The unexpected side effect of COVID-19 lockdowns
21 December 2024
In the wake of the global response to the COVID-19 pandemic, an unexpected side effect has been observed: the apparent disappearance of the B/Yamagata lineage of the influenza B virus. Since March 2020, this strain has gone undetected, leading some to speculate about its extinction.
Influenza viruses are categorized into types A, B, C, and D, based on their core proteins. Type A viruses are further divided into subtypes according to the combinations of two proteins on their surface: hemagglutinin (H) and neuraminidase (N). There are 18 different H subtypes and 11 N subtypes known, leading to numerous combinations, primarily circulating in birds. A few that managed to jump species are now circulating in humans: the H1N1 and the H3N2 subtypes. Contrary to A viruses, type B influenza viruses primarily circulate in humans and a few other mammals. The do not have subtypes, but are split into two lineages: B/Yamagata and B/Victoria, based on genetic and antigenic differences. Influenza types C and D are less significant for human health, with type C causing mild respiratory illness and type D primarily affecting cattle.
The onset of the COVID-19 crisis brought about drastic measures to curb the spread of SARS-CoV-2 (the virus behind the COVID-19 illness) including mask mandates, social distancing, travel restrictions, and enhanced hygiene practices. These interventions not only significantly reduced the transmission of the coronavirus but also had a profound impact on other respiratory viruses, including influenza. Data from around the world indicated a dramatic drop in influenza cases following lockdowns, with influenza B/Yamagata being notably absent from reports since the early stages of the global lockdown in 2020.
The lack of detection of the B/Yamagata lineage has led to a change in the vaccine formulation for the 2024-2025 season. Vaccines for the season in the Northern Hemisphere are now excluding this strain, focusing instead on three main strains: Influenza A (H1N1 and H3N2) and the B/Victoria lineage. This decision was based on the absence of B/Yamagata in influenza surveillance systems worldwide, suggesting its possible extinction.
However, the notion of viral extinction in the context of influenza, a virus known for its antigenic drift and shift, is complex. Influenza B/Yamagata does not have an animal reservoir like some influenza A strains, making it potentially more vulnerable to eradication. Still, the virus has shown resilience in the past, with B lineages known to fluctuate in prevalence over time.
Despite the compelling evidence for the disappearance of B/Yamagata, there's a significant caveat: global influenza surveillance isn't uniform or comprehensive enough to definitively claim extinction. In regions near the equator, where influenza can circulate year-round due to consistent climate conditions, testing capabilities might be limited. This raises the possibility that the virus could still be circulating undetected in these areas or in populations that are less accessible to health surveillance systems.
Countries with lower health infrastructure might not report cases accurately or might not have the resources to conduct widespread testing for influenza. Moreover, the focus on SARS-CoV-2 during the peak of the pandemic might have shifted resources away from other respiratory virus surveillance, potentially missing cases of influenza B/Yamagata.
Even if B/Yamagata has indeed vanished from most parts of the world, its potential re-emergence or hiding in lower surveillance areas cannot be ruled out without comprehensive monitoring. The potential extinction of a major influenza strain could simplify flu vaccine composition and reduce disease burden. On the other, it might lead to complacency in surveillance and preparation for other influenza strains that could evolve or re-emerge.
Only time will confirm us whether the virus has been effectively eradicated.