Health restrictions in 2020 led to a dramatic drop in flu cases, causing the B/Yamagata lineage to disappear. Could this change the course of flu epidemics?
In 2020, Covid-19 and health restrictions severely limited cases of seasonal flu, the number of which fell by more than 95%. So much so that one of the lineages of flu viruses usually circulating seems to have disappeared… This is what a study published in Science, carried out by Chinese, Belgian and English researchers.
In France, the flu epidemic generally begins around the end of December and affects 2 to 8 million people, causing up to 6,000 deaths. Circulating viruses are divided into two main types, themselves divided into subtypes or lineages: type A viruses exist in many animal species, type B viruses mainly affect humans. Every winter, the two types of flu circulate at the same time, in varying proportions depending on the year. But if we know approximately which strain dominates at what time of the epidemic, “we do not have the exact figures for deaths linked to a particular lineage”, explains Mustapha Si-Tahar, director of the Center for the Study of Respiratory Pathologies at Inserm (National Institute of Health and Medical Research).
Among type B influenza viruses, two lineages coexist, named B/Yamagata and B/Victoria. “Certain years in France, the B/Yamagata strain circulated more than the B/Victoria line», Indicates Olivier Terrier, research director at the CNRS. And yet, already in decline before the Covid pandemic, it now seems to have completely disappeared thanks to health restrictions such as confinement, wearing a mask or even social distancing.
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The virus continued to circulate in South and West Asia
You might think it’s good news, but you shouldn’t rejoice too quickly. According to the study, the disappearance of the B/Yamagata lineage could increase infections by the B/Victoria lineage. Indeed, infections with one of the influenza B lineages offer partial protection against the other. Without B/Yamagata, fewer people will benefit from this cross-immunity, which could promote the spread of B/Victoria, which can cause more severe symptoms in young children. “ Even though B/Yamagata mainly infected adults and the elderly, it indirectly helped immunize children against this strain and type B influenza in general, without them developing symptoms. With its disappearance, children could become more vulnerable to the flu, because they would not have benefited from this cross-immunity », specifies Olivier Terrier. The researchers point out that although this hypothesis of an increase in infections due to B/Victoria viruses is not confirmed, it remains a serious possibility that deserves to be explored.
This study reveals that certain regions served as an “evolutionary reservoir” for the virus. “ In some areas, for example in South and West Asia, the circulation of influenza types A and B/Victoria continued » during the Covid pandemic, write the authors. The preeminence of this or that strain lasted even longer, probably due to the reduction in long-distance travel that usually allows for the replacement of flu types. The researchers concluded that international travel is mainly responsible for the spread of influenza lineages, but also probably of other respiratory pathogens. However, ” we do not think that one lineage will become dominant over the others even if some disappear. Indeed, we have now returned to a diversity of strains similar to the pre-pandemic period, with the exception of the disappearance of B/Yamagata », continues Moritz Kraemer, professor of epidemiology and data science at Oxford.
« The return of the various flus even appeared more quickly than we had anticipated », explains Moritz Kraemer. “ What is important for future planning, according to this study, is to implement increased surveillance in areas with high antigenic drift, that is to say the process of evolution of the virus, areas where the emergence of new lineages which then become global », continues the researcher.
