Epidemiology

Highly pathogenic avian influenza (HPAI) H5N1 virus strains have infected poultry or wild birds in more than 50 countries since 2003. Sixteen countries have reported detection of cases of human HPAI H5N1 virus infection since 2003. Between 2003 and 2018, 860 cases were reported, with 454 deaths (a case fatality rate of 53%).[26]

WHO: cumulative number of confirmed human cases of avian influenza A(H5N1) reported to WHO external link opens in a new window

In 2019, only one human case was reported to the World Health Organization.[27] In October 2020, the Lao People’s Democratic Republic reported a case of human infection in a one-year-old female.[28] Most human HPAI H5N1 cases have been among previously healthy children and young adults. The median age of patients is approximately 20 years, with an age range for all patients from under 1 year to 81 years.[29] The ratio of male to female cases is about equal; however, there is a higher case-fatality proportion in females, which may be due to many different epidemiological factors, such as delay in accessing healthcare, case age, and physician testing patterns.[18] From 2003 to 2010, patients under 20 years of age had a significantly lower risk of dying than those aged over 20 years (case-fatality proportions: 52% vs. 66%).[18] Mortality is associated with delayed recognition of disease and hospitalisation after symptom onset.[18] One study reported that the presence of rhinorrhoea appeared to indicate a better prognosis for children with HPAI H5N1.[24]

In January 2014, Canada reported identifying a human case of HPAI H5N1 in a person who travelled from China to Canada, where the patient was hospitalised and died.[30] While there is no indication that the infection was acquired within North America, this case highlights that clinicians should remain vigilant for influenza virus infections, including HPAI H5N1 and other novel influenza A viruses, in critically ill patients. Also in 2014, the United Nations Food and Agriculture Organization (FAO) reported that there were at least 6 countries with endemic HPAI H5N1 virus circulation among poultry populations: Bangladesh, the People’s Republic of China, Egypt, India, Indonesia, and Vietnam, with sporadic poultry outbreaks in other countries.[31] One systematic review and meta analysis of human seroprevalence of H5N1 in China detected an overall seroprevalence of 2.45%. A higher seroprevalence of 7.32% was detected in central China.[32] A cohort study of human avian influenza virus infections in households raising backyard poultry in Egypt found a very low prevalence of H5N1 (0.4% at baseline and 0.2% at follow up).[33] Asian lineage HPAI H5N1 virus strains have not been detected in domestic poultry in North or South America to date, though antigenically distinct HPAI H5N1 viruses not associated with human infection have been identified in birds in the US.[34]

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