Influenza-associated excess mortality associated with influenza B in Hong Kong, 2014-2023

Background Influenza B epidemics can have substantial public health impact. We aimed to estimate the mortality burden associated with influenza B virus infections over a 7-year period in Hong Kong. Methods Age- and cause-specific (i.e., respiratory diseases, circulatory diseases, renal diseases and other causes) and all-cause mortality rates in Hong Kong from 2014 through 2023 were fit to linear regression models with influenza B virus lineages as covariates. The influenza-associated excess mortality from influenza B viruses was estimated as the difference between fitted death rates with or without influenza B virus activity. Results Between 2014 and 2023, B/Yamagata predominated in four seasonal epidemics but eventually disappeared in 2020. In contrast, B/Victoria was predominant only in 2016, with influenza A(H1N1) and B/Yamagata co-circulating during that year. The annual respiratory excess mortality rate associated with influenza B was 3.5 (95% credible interval (CrI): 2.4, 4.6) per 100,000 person-years. We estimated an average of 260 (95% CrI: 180, 340) excess deaths associated with influenza B annually from 2014 through 2023, with a majority of the excess deaths occurring in adults ≥65 years of age. Influenza B/Yamagata epidemics were associated with more excess deaths than influenza B/Victoria, and the majority of influenza-associated deaths were from respiratory causes. Conclusions Influenza B was associated with mortality burden each year, mainly among older adults, from 2014-2023. The disappearance of influenza B/Yamagata since 2020 suggests that influenza B burden will be lower in the future.