No, we can’t consider it a seasonal virus yet. Sars-CoV2 has given rise to peaks in the incidence of infections that have not yet reached regularity. A virus is defined as seasonal, in fact, when these peaks are regular: some have winter peaks, others spring for example. Sars-CoV2 still lacks such regularity, although it will probably reach it over time, but even today we observe more than one peak every year. This is probably linked to the fact that population immunity has not reached a level such as to slow down the circulation of the virus even more and, above all, because new subvariants are involved. If a sub-variant intervenes which quickly replaces the previous one, in a period of less than twelve months, it is clear that any seasonal factors that have emerged are interrupted. This is both because we have a population that is not completely immune, and because the subvariant escapes the immune system.
For Sars-CoV2 to become seasonal it could take several years: in previous flu pandemics we had one or two pandemic peaks and then the worldwide circulation of the virus synchronized, the virus became seasonal more or less after two seasons. We have not seen this with the coronavirus, probably due to the ability of the virus, especially in the omicron version, to produce subvariants with a strong immune escape, which thus manage to escape the previous immune defenses. In any case, the more the virus circulates, the more population immunity tends to increase, regardless of the subvariants: therefore it is true that we can have peaks even twice a year, but the intensity, as a disease impact, over time decreases and the share of the population that has a severe form of the disease tends to decrease.
*Pierluigi Lopalco is professor of Hygiene at the University of Salento
