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Biosecurity BriefingStudy Suggests Increase of H5N1 Subclinical Infections By Kunal Rambhia, February 1, 2008 On January 25, 2008, the Center for Infectious Disease Research & Policy (CIDRAP) reported on the Bangkok International Conference on Avian Influenza, which was attended by 500 experts from 40 countries from January 23-25. According to CIDRAP, one of the topics covered during the conference was a Cambodian study (not yet published) which tested 674 people who were exposed to H5N1 and found that seven of those who were tested already had antibodies for the virus, suggesting previous infection.1 On January 24, Bloomberg News reported that the Cambodian study “indicates more people, particularly children and adolescents, may be contracting the virus without developing the high fever and severe pneumonia that’s the hallmark of H5N1 in people.”2 This study contradicts previous research findings on this topic. A review from the New England Journal of Medicine (NEJM) published January 17, 2008, indicated that since 2003 asymptomatic or mild cases of H5N1 infection have been “rare.”3 Additionally, a previous study in Cambodia published in Emerging Infectious Diseases in 2006 tested 351 people and found that none had antibodies for H5N1.4 According to CIDRAP, the mild cases of H5N1 observed in this new Cambodian study “suggest that the virus is improving its ability to spread among humans, while becoming less virulent.”1 In the Cambodian study, those with antibodies for the virus ranged in age from 4-18 years.2 The NEJM review noted that of all the recorded cases of human H5N1 infection, the highest fatality rate has occurred in persons aged 10-19 years. Additionally, 90% of infections have occurred in people under 40 years old.3 According to Malik Peiris, microbiologist at the University of Hong Kong, children are thought to be more susceptible to infection because they “tend to have a greater number than adults of a specific type of receptor in their upper airway that’s more ‘avian-like’ and which enables the H5N1 virus to more easily invade the body.”2 Pathologist John Nicholls at the University of Hong Kong also added that, “[children’s] shorter respiratory tract means the virus needs to travel a shorter distance to invade the lungs” and “mild infection could be the result of fewer viral particles entering the body.”2 References
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