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H1N1 (Swine Flu): WHO Update On Human Cases Of 2009 Avian Influenza (H5N1) Infections

February 16, 2010

The World Health Organization (WHO) just issued the report of the 2009 laboratory-confirmed cases of human infection with highly pathogenic avian influenza A (H5N1) virus.  There were 73 cases reported to the WHO from five countries.

Geographical Distribution

The 73 laboratory-confirmed cases of H5N1 virus infection were reported from:

  • Cambodia (1 case)
  • China (7)
  • Egypt (39)
  • Indonesia (21)
  • Viet Nam (5)

All of these countries had reported human cases of H5N1 previously.

Circulation of highly pathogenic H5N1 virus in poultry is considered to be endemic in these countries, with the exception of Cambodia, where poultry outbreaks occur sporadically. As in previous years, an increase in cases was reported during the northern hemisphere’s winter and spring seasons. Reports of outbreaks among animals also tended to increase during the same period.

Distribution By Sex And Age

In 2009, the overall ratio of male cases (n=35) to female cases (n=38) was almost even (ratio of males to females, 0.92). However, Indonesia reported twice as many cases in females as in males (ratio, 0.5). Cases ranged in age from 6 months to 57 years, with a median age of 5 years. The low median age was due primarily to the high proportion of cases occurring among children in Egypt; Egypt accounted for 53 percent of all cases worldwide.

The age of cases in Egypt was notably younger than elsewhere, with a median age of 3 years; 80 percent of cases occurred in children aged less than 10 years.

Outcome – Case Fatality Ratio (CFR)

The overall case-fatality ratio (CFR) for 2009 was 44 percent, lower than the previous 2 years but similar to the CFR in 2005. CFRs varied widely among countries.

  • Egypt reported its lowest CFR (10 percent) since 2003.
  • In all countries, the CFR was lower for cases aged less than 10 years than for cases aged greater than or equal to 10 years (24 percent versus 71 percent; odds ratio.
  • In contrast to previous years, the CFR for females in all countries was higher than for males, but the difference was not significant (50 percent versus 37 percent.
  • This pattern was not consistent among countries.

Discussion

With almost double the number of human cases of H5N1 reported in 2009 compared with 2008, and with continuing circulation of the virus among certain poultry populations, it is clear that  H5N1 remains a concern for both animal health and public health. Although the CFR remains high, there are differences among affected countries.

These data do not allow a determination of the causes of this difference to be made. Possible explanations for the disparities among countries might include differences in the type or intensity of exposure, in health-seeking behavior, in testing or treatment practices, or even in the virulence of virus strains. The marked difference in the age distribution of cases among countries makes a direct comparison of risk factors difficult, given the relatively small total number of cases available for analysis. At present, this remains an avian virus that has not demonstrated a facility for human-to-human transmission, and human infections remain rare and sporadic.

A total of 3 clusters, each involving 2 family members and without sustained human-to-human transmission, were documented in 2009. It is notable that all cases have occurred in countries with ongoing circulation or reintroduction of highly pathogenic avian influenza A(H5N1) viruses in poultry. Globally, better management of outbreaks in poultry in many countries has led to a decrease in virus circulation and a decreased risk of human exposure. However, human exposure and cases can continue to be expected whenever the virus is circulating.

Efforts should be intensified to decrease both circulation in poultry and the risk of human exposure. Influenza viruses mutate constantly, and vigilance must be maintained. Surveillance of influenza in humans and animals should be strengthened to enable timely detection of epidemiological, clinical and virological changes. The rapid sharing of information globally is essential to ensure a quick and comprehensive assessment and global response.

ProMED Digest V2010 #69 www.promedmail.org

WHO Weekly epidemiological Record Bulletin,  vol. 85, 7 (pp 49-56) http://www.who.int/wer/2010/wer8507.pdf

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