Study of Hospitalizations in Canada Quantifies Benefit of COVID-19 Vaccine to Reduce Death, ICU Admissions

Richard Mark Kirkner

May 08, 2023

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 4035 pediatric admissions for COVID-19. Although the share of COVID-19 admissions increased in the fifth and sixth waves, from December 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 (ICU) was significantly lower, at 8.7% vs 21.8% (odds ratio, 0.35; 95% CI, 0.32 – 0.36).

Dr Charles Frenette

"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 healthcare system were much higher than the previous waves," study author Charles Frenette, MD, director of infection prevention and control at McGill University Health Center in Montreal and chair of the study's adult subgroup, told Medscape Medical News. "But here we were able to show the benefit of vaccinations, particularly the boosting dose, in protecting against those severe outcomes."

The study, published April 20 in JAMA Network Open, used the Canadian Nosocomial Infection Surveillance Program (CNISP) 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 Medscape.

"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 Thampi, an associate professor and infectious disease specialist at Children's Hospital of Eastern Ontario in Ottawa.

"That's a particular strength of this surveillance program as well. We would see this data on a biweekly basis, and that allows for 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 September 1, 2020, to February 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 December 25, 2021, when the Alpha variant was dominant. The Omicron variant dominated during waves five (December 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 2164 during wave four, and all-cause deaths peaked at 1663 during wave two.

The investigators also analyzed how unvaccinated patients fared in comparison 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 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, 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).

Dr Nisha Thampi

"What's important is how pediatric hospitalizations changed over the course of the various waves," said 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."

Doing More With the Data

Commenting on the study for Medscape, David Fisman, MD, MPH, a professor of epidemiology at the University of Toronto, said, "This is a study that shows us how tremendously dramatic the effects of the COVID-19 vaccine were in terms of saving lives during the pandemic." Fisman was not involved in the study.

Dr David Fisman

But CNISP, which receives funding from Public Health Agency of Canada, could do more with the data it collects to better protect the public from COVID-19 and other nosocomial infections, Fisman said.

"The first problematic thing about this paper is that Canadians are paying for a surveillance system that looks at risks of acquiring infections, including COVID-19 infections, in the hospital, but that data is not fed back to the people paying for its production," he said.

"So, Canadians don't have the ability to really understand in real time how much risk they're experiencing via going to the hospital for some other reason."

The study was independently supported. Frenette and Thampi report no relevant financial relationships. Fisman has disclosed financial relationships with Pfizer, AstraZeneca, Sanofi, Seqirus, Merck, the Ontario Nurses Association and the Elementary Teachers' Federation of Ontario.

JAMA Netw Open. Published April 20, 2023. Full text

Richard Mark Kirkner is a medical journalist based in the Philadelphia area.

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