the article, which was published online in the WHO´s Western Pacific Surveillance and Response Journal.

The WHO experts also found a distinct difference in the case-fatality rate (CFR) between elderly men and elderly women. For all ages, the CFR for males was the same as for females: 22% (10 of 45 and 4 of 18). But for those 60 and older, the CFR for men was 20% (6 of 30), while for women it was zero (0 of 9).

As others have pointed out, the H7N9 age distribution differs from that of the 45 H5N1 cases recorded in China, the WHO authors note. Most of the H5N1 patients were young, working-age adults, with a median age of 26, the WHO experts found.

They list three potential reasons for the age and sex distribution in H7N9: gender-related differences in exposures to the virus, gender-linked biological differences in the clinical course of illness, and gender-related differences in healthcare-seeking behavior or healthcare access.

Concerning possible exposure differences, the article notes that live-bird markets (LBMs) have been the primary sites where the H7N9 virus has been found in poultry and environmental samples in affected areas. However, "age- and sex-specific LBM visit patterns are unknown."

The authors go on to suggest that pet birds may be worth investigating as a possible source of the virus: "Elderly Chinese men are also well-known to be hobbyists of ornamental pet songbirds and take frequent and extended walks with their caged birds, congregating together in parks."

They add, "A better understanding of the social norms and behaviors among elderly Chinese men in affected areas may better guide us in the investigation (eg, by identifying hypotheses for case-control series)."

The article offers no hypotheses about the role of possible biological differences. But it suggests that useful information might be gained from serologic investigations among close contacts of patients and studies of underlying conditions and other risk factors among the patients themselves.

The authors doubt that H7N9 cases in elderly men are more likely to be detected than cases in other subgroups, given the high severity of illness in most cases, high public awareness of the outbreak, and nationwide enhanced surveillance for flu-like illness.

"At this time, it is clear that there are more questions than answers," the WHO experts write. They suggest that investigators focus on three avenues of inquiry:

• The "societal norms and common social practices" of elderly men in the affected provinces
• The age and sex distribution of severe acute respiratory illness and key risk factors for respiratory illness in the general population in the affected region
• The age and sex distribution of healthcare use in the general population in the affected provinces

See also:

Apr 22 WHO WPRO perspective article

Apr 19 CIDRAP News story "H7N9 mystery: Why does age profile tilt older?"