A study involving multiple Quebec researchers has found that a third COVID-19 vaccine dose offered no additional protection to health-care workers who were infected with the Omicron variant and then received two doses of a messenger RNA vaccine.
This "hybrid protection" appeared to protect these workers from re-infection with Omicron and its sub-variants for an extended period of time, the researchers write in 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 Quebec Institute of Public Health (INSPQ).
The study included all health-care workers (physicians, nurses, pharmacists and others) aged 18 years and older who work in the Quebec public health system. The researchers compared participants who tested positive for coronavirus during the study period with those who tested negative.
They then examined the vaccination status of the participants and determined how many had been reinfected with variant BA.2 after an initial infection with variant BA.1 (Omicron).
About 3 per cent of infected participants had an initial infection and two doses of vaccine, and about 4 per cent 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 re-infection with BA.2 was reduced by 96 per cent, and the risk of symptomatic re-infection by 98 per cent. The addition of a third dose of vaccine did not improve this protection, which lasted at least five months after primary infection.
"Omicron has completely changed the picture," said Carazo. "The proportion of the population infected with Omicron was so large that the effect of this infection on protection becomes very important for public health."
Vaccination is still the best weapon available 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 Omicron variant.
The study authors note that their work involved health-care workers aged 18 and older, so it would be difficult to extrapolate their results to children, seniors or immunosuppressed individuals.
Similarly, it is not clear whether the protection seen against re-infection with the BA.2 variant would persist against newer variants, such as BA.4 and BA.5.
However, preliminary studies in Portugal and Qatar conclude that the hybrid protection from BA.1 infection and vaccination remains high, at 76 per cent to 80 per cent.
This report by The Canadian Press was first published in French on Sept. 22, 2022.
