“Hybrid immunity” does not always protect the elderly in residential settings

A new study published Monday indicates that older adults living in long-term care centers or seniors’ homes contracted COVID-19 despite being infected a few months earlier with a sub-variant of Omicron.

Dawn Bodish, the study’s lead author and Canada Research Chair in Aging and Immunity at McMaster University, Hamilton, Ontario, said these findings came as a “big surprise” to the researchers because they challenge the concept of “hybrid immunity” to the coronavirus.

Hybrid immunity occurs when people are vaccinated against SARS-CoV-2 and also become infected with the coronavirus.

This hybrid immunity is believed to provide increased protection against reinfection and serious disease, according to several experts in public health, infectious diseases and immunology.

However, this observational study found the opposite in 750 seniors living in 26 long-term care and seniors’ homes in Ontario, Bodish said.

Professor Bowdish said vaccinated elderly people who were infected with the Omicron variants in early 2022 – and therefore had hybrid immunity – were about 20 times more likely to be reinfected with another Omicron variant later that year than older people who had been vaccinated and had not been infected. never. .

Many unknowns

Bowdish said the study, which was reviewed and published in The Lancet’s open-access journal eClinicalMedicine, shows that much remains unknown about how this virus infects people.

“The vaccination strategy (in Canada) is based on this assumption that a recent infection will protect you from infection for at least a short time. “Our study shows that for some variants, in some people, this isn’t true,” she said.

The National Advisory Committee on Immunization (NACI) currently recommends that boosters be given at least six months after a previous dose or known infection – whichever is more recent.

This recommendation generally applies to people age 65 or older — including those who live in long-term care centers and seniors’ homes. However, based on the results of the study, one could consider more frequent booster doses for these high-risk populations, Professor Bowdish suggested.

Study limitations

One limitation of the study, she said, is that it is not clear whether the same reinfections would occur in the younger population or if the phenomenon was confined to the elderly.

Another limitation, she said, is that the study only looked at specific sub-variants of Omicron — those that were in circulation at the time the research was conducted. We have already found out that all residents who have been infected by the variants of Omicron BA.1 or BA.2 since 2022 have been reinfected by a single variant of Omicron, the BA.5, with three of them. later.

There is no way to know without further study whether a previous infection increases or decreases the risk of reinfection with other Omicron sub-variants, such as the XBB and EG.5 currently in circulation, Ms. Bowdish.

“But if the virus develops this ability (to re-infect) once, there is no saying that another strain cannot have this ability as well,” she added.

Other factors

Bowdish said the research team ruled out as many variables as possible.

But Dr. Allison McGeer, an infectious disease specialist and microbiologist at Mount Sinai Hospital in Toronto, who was not involved in the research, said many factors could influence the results of the study.

“One of the big problems with non-randomized coronavirus studies is that not only can you not control exposure, you often can’t measure exposure,” Ms McGeer said.

She said identifying and tracking variants of exposure to COVID-19 in long-term care facilities and seniors’ homes is “extremely difficult.”

“It depends on where you eat, who you are with. It depends on how long it takes the elevator to come down to eat, how long it takes to get there, who you do the activities with or how many caregivers (…) these are all Things we can’t measure.”

She acknowledged that McMaster’s findings were plausible, but the incidence of reinfection could also be attributed to differences in exposure in those living environments.

Angela Rasmussen, a virologist at the University of Saskatchewan, said the study results “are consistent with our understanding of immunity conferred by infection alone” without vaccination.

“Some people will mount an immune response sufficient to provide protection equivalent to vaccination, but others will not, likely due to a combination of variables, including the severity or strength of the initial infection, and the response [immunitaire] Ms. Rasmussen, who was also not involved in the study, wrote in an email.

“In general, my opinion is that hybrid immunity should not be relied upon to provide additional protection, in addition to vaccines,” she said. Hybrid immunity may be beneficial for some people, but it is not a sure way to protect against infection.”

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