Is influenza B/Yamagata extinct and what public health implications could this have? An updated literature review and comprehensive assessment of global surveillance databases.

IntroductionEarly after the start of the COVID-19 pandemic, a major drop in the number of influenza B/Yamagata detections was observed globally. Given the potential public health implications, particularly with regards to influenza vaccination, we conducted a systematic review of influenza B/Yamagata virus circulation data from multiple complementary sources of information. MethodsWe searched articles published until 20th March 2023 in PubMed and EMBASE; examined epidemiological and virological influenza data for 2020-2023 contained in the publicly available WHO-FluNet and GISAID (Global Initiative on Sharing All Influenza Data) global databases, or collected by the multi-national Global Influenza Hospital Surveillance Network (GIHSN) study; and looked for influenza data in the webpages of respiratory viruses surveillance systems from countries worldwide. ResultsHighly consistent findings were found across all sources of information, with a progressive decline of influenza B/Yamagata detections from 2020 onwards across all world regions, both in absolute terms (total number of cases), the positivity rate, and as a fraction of influenza B detections. Isolated influenza B/Yamagata cases continue to be sporadically reported, and these are typically vaccine-derived, mistaken data entries or under investigation. DiscussionWhile it is still too early to conclude that B/Yamagata is (or will soon become) extinct, the current epidemiological and virological data call for a rapid response in terms of influenza prevention practices, particularly regarding the formulation of influenza vaccines. The current epidemiological situation is unprecedented in recent decades, underlying the importance of continuously and carefully monitoring the circulation of influenza viruses (as well as SARS-CoV-2 and the other respiratory viruses) in the coming years.