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Study of hospitalizations in Canada quantifies benefit of COVID-19 vaccine to reduce death, ICU admissions


A cohort study of more than 1.5 million hospital admissions in Canada through the first 2 years of the COVID-19 pandemic has quantified the benefit of vaccinations. Unvaccinated patients were found to be up to 15 times more likely to die from COVID-19 than fully vaccinated patients.

Investigators analyzed 1.513 million admissions at 155 hospitals across Canada from March 15, 2020, to May 28, 2022. The study included 51,679 adult admissions and 4,035 pediatric admissions for COVID-19. Although the share of COVID-19 admissions increased in the fifth and sixth waves, from Dec. 26, 2021, to March 19, 2022 – after the full vaccine rollout – to 7.73% from 2.47% in the previous four waves, the proportion of adults admitted to the intensive care unit was significantly lower, at 8.7% versus 21.8% (odds ratio, 0.35; 95% confidence interval, 0.32-0.36).

“The good thing about waves five and six was we were able to show the COVID cases tended to be less severe, but on the other hand, because the disease in the community was so much higher, the demands on the health care system were much higher than the previous waves,” study author Charles Frenette, MD, director of infection prevention and control at McGill University, Montreal, and chair of the study’s adult subgroup, said in an interview. “But here we were able to show the benefit of vaccinations, particularly the boosting dose, in protecting against those severe outcomes.”

The study, published in JAMA Network Open, used the Canadian Nosocomial Infection Surveillance Program database, which collects hospital data across Canada. It was activated in March 2020 to collect details on all COVID-19 admissions, co-author Nisha Thampi, MD, chair of the study’s pediatric subgroup, told this news organization.

“We’re now over 3 years into the pandemic, and CNISP continues to monitor COVID-19 as well as other pathogens in near real time,” said Dr. Thampi, an associate professor and infectious disease specialist at Children’s Hospital of Eastern Ontario.

“That’s a particular strength of this surveillance program as well. We would see this data on a biweekly basis, and that allows for [us] to implement timely protection and action.”

Tracing trends over six waves

The study tracked COVID-19 hospitalizations during six waves. The first lasted from March 15 to August 31, 2020, and the second lasted from Sept. 1, 2020, to Feb. 28, 2021. The wild-type variant was dominant during both waves. The third wave lasted from March 1 to June 30, 2021, and was marked by the mixed Alpha, Beta, and Gamma variants. The fourth wave lasted from July 1 to Dec. 25, 2021, when the Alpha variant was dominant. The Omicron variant dominated during waves five (Dec. 26, 2021, to March 19, 2022) and six (March 20 to May 28, 2022).

Hospitalizations reached a peak of 14,461 in wave five. ICU admissions, however, peaked at 2,164 during wave four, and all-cause deaths peaked at 1,663 during wave two.

The investigators also analyzed how unvaccinated patients fared, compared with the fully vaccinated and the fully vaccinated-plus (that is, patients with one or more additional doses). During waves five and six, unvaccinated patients were 4.3 times more likely to end up in the ICU than fully vaccinated patients and were 12.2 times more likely than fully vaccinated-plus patients. Likewise, the rate for all-cause in-hospital death for unvaccinated patients was 3.9 times greater than that for fully vaccinated patients and 15.1 times greater than that for fully vaccinated-plus patients.

The effect of vaccines emerged in waves three and four, said Dr. Frenette. “We started to see really, really significant protection and benefit from the vaccine, not only in incidence of admission but also in the incidence of complications of ICU care, ventilation, and mortality.”

Results for pediatric patients were similar to those for adults, Dr. Thampi noted. During waves five and six, overall admissions peaked, but the share of ICU admissions decreased to 9.4% from 18.1%, which was the rate during the previous four waves (OR, 0.47).

“What’s important is how pediatric hospitalizations changed over the course of the various waves,” said Dr. Thampi.

“Where we saw the highest admissions during the early Omicron dominance, we actually had the lowest numbers of hospitalizations with death and admissions into ICUs.”


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