Severity of human infections of avian influenza A(H7N9) in China between 2014 and 2018

      Purpose: Avian influenza A(H7N9) is considered an important zoonotic pathogen, human cases of which have been increasing in multiple epidemics waves since it was idnetified in 2013. Characterising the temporal pattern of mortality or clinical severity of human infection can identify changes in viral pathogenicity and help inform emergence and pandemic risk assessments. Here, we consider how the severity of A(H7N9) in humans has changed by epidemic wave, after adjusting for demographic and spatial factors, and including cases up to June 2018.
      Methods & Materials: Using data published by the Hong Kong Centre for Health Protection, the association of epidemic wave with death and being clinically severe (defined as a case being characterised as fatal or critical) was estimated using generalised additive models, adjusting for age, sex and province of cases.
      Results: We found significant changes in case fatality between waves, but no evidence that the largest epidemic wave (2016–17) was associated with increased mortality compared to previous waves. Mortality was significantly associated with age, with older ages tending to have higher mortality, and with province. The risk of being clinically severe in more recent waves (2015–16 and 2016–17) was significantly lower than during the previous two waves; there was a significant association with age, and significant differences between provinces.
      Conclusion: We found no evidence of an increase in mortality of human cases of A(H7N9) in the most recent waves, despite a marked increase in the number of cases between 2016 and 2017 and the emergence of a highly pathogenic viral varient, suggesting that the average pathogenicity of the virus has not changed. However, the risk of being a clinically severe case was lower in more recent waves, possibly due to improved clinical care or more rapid diagnosis and treatment.