Should people who have already been infected be vaccinated?

More than a year after the start of the COVID-19 pandemic, it remains difficult to control the virus’ spread despite the development of vaccines. Progress has been slowed due to the emergence of variants that are more transmissible and more resistant to antibodies. In this combat, it is important to know whether long-term immune protection develops after natural infection, and whether people who have been infected should nevertheless be vaccinated. Researchers at Rockefeller University (New York, USA) have studied the immune responses of vaccinated and non-vaccinated individuals infected by SARS-CoV-2 up to one year after infection.

The researchers studied a cohort of 63 individuals (of whom 43% were women), aged between 26 and 73 years old, who had been infected by SARS-CoV-2. 41% had been vaccinated with at least one dose of an mRNA vaccine (Pfizer-BioNTech or Moderna). Blood samples were taken 1, 3 and 12 months after their infection to analyse their anti-SARS-CoV-2 antibody and B lymphocyte responses.

In non-vaccinated individuals, the level of anti-spike (the surface protein of the virus) antibodies was stable between 6 and 12 months. In vaccinated patients, antibody levels were found to be about 30 times higher.

But are these antibodies effective? Their neutralisation capacities were tested and it was found that this activity seemed to be reinforced by vaccination. As has been noted in other studies of infected persons, the effectiveness of antibody neutralisation against the Alpha variant (⍺ or UK), Beta variant (β or South African), and Gamma variant (ɣ or Brazilian) is lower than that obtained against the original virus. On the other hand, neutralising activity is higher against these variants in vaccinated individuals. In non-vaccinated infected individuals, numbers of anti-spike memory B cells reduce only a little between 6 and 12 months. However, their numbers are 9 times greater in those people vaccinated after infection.

Individuals who are vaccinated after having been infected therefore benefit from better protection than those who are non-vaccinated. This holds true against all variants.