Pre-Omicron COVID-19 infection greatly reduces hospitalization risk due to new variants: study

A Quebec study has found that people who have been infected with a pre-Omicron variant of COVID-19 are highly protected against hospitalizations caused by the Omicron variant.

The study was published Friday and conducted last winter by a team from the Quebec Institute of Public Health (INSPQ), Laval University and other Canadian research institutions. It adds that in unvaccinated people, a pre-Omicron infection reduces the risk of Omicron infection by 44 per cent and lowers the risk of Omicron hospitalization by 81 per cent.

Protection is higher in people who have been vaccinated. Indeed, the combination of pre-Omicron infection and vaccination provides the highest protection against hospitalization: 86 per cent with one dose, 94 per cent with two doses and 97 per cent with three doses.

The study also found that the protection against Omicron conferred by a previous infection decreased over time.

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Study leader Gaston De Serres, a professor at Laval University and head physician of the scientific group on immunization at the INSPQ, points out that vaccinated people who have not been infected by the virus have less lasting protection against the risk of hospitalization.

If they are 60 years or older or have a chronic illness, it is important that they receive the prescribed booster doses, he said.

De Serres adds in an article published by Laval University that if future studies show that previous infection is protective against the main variant now circulating in Quebec, recommendations could be revised to take into account the protective effect of infection with COVID-19.

The study involved nearly 697,000 people aged 12 and older and was conducted between December 26 and March 12.

This report by The Canadian Press was first published in French on Oct. 17, 2022.