Bird flu most deadly in teens, young adults: WHO

Updated Fri. Jun. 30 2006 11:27 PM ET

Canadian Press

The H5N1 avian flu virus has exacted an alarmingly high death toll among adolescents and young adults -- an eerie echo of the infamous Spanish flu, a new analysis of cumulative cases by the World Health Organization confirms.

"The differences in the age-related case-fatality distribution among H5N1 cases are reminiscent of those observed during previous pandemics, particularly in 1918, where case-fatality rates were higher among young adults," the review said.

The report, published Friday in the WHO's publication the Weekly Epidemiolgical Record, also urges countries to share data on avian flu cases, saying doing so "will collectively defend all countries against a common threat.''

One of very few reviews drawing together details of accumulated H5N1 cases, the report looked at the trends evident in the 203 confirmed cases in nine countries that occurred between December 2003 and the end of April 2006. Of that number, 113 people or 56 per cent died.

A number of infections have occurred since, bringing the global total to 228 cases and 130 deaths in 10 countries.

The age group with the highest fatality rate was 10-to-19-years olds; 73 per cent of cases in that age range who contracted the virus died from it, noted the authors. (As is the practice of the Weekly Epidemiological Record, authors are not listed by name.)

Sixty-two per cent of 20-to-29-year olds and 61 per cent of 30-to-39-year olds who tested positive for the virus succumbed to the infection, said the report.

By age 50 and older, the fatality rate dropped to 18 per cent, though the overall number of infections in older adults is low in comparison with younger age groups. In the very young -- under age five, and five to nine years of age -- the fatality rates were 43 per cent and 41 per cent respectively.

Adolescents and young adults weren't just more likely to die from the virus; they were also more likely to become infected in the first place, the review confirmed. The highest proportion of cases occurred in people aged 10 to 29 years.

In part, that might relate to the fact that many of the countries which have seen human cases have young populations, the authors said.

Exposure patterns in adolescents and young adults could also help explain the spike in infections in those aged 10 to 29, the report said, noting that young girls and women might be more at risk because they are often involved in culling, defeathering and preparing chickens for consumption. There were slightly more female cases than male, 106 to 97.

The report cautioned against drawing too many conclusions on spotty evidence.

"The incomplete nature of the data on exposure make it difficult to infer a link between age and exposure, and further studies are needed, especially to assess whether younger people or other groups (such as pregnant women) have an increased risk of contracting the infection," the report said.

British influenza expert Dr. Angus Nicoll recently bemoaned the lack of detailed data on the human cases and disease outbreaks, calling it "a collective failure ... that must be overcome."

Nicoll, who co-ordinates influenza activities at the European Centre for Disease Prevention and Control in Stockholm, complained that the number of analytical reports of outbreaks is "embarrassingly small."

"Consequently little more is known now than in 1997 about an infection that seemingly remains hard for humans to acquire, but is highly lethal when they do," he wrote in an editorial in the May issue of Eurosurveillance, an online publication on European communicable disease surveillance and control.

The lack of good follow-up studies after outbreaks are contained means the world still isn't clear if some people are getting the disease but only experiencing mild or virtually no symptoms, for instance. Asymptomatic infections, as they are called, occur with many infectious diseases, though not all.

This piece of information is crucial as it would indicate whether calculating the fatality rate based on recorded cases overestimates the lethality of the disease.

If studies testing the blood of exposed people showed many had antibodies to the virus -- signalling they had been infected and survived -- it would be proof the fatality rate was actually much lower.

Few such studies have been done and fewer still have been published in the scientific literature. Those that are in the public domain suggest mild, missed cases are not occurring.

The many knowledge gaps about H5N1 and its infection and fatality pattern highlight the need for countries which have human cases to share information with the global community, the report said.

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