A third dose of COVID-19 vaccine offered no additional protection to healthcare workers who had been infected with the Omicron variant and then received two doses of a messenger RNA vaccine, a study found. collaborated with multiple Quebec researchers.
This “hybrid protection” appeared to protect these workers against reinfection with Omicron and its subvariants for an extended period of time, the researchers write in the pages of the medical journal The Lancet: Infectious Diseases.
“Vaccination with a previous infection gives very good protection,” summarized the study’s lead author, Dr. Sara Carazo of the National Institute of Public Health of Quebec.
If this protection persists in the face of future variants, the authors add, future doses of the vaccine may be of only limited use for people who have such hybrid immunity.
The study focused on all health workers (doctors, nurses, pharmacists and others) aged 18 and over who work in the public network of Quebec. Researchers compared participants who tested positive for coronavirus during the study period to those who tested negative.
They then looked at the vaccination status of the participants and then determined how many of them had been reinfected with the BA.2 variant after an initial infection with the BA.1 variant (Omicron).
About 3% of infected participants had an initial infection and two doses of vaccine, and about 4% had an initial infection and three doses of vaccine.
Among subjects infected with the Omicron variant before receiving two doses of messenger RNA vaccine, the risk of reinfection with BA.2 was reduced by 96%, and the risk of symptomatic reinfection by 98%. Adding a third dose of vaccine did not improve this protection, which lasted at least five months after the primary infection.
“Omicron completely changed the picture,” said Dr. Carazo. The proportion of the population infected with Omicron was so large that the protective effect of this infection becomes very important for public health.”
Vaccination is still the best weapon we have against the virus, she added, especially for people who have not had a previous infection or whose previous infection occurred before the arrival of the virus. Omicron variant.
The study authors point out that their work focused on healthcare workers aged 18 and older; it would therefore be difficult to extrapolate their results for children, the elderly or immunosuppressed individuals.
Similarly, it is not known with certainty whether the protection noted in the face of reinfection by the BA.2 variant would persist in the face of new variants, such as BA.4 and BA.5. Preliminary studies in Portugal and Qatar conclude, however, that the hybrid protection conferred by BA.1 infection and vaccination remains high, at between 76% and 80%.