The failed flu waves of 2020 and 2021 have far-reaching evolutionary consequences for the virus. The dramatically lower number of infections worldwide presumably caused a significant number of virus lines to disappear – including possibly even a variant contained in the current flu vaccination. This is reported by a working group led by Vijaykrishna Dhanasekaran from the University of Hong Kong in a preliminary publication. The team analyzed data from the World Health Organization's global influenza surveillance and control system (GISRS), which incorporates global surveillance and sequence data. Among other things, it turned out that the Yamagata line of influenza B, which will also be vaccinated against in the 2021/2022 season, may have become extinct since April 2020. The cause of variant deaths in the flu are the worldwide infection protection measures and travel restrictions against corona. As a result, many chains of infection are running dead and regional outbreaks are not spreading to other areas. In particular, the seasonal flu waves of temperate latitudes are based on the fact that the virus is introduced in winter from other parts of the world where it previously circulated. This is especially true for the subtype H3N2, which dies out every year in the summer and is only distributed permanently in Southeast Asia. This is probably also the reason why three of the eight most important variants of this virus disappeared in the pandemic.
#Preprint: Human Seasonal influenza undercovid-19 and the potential consequences of influenza lineage elimination. pic.twitter.com/vqsvq5jj9t
The fate of the Yamagata line of influenza B, of which no sequence data were available for analysis, is puzzling. Only individual cases were registered by the worldwide monitoring system. But these are so few that they may be false positive cases, Dhanasekaran speculates on Twitter. But it is also conceivable that the virus simply slips through the gaps in surveillance. Under certain circumstances, the line circulates unnoticed in China, where there were no such strict nationwide corona measures, writes virologist Kevin R. McCarthy of the University of Pittsburgh. This certainly applies to the second main line of influenza B, named after the Australian state of Victoria, which even triggered major epidemics in China.
The disappearance of the virus lines is only conditionally good news. Most experts agree that the disappearance of the seasonal flu during the pandemic can make the future waves much more severe after the end of the corona measures. On the one hand, the immune protection decreases over time due to previous infections, and on the other hand, the virus lines are very unevenly distributed regionally. It is possible that, at least in some parts of the world, the next big wave of influenza will come from previously rare virus lines that are poorly covered by the vaccine and against which there is little protection in the population from previous infections.
