COVID-19

Covid-19, vaccines and protection by T cells – Sciences et Avenir

Doubt hovered, evidence was lacking. Given the great immune response generated by RNA vaccines, their protection should continue to be valid in the long term. However, the rapid decline in antibodies seen after six months and the decreased protection against infection seen at the end of this period raised concerns that the efficacy of the vaccines was rapidly becoming insufficient. Now a new study shows that despite this drop in antibody level, we remain very well protected against severe forms of the coronavirus for at least nine months. And this thanks to the action of T lymphocytes. These immune cells remain mobilized for a long time after vaccination and also protect us from the Omicron variant: it escapes antibodies but not lymphocytes.

Two doses of RNA vaccine protect against severe forms for at least 9 months

Researchers from the University of North Carolina (USA) analyzed data from 10 million inhabitants of this EAmerican tat, about half of which had been vaccinated with two doses of an RNA vaccine. His article was published on January 12, 2022 in the NEJM newspaper. The scientists found that protection against symptomatic infection dropped from 95% to 66% for Pfizer and 80% for Moderna after seven months. A drop correlated with the drop in the number of neutralizing antibodies detected (nearly fourfold less six months after the second dose, according to an Israeli study published Dec. 9, also in NEJM).

However, protection against hospitalization remained high after seven months: 89% for Pfizer and 94% for Moderna. This shows that protection against severe forms, the main objective of vaccines, remains high with only two doses. This phenomenon is explained despite the fall in antibodies thanks to the rest of the immune response, in particular T lymphocytes. But this efficacy decreases more rapidly in people over 65 years of age, a population in which the booster dose It is essential to maintain optimal protection.

The antibody study alone does not allow to evaluate the efficacy of the vaccine

The Omicron variant poses an additional problem for vaccines, as its mutations allow it to evade antibodies generated by vaccination. According to in vitro studies, the antibodies produced by two doses of the vaccine barely neutralize this variant.

However, one study (published in prepress therefore not yet peer-reviewed) has just shown that protection against Omicron infection is approximately 30% at these two doses and 62% after the third. Because, unlike antibodies, T cells continue to recognize Omicron very well despite its mutations, which only affect a minority of the sites recognized by these cells (between 20 and 30% according to two preprints of U.S and of theSouth Africa). This immune response of the T lymphocytes would therefore protect against Omicron infection, but above all against the severe forms that this variant could cause. The protection of vaccines is therefore broader and more durable than the simple study of antibodies would lead us to believe.

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