Some seniors with early Omicron infection may have been more susceptible to reinfection: Canadian study

A new study with Canadian researchers has found that early Omicron infection may have made some seniors more susceptible to reinfection by a later version of the COVID-19 variant.

Researchers at McMaster University in Hamilton, Ontario, published a study Monday in eClinicalMedicinea journal of The Lancet that studied 750 vaccinated seniors in Ontario's retirement and long-term care homes, including public and private, nonprofit and for-profit organizations.

The researchers say that all initial infections occurred during the early 2022 wave of Omicron subvariants BA.1 and BA.2.

All reinfections, meanwhile, occurred later in the summer when BA.5 was the dominant subvariant.

“We found that some individuals had normal immune responses after initial infection, while others had very low levels of protective antibodies, which we think was one of the reasons they became reinfected,” said McMaster immunologist Dawn Bowdish, corresponding author on the study and a Canadian Research Chair in Aging and Immunity, said in a message from the university.

The study also found that those infected with an early omicron variant were more likely to be reinfected than those who never had an infection.

The researchers write in their study that the results “challenged the concept that hybrid immunity increases protection against subsequent SARS-CoV-2 infection with omicron subvariants.”

All seniors included in the study had four mRNA vaccine doses as of July 1, 2022, and had not had a COVID-19 infection within seven days of their fourth vaccination.

The median age was 87 years, about 64 percent were female and about 57 percent were in a nursing home.

The researchers note that they could not identify exactly which omicron subvariant an individual had, but say all cases occurred during waves in which those particular subvariants were responsible for the vast majority of infections.

They also warn that people were not matched by age or sex and that asymptomatic infections may have been “missed,” adding that “the data should not be interpreted to suggest a causal relationship.”

And while they found that a person's frailty didn't increase their risk of infection, the way a person required daily care might have increased their interactions with others, which might have increased their risk of exposure, and therefore infection.

However, Andrew Costa, an epidemiologist and associate professor at McMaster who served as senior co-author on the study, said the study strongly suggests that more research is needed to understand whether the broader population might also share the same susceptibility .

“Until we know more, we think it's wise for everyone to protect themselves,” Costa said.

The researchers posit that this could also include receiving a booster shot against COVID-19 in the fall.

“This research underscores the need for continued vigilance and underscores the importance of ongoing preventive measures against COVID-19,” Bowdish said.

“We must remain cautious and proactive in our approach to protecting public health.”

The study also notes that antibodies from BA.1 infection are reported to be less effective against BA.5, while three or four doses of the Pfizer-BioNTech and Moderna vaccines may have provided only short-term protection against the subvariant.

“Therefore, consideration of the SARS-CoV-2 variant-specific infection history as well as the circulating omicron sublineage is likely important when examining associations between omicron-associated hybrid immunity and vaccine efficacy,” the study states.