In the spring of 2020, as the COVID-19 pandemic broke out, draconian public health measures were put in place in every country around the world. In particular, air traffic was interrupted. This considerable change in human behavior had important consequences on the dispersal and evolution of seasonal influenza viruses, found researchers in a study published in the journal Science.
Non-pharmacological interventions, such as mask wearing, hand washing, social distancing and travel restrictions, including the suspension of international flights, which have been imposed in the majority of countries in the Northern Hemisphere and the Southern Hemisphere to curb the spread of COVID-19 have proven to be very effective in countering the flu, researchers have noted. Seasonal flu viruses completely disappeared from radar in these countries during the acute phase of the pandemic.
During this same period, however, outbreaks occurred in some regions of Asia and Africa with a tropical climate, which allows the circulation of influenza viruses throughout the year and where the restrictions associated with the pandemic were less severe. Influenza viruses thus continued to circulate and evolve in these few regions. Specifically, the two A subtypes (H1N1pdm09 and H3N2) persisted in South Asia, while the Victoria type B lineage became “confined” to West Asia and Africa.
Since March 2020, since the very beginning of the pandemic, the Yamagata type B lineage (B/Yamagata) has never again been detected in the world. “Obviously this lineage has become extinct,” says Dr.r Donald Vinh, microbiologist-infectious disease specialist at the McGill University Health Center (MUHC).
According to the study’s researchers, its extinction could be explained by a combination of factors, such as significant changes in human behavior during the COVID-19 pandemic, a decrease in the number of humans likely to be infected in due to the immunity that they would have acquired during a large outbreak in 2017-2018, as well as the slow evolution of the virus.
For this reason, the WHO has recommended no longer including this lineage in vaccines prepared for the autumn and southern winter of 2024 in the Southern Hemisphere and for the 2024-2025 influenza season in the Northern Hemisphere.
Then, in 2023-2024, with international air transport returning to its pre-pandemic frequency, the spread of influenza viruses between different regions of the world began again and generally returned to pre-pandemic levels. “Once air transport resumed, the new strains which had emerged in the regions of the world where the virus continued basal activity during the pandemic then spread to other countries,” summarizes the Dr Vinh.
The researchers also noted that in the few regions where influenza viruses continued to thrive during the pandemic, the dwell time of these viruses, that is, the time during which the viruses remained confined to the within the same population, has become longer than what normally happens. This longer residence time may have induced “a greater local diversification of viral lineages and, therefore, the emergence of variants”, which increased “the risk that the strains chosen for the composition of the vaccines were not adequate for the populations who would receive them.
“The fact of circulating for a long time within the same population of people allowed the strains to “refine” themselves by accumulating mutations which made them capable of infecting people who had become more immune than elsewhere. The viruses of these strains have thus learned to escape the immunity developed by the population. But this dynamic could also lead to the disappearance of a lineage, as was the case for B/Yamagata which perhaps did not manage to evolve quickly enough to overcome people’s immunity,” explains Dr.r Vinh.
The researchers also suggest that populations who were not exposed to influenza during the pandemic probably have less immunity to this pathogen due to the few natural infections and a drop in vaccinations against influenza during this pandemic. period, which could “induce larger influenza epidemics in the future”. This phenomenon could thus explain this major epidemic that Hong Kong experienced in 2023, they emphasize.
“Population immunity in our regions is almost zero because there have been almost no infections [les virus de la grippe n’étant pas présents]and very few people have received the vaccine during the pandemic. Subsequently, because of vaccine fatigue, fewer people will get vaccinated for the flu. These three factors make people truly “naive” immunologically. They therefore risk being really ill if they catch the flu,” specifies the Dr Vinh.
The population has “necessary catching up” to do to fill “this little hole in our immune system”. “We would need a good percentage of people to receive the vaccine. With a vaccination rate of 40% to 50% as is usual, this catch-up could take longer,” he predicts.
According to two researchers from the University of Georgia, in the United States, who published a Perspective in the journal Science“this study reinforces the observation that non-pharmacological interventions can prove incredibly effective in disrupting or even interrupting the transmission of viruses, and disrupting the diversification of these pathogens and their antigenic evolution. And that they are undoubtedly more effective than vaccination alone.”
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