Antibodies, B cells, recalls: certainties and open questions on the duration of protection offered by infections and vaccines against covid. And the answers of science.
Establishing with certainty the duration of immunity to an infection is never easy; even more so it is not so for covid, a new and relatively little known disease. For now we have reason to believe that after a vaccine or a natural infection, protection from new encounters with SARS-CoV-2 will last at least a few months, perhaps a couple of years.
But to give more certain answers, there are still important issues to be solved , such as that of the “longevity” of the antibodies that target the spike protein of the coronavirus, or the role of B and T lymphocytes , cells that combine to form immunity. to covid and that in the first months of the pandemic were somewhat underestimated. A review published in the British Medical Journal takes stock of the information we have on covid immunity.
HOW LONG DO THE ANTIBODIES STAY IN CIRCULATION? The antibodies produced against SARS-CoV-two appear to last for a few months in the body, before starting to gradually decline. According to a study of nearly 6,000 covid cured people published in November 2020 on Immunity , antibodies were present in the blood even seven months after infection, both in convalescents from mild cases, and in veterans from severe forms (who, however, had in larger quantities).
Approved covid vaccines also stimulate significant antibody production: Participants in Moderna’s vaccine clinical trial still had high levels of coronavirus antibodies six months after the second dose, and those who received the AstraZeneca vaccine had large amount of antibodies with “minimal signs of decline” three months after the first injection.
While neutralizing antibodies are naturally destined to decrease once the acute infection passes, polyfunctional antibodies – capable of both directly neutralizing the virus and assisting T cells in destroying infected cells – seem to stay longer and build protection against long duration. Finally, recent studies that have examined the amount of neutralizing antibodies produced in response to vaccines and estimated their respective decline rates have concluded that, even without booster, vaccinated individuals should be protected for a very long time from severe or lethal covid, even if remaining susceptible to mild infections.
HOW LONG DO B AND T CELLS LAST? The real architects of long-lasting protection against infections are not so much antibodies but B and T lymphocytes, some components of which act as memory cells : they remain in the body for years, even decades, ready to trigger a new response. immune system if the virus is encountered again. According to some studies on the healed , even though the antibodies drop over time, the B and T cells are still present eight months after healing.
What we know about previous covid diseases bodes well. In 2008, a study published in Nature showed that 32 people born in 1915 or earlier still retained, 90 years later, some immune protection against the strain of flu virus that caused the Spagnola in 1918 . In 2020, an analysis of SARS patients revealed that these people still had certain types of T cells 17 years after infection. Furthermore, some of them reacted well to SARS-CoV-2, in a form of cross-immunity.
How important is the resistance of B and T cells if antibodies drop over time? Can we rely on them alone to be protected? An answer is not yet possible.
NATURAL IMMUNITY VS VACCINE IMMUNITY: IS THERE A DIFFERENCE? Vaccine-induced immune protection is more homogeneous and safer: most vaccinated people develop excellent levels of immune cells. Natural immunity is more subjective and linked to the characteristics of the immune system and the severity of the contracted infection. Vaccination “seals” and refines the natural immunity developed by the recovered, providing protection from covid that is likely to last for life.
HOW DOES IMMUNITY CHANGE AFTER THE FIRST AND SECOND DOSE? Most of the studies so far have only considered the level of antibodies at different stages of the vaccination cycle . A British study of over 50,000 people found that 96.4% of them had anti-covid antibodies one month after the first dose of Pfizer or AstraZeneca, and that 99.1% of them had them between the ages of seven and 14. days after the second dose. Antibodies skyrocket two weeks after the second dose – at which point one is also highly covered by the coronavirus variants. The temporal distance between first and second doses influences the antibody response, but could encourage the evolution of more resistant variants of SARS-CoV-2.
DOES IMMUNITY AFFECT REINFECTIONS? Definitely yes: reinfections in recovered or fully vaccinated are rare , and when they occur they do so in a mild form. This does not mean that vaccinated and cured cannot in turn transmit the covid to those who have not had the vaccine.
VACCINES: WILL A BOOSTER BE NEEDED? Probably yes for elderly people (more at risk and with a less reactive immune system) or immunosuppressed. The recalls could also become necessary, in the coming months, to counter the emerging variants of the virus; although, for the time being, approved vaccines also offer excellent protection from new versions of SARS-CoV-2 – including Delta . The best defense against the virus in any form is … the attack: a rapid and homogeneous vaccination campaign, with the responsible adhesion of all, removes the hosts to infect from the virus. And a virus that doesn’t circulate loses the ability to mutate.
