Clinical severity of human infections with avian influenza A(H7N9) virus, China, 2013/14

Abstract

Assessing the severity of emerging infections is challenging because of potential biases in case ascertainment. The first human case of infection with influenza A(H7N9) virus was identified in China in March 2013; since then, the virus has caused two epidemic waves in the country. There were 134 laboratory-confirmed cases detected in the first epidemic wave from January to September 2013. In the second epidemic wave of human infections with avian influenza A(H7N9) virus in China from October 2013 to October 2014, we estimated that the risk of death among hospitalised cases of infection with influenza A(H7N9) virus was 48% (95% credibility interval: 42-54%), slightly higher than the corresponding risk in the first wave. Age-specific risks of death among hospitalised cases were also significantly higher in the second wave. Using data on symptomatic cases identified through national sentinel influenza-like illness surveillance, we estimated that the risk of death among symptomatic cases of infection with influenza A(H7N9) virus was 0.10% (95% credibility interval: 0.029-3.6%), which was similar to previous estimates for the first epidemic wave of human infections with influenza A(H7N9) virus in 2013. An increase in the risk of death among hospitalised cases in the second wave could be real because of changes in the virus, because of seasonal changes in host susceptibility to severe infection, or because of variation in treatment practices between hospitals, while the increase could be artefactual because of changes in ascertainment of cases in different areas at different times.

Figure 1

Incidence of laboratory-confirmed human cases of avian influenza A(H7N9) virus infection in China, by date of hospitalization. The first wave of infections in 2013 is divided into two parts, before and after the announcement of human cases on March 31, 2013 because of the potential for under-ascertainment of less severe cases in the earlier period.

Figure 2

Estimates and 95% credibility intervals of the risk of serious outcomes among confirmed H7N9 cases hospitalized for medical reasons, by age and wave. Panel A: the risk of death. Panel B: the risk of death or mechanical ventilation. Panel C: the risk of death or mechanical ventilation or ICU admission. The periods covered by waves 1A, 1B and 2 are shown in Figure 1.

Figure 3

Comparisons of epidemiologic distributions between waves. Panel A: the time from illness onset to hospital admission. Panel B: the time from illness onset to laboratory confirmation. Panel C: the time from hospital admission to death. Panel D: the time from hospital admission to discharge. The periods covered by waves 1A, 1B and 2 are shown in Figure 1.