Judith Mueller is an epidemiologist, professor at the School of Advanced Studies in Public Health (EHESP) and researcher at the Institut Pasteur. She returns to the interest of the fourth dose (second booster), 18 months after the launch of the vaccination against Covid-19 and following the seventh wave of the epidemic with the Omicron BA.4 and BA variants. 5 , and provides an update on the effectiveness of current vaccines against Omicron variants.

The Conversation: The vaccines currently in use were designed from the initial strain of SARS-CoV-2 that emerged in Wuhan. At present, what is known about their effectiveness against Omicron variants?

Judith Mueller: Immunological research has shown that the Omicron variants, and in particular BA.4 and BA.5, escape more neutralizing antibodies and even, a little, immunity linked to T lymphocytes , developed following vaccination. An identical phenomenon is observed for antibodies acquired following an infection.

It is a normal natural mechanism  : viruses evolve according to the antibodies they encounter, and more specifically given antibodies protecting against this infection. Genetic evolution in these viruses selects structures that are less well recognized by antibodies, with the consequence of maintaining the capacity for infection in an immune population – this is immune escape.

On the other hand, it is not “useful” for a virus like SARS-CoV-2 to be more dangerous. This corresponds to the observation that, for the moment, the successive variants have not really diminished the protection against the risk of a severe form of Covid-19 that vaccination provides .

The question is therefore rather this: could we, in the future, because of these genetic adaptations, see the emergence of a variant against which vaccination no longer or little protected against severe forms? It would rather be an unfortunate coincidence – it cannot be ruled out, but it is unlikely.