Previous infection with an Omicron variant of COVID-19 did not protect seniors in long-term care and retirement homes from getting reinfected within a few months, a new study published Monday has found.
The findings were “a huge surprise” because they challenge the current thinking about hybrid immunity to the virus, said Dawn Bowdish, senior author of the study and Canada Research Chair in Aging and Immunity at McMaster University in Hamilton.
Hybrid immunity, which occurs when people have been vaccinated against the virus and have also been infected, is believed to provide increased protection against reinfection and serious illness, according to many public health, infectious disease and immunology experts.
But this observational study found the opposite among 750 seniors living in 26 long-term care and retirement homes across Ontario, Ms. Bowdish said.
Vaccinated seniors who had been infected with Omicron variants in early 2022 were about 20 times more likely to be reinfected with another Omicron variant later that year than seniors who had been vaccinated but not infected, she said.
The study, which was peer-reviewed and published in The Lancet’s open access journal eClinicalMedicine, shows that a lot is still unknown about how the virus infects people, Ms. Bowdish said.
“[Canada’s] vaccination strategy is predicated on this assumption that having had a recent infection will protect you from an infection at least for a short period of time. And our study shows that for some variants that’s just not true in some people,” she said.
The National Advisory Committee on Immunization (NACI) currently recommends COVID-19 booster shots “at least 6 months from the previous COVID-19 vaccine dose or known SARS-CoV-2 infection [whichever is later].”
That recommendation includes people age 65 and over in general, as well as seniors living in long-term care and retirement homes.
Based on the study findings, more frequent boosters for those vulnerable populations might be worth considering, Ms. Bowdish said.
One of the limitations of the study is that it’s not known whether the same reinfections would happen in a younger population or if the phenomenon is specific to seniors, she said.
Another limitation, she said, is that the study only looked at specific subvariants, given those were circulating at the time the research was conducted. It studied whether or not residents who were infected with the Omicron BA.1 or BA.2 variants in early 2022 were reinfected with another Omicron variant – BA.5 – three to six months later.
There’s no way to know without further study whether previous infection would increase or decrease the risk of reinfection in other Omicron subvariants, such as the currently circulating XBB and EG.5, Bowdish said.
However, “if the virus develops this capacity [to reinfect] once, there’s nothing to say another variant couldn’t have this capacity,” she said.
The research team ruled out as many other variables as possible, Ms. Bowdish said, including whether seniors who were reinfected were more likely to be on immunosuppressing drugs and whether they were in homes with more COVID-19 outbreaks. Through blood tests, they also looked at whether or not the antibody response to vaccination varied between those who were reinfected compared to those who were not and didn’t find a difference.
But there are many factorsthatcould have influenced the study’s findings, said Dr. Allison McGeer, an infectious diseases specialist and microbiologist at Mount Sinai Hospital in Toronto who was not involved in the research.
“One of the huge problems with doing anything with COVID that is not randomized is that not only can you not control for exposure, but you often can’t measure exposure,” Dr. McGeer said.
It’s “particularly challenging” to identify and track COVID-19 exposure variables in long-term care and retirement home settings, she said.
“It depends where you eat lunch. It depends who you eat lunch with. It depends how long you wait for the elevator to get down to lunch or if you wait for the elevator to go to lunch, or who you do activities with or how many caregivers you have … and all those are things that we can’t measure,” Dr. McGeer said.
The McMaster University findings are plausible, she said, but differences in exposure could be another explanation for the reinfections.
But Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization (VIDO) at the University of Saskatchewan, said the study results make sense.
“I think this is consistent with our understanding of immunity from infection alone (without vaccination): some people will mount a sufficient immune response to provide protection equivalent to vaccination and some won’t, likely due to a combination of variables including the severity/robustness of the initial infection and the response of the person infected,” Dr. Rasmussen, who was also not involved in the study, wrote in an e-mail.
“Overall, my take-home is that we shouldn’t rely on ‘hybrid immunity’ to provide additional protection on top of vaccines.”
“Hybrid immunity may be beneficial for some people, but it’s not a surefire way to protect against infection,” Dr. Rasmussen said.
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