Omicron infection may increase risk of reinfection, study finds

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Omicron infection may increase risk of reinfection, study finds

McMaster University researchers found that rather than conferring immunity against future infections, infection during the first Omicron wave of COVID made the elderly they studied much more vulnerable to re-infection during the second Omicron wave.

The surprising finding from a study of 750 vaccinated seniors in Ontario retirement homes and long-term care facilities suggests we don’t understand how some variants of Omicron can evade the immune system, according to Dawn Bowdish, an immunologist who holds the Canada Research Chair in Aging and immunity.

There have been four major waves of Omicron infections in Ontario, Canada, and the researchers found that infection during the first wave caused by Omicron variants BA.1 and BA.2 made older people more susceptible to infections in the third wave that was caused by the Omicron BA.5 variant. Surprisingly, people who had an infection with an early variant of Omicron were much more susceptible to re-infection than people who had no infection at all.

“This research underscores the need for continued vigilance and underscores the importance of ongoing preventive measures against COVID-19,” said Bowdish, who authored a study published today in eClinicalMedicineopen access journal published by The lancet. “We must remain cautious and proactive in our approach to protecting public health.

Senior co-author Andrew Costa, an epidemiologist and associate professor in McMaster’s Department of Health Research Methods, Evidence and Impact, says the findings should serve as a warning that we don’t know how previous infections will affect susceptibility to the variants now in circulation.

These findings strongly suggest that more extensive research is needed to understand whether the broader population shares the same susceptibility as the elderly studied by our group. Until we know more, we think it’s prudent for everyone to protect themselves.”

Andrew Costa, epidemiologist and associate professor, McMaster’s Department of Health Research Methods, Evidence and Impact

Long-term care residents are easier to study because COVID-19 infections have until recently been more closely monitored, Bowdish explains, and while the results may or may not be the same in the broader population, it’s important to learn more and for everyone to consider a COVID vaccine booster this fall.

Although the researchers were unable to identify which Omicron variant a person had, all initial infections occurred during the BA.1/BA.2 wave, and all re-infections occurred in the summer of 2022, when the BA.5 variant is responsible for the majority of infections.

“We found that some individuals had normal immune responses after the first infection, while others had very low levels of protective antibodies, which we believe is one of the factors contributing to their re-infection,” says Bowdish.

Researchers urge people not to assume immunity from previous Omicron infection and to remain vigilant to prevent further spread of the virus.

She and Costa emphasize the urgency of considering COVID vaccine boosters this fall to protect against potential reinfections.

“Our current vaccine schedules are based on the assumption that past infection provides some level of protection against future infections, but our study shows that this may not be true for all variants in all people,” says Bowdish.

Despite the significance of the findings, Bowdish highlights some caveats. The study focused on an older adult population, many of whom are frail and have chronic health problems, and the results may not directly apply to younger individuals.


Journal reference:

Breznik, Ya.A., et al. (2023) Early Omicron infection is associated with increased risk of reinfection in older adults in long-term care and retirement facilities. eClinicalMedicine.

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