These two works, published in the Lancet Infectious Diseases, provide some answers to a hotly debated scientific question: are we better protected against the disease after being infected or after being vaccinated?
The answer is not necessarily binary, but these studies underline that a former Covid patient has every interest in being vaccinated to strengthen the immunity already acquired by his past infection.
The first of these works is based on the health data of more than 200,000 Brazilians, whose country has been one of the hardest hit in the world by the Covid pandemic.
In this sample, some of the individuals were infected without being vaccinated.
Among those who survived the disease, some received a vaccine - Pfizer/BioNTech, AstraZeneca, Sinovac or Johnson & Johnson/Janssen - and some did not.
However, “these four vaccines prove to give additional and substantial protection to people already infected with Covid-19 before”, remarks one of the authors, Julio Croda.
This protection is more or less important: the risk of hospitalization or death is reduced by 90% with Pfizer/BioNTech and AstraZeneca, around 80% with Sinovac, but only a little more than half with Johnson & Johnson.
The second study, carried out using Swedish data, goes in the same direction.
It shows that former Covid patients retain high immunity for up to twenty months.
But it also indicates that these people see the risk of reinfection drop even more - by two thirds, approximately - if they were vaccinated after their first infection.
However, these two studies have a weakness: they cover a period which precedes the emergence of the Omicron variant, which is clearly more contagious and resistant to vaccines than its predecessors.
However, they suggest that "hybrid immunity, acquired by being exposed to both infection and vaccination, (...) could give long-lasting protection, including against new variants", estimates the Indian researcher Pramod Kumar, who did not participate in these studies.
© 2022 AFP