It is now well established that the Omicron variant is less sensitive to antibodies produced by two doses of the COVID-19 vaccine. Fortunately, studies indicate that the other aspect of immunity, cellular immunity, is not affected by mutations in the variant, which can help maintain excellent protection against more severe forms of the disease. This undoubtedly explains the fact that despite the dazzling increase in the number of new positive cases, the number of hospital admissions is low, mainly in the vaccinated population.
Antibodies play an important role in the immune response induced by vaccination against COVID-19: by binding to certain areas of the coronavirus on the surface of the S protein (epitope), the antibodies neutralize the transmitted virus and thereby significantly reduce the severity of infection and / or disease.
A very large number of people have these neutralizing antibodies, either because they have been vaccinated or because they have been infected with the virus in the past. This high prevalence of antibodies creates enormous evolutionary pressure on the coronavirus to select mutations that allow the immune system to escape this neutralization.
The Omicron variant, which has recently appeared, seems to represent a real big upside in acquiring this ability for immune avoidance by virus: studies conducted so far suggest a very strong reduction (30 times and plus) neutralization by antibodies. Produced by all current vaccines(1).
The extraordinary speed with which this variant is currently spreading around the world clearly shows that the combination of these mutations has a significant evolutionary advantage over the virus and that this variant will soon become the main form of the transmitted coronavirus.
Despite all this the vaccines are still effective
The greater resistance of the omicran variant to antibodies clearly affects the effectiveness of the vaccine, but much less than initially feared. For example, data collected in South Africa, where the omicran variant originated, indicate that the vaccine reduces the risk of serious complications from COVID-19 by 70% and that those who are vaccinated generally appear to be less ill. Cases observed in previous waves(2).
It has also been shown that a third dose (booster) of mRNA vaccines can be used to prevent the loss of immunity and protection against the original viral strain, i.e. up to almost 90% replacement. Individuals at high risk for complications from Kovid-19, due to age or co-morbidity, may be able to adequately protect themselves from the omicran variant with a third dose of the vaccine.
Cellular immunity of killer cells
Despite the decline in antibody neutralization activity, this protection against Omicron has been observed, which is largely due to the action of T lymphocytes. Battalion of T lymphocytes, which also have the ability to neutralize the virus: This is called cellular immunity. Some of them, called killer T cells (or CD8 + T cells), completely destroy the infected cells. Others, called helper T cells (or CD4 + T cells), are important for a variety of immune functions, including stimulating the production of antibodies and killer T cells.
Two characteristics of T lymphocytes make them very important in fighting the virus we are currently facing:
- These cells have long memories: after being activated by a virus (either by vaccination or by following an infection), some clones become aware of the presence of the virus and quickly reactivate when a new infection occurs. According to a recent study, this T cell memory is still active for more than a year after infection with a coronavirus.(3).
- T cells target different parts of the virus from those identified by antibodies, and these identified sites may vary from person to person. As a result, there are many types of T cells in the population that can neutralize the virus, making it very difficult for the virus to select for mutations that allow it to escape this cellular recognition. Furthermore, it has recently been shown that most epitopes identified by T lymphocytes are different from mutations in the omicron variant and, consequently, do not interfere with the neutralization mediated by T cells.(4).
So the general picture of immunity to omicron is as follows: On the one hand, a decrease in the effect of antibodies against omicron means that the immune system can neutralize the virus through transmission, which reduces the effectiveness of vaccines against infections. For this variant (this effect can be detected with a third dose).
T cells, on the other hand, cannot prevent infection because they begin only after the virus has infiltrated our cells. However, since they are not affected by virus mutations, they have the property of removing the infected cells with the variant and thereby preventing the virus from spreading within the body. The net result is that in the majority of cases the infection remains mild and the disease does not progress to the acute stages of hospitalization.
(1) Cameroni et al. Widely neutralizing antibodies outperform SARS-CoV-2 omicron antigenic change. bioRxiv Filed in pre-publication, December 14, 2021.
(3) Adamo S et al. Signature of chronic memory CD8 + T cells in acute SARS-CoV-2 infection. Nature, published December 7, 2021.
(4) Red AD and call. Minimal cross-over between mutations associated with the Omicron variant of SARS-CoV-2 and CD8 + T cell epitopes was observed in individuals with COVID-19 recovery. bioRxiv, déposé en prepublikation le 9 December 2021.