Vaccines have been proven effective in protecting those who had never been infected by the coronavirus, but in the case of those who have already had Covid, efficacy has up until now been less certain.
But a Brazilian study published yesterday in the scientific journal The Lancet demonstrated the effectiveness of four different vaccines in the immunization of people who have already been infected. The four are those most applied in the country: CoronaVac, Oxford-AstraZeneca, Janssen, and Pfizer-BioNTech.
The study found evidence that the four immunizers provide protection against symptomatic SARS-CoV-2 infection and severe outcomes in individuals who were previously infected, with a second dose conferring significant additional benefits.
The risk of hospitalization and death was reduced by 90% with Pfizer/BioNTech and AstraZeneca immunizers, about 80% with Sinovac, and a little over 50% with the Janssen vaccine. For vaccines with two-dose regimens, the second dose provided significantly increased effectiveness compared with one dose alone.
The researchers used the data of more than 200,000 Brazilians from a national database of Covid-19 cases, laboratory testing, and vaccination in Brazil.
The study was funded by Brazilian National Research Council, Fundação Carlos Chagas, Oswaldo Cruz Foundation, JBS, Instituto de Salud Carlos III, the Spanish Ministry of Science and Innovation, and Generalitat de Catalunya.
The journal also published another study on the subject, carried out using data collected in Sweden. It shows that people who have been infected with Covid-19 maintain high immunity for about 20 months, but the risk of reinfection decreases by about two-thirds if individuals were vaccinated after the first infection.