The Perfect Enemy | Canadian study shows more hospitalizations—but fewer severe outcomes—during recent COVID waves
April 15, 2024

Canadian study shows more hospitalizations—but fewer severe outcomes—during recent COVID waves

The study was based on patients with lab-confirmed COVID-19 seen at 155 Canadian hospitals.

Today JAMA Network Open published a study showing that more Canadian adults and children were hospitalized in later waves of the COVID-19 pandemic compared to initial waves, but outcomes were less severe.

The study was based on patients with lab-confirmed COVID-19 seen at 155 Canadian hospitals and reported to the Canadian Nosocomial Infection Surveillance Program (CNISP) between March 15, 2020, and May 28, 2022.

The authors of the study looked at the vaccination status and outcomes of patients, including intensive care unit (ICU) admission, mechanical ventilation, and death, in six waves of virus activity: Wave 1 (March 15 to August 31, 2020 [wild-type variant dominant]), wave 2 (September 1, 2020, to February 28, 2021 [wild-type variant dominant]), wave 3 (March 1 to June 30, 2021 [mixed Alpha, Beta, and/or Gamma variants]), wave 4 (July 1 to December 25, 2021 [Alpha variant dominant]), wave 5 (December 26, 2021, to March 19, 2022 [Omicron variant dominant]), and wave 6 (March 20 to May 28, 2022 [Omicron variant dominant]).

Both pediatric and adult patients were included in the study. The weekly proportion of severe outcomes was calculated as the number of new COVID-19–positive patients per 1,000 patient admissions.

During the study period there were a total of 1,513,065 admissions reported, including 51,679 adult and 4,035 pediatric patients hospitalized with laboratory-confirmed COVID-19, with 8,683 adults (16.8%) and 498 children (12.3%) admitted to an ICU.

More admissions during Omicron

There were significantly more COVID-19 admissions in waves 5 and 6 compared to earlier waves of the virus. The authors found the proportion of COVID-19 hospitalizations (adult and pediatric patients) for waves 1 through 4 combined was 24.7 per 1,000 patient admissions, compared with 77.3 per 1,000 patient admissions for waves 5 and 6.

The highest rate of hospitalizations was 146.8 for adults and 96.3 per 1,000 pediatric admissions, during the week of January 16, 2022 (wave 5), the height of Omicron activity in Canada.

But adult ICU admissions during waves 5 and 6 were much lower compared to waves 1 through 4, despite the higher proportion of hospitalizations. A total of 8.7% of patients required ICU care in waves 5 and 6, compared to 21.8% in waves 1 through 4.

Mechanical ventilation among adults admitted to the ICU was also significantly lower in waves 5 and 6 compared to the earlier waves.

The same trend was documented in pediatric patients. The proportion of pediatric patients hospitalized with COVID-19 who were admitted to an ICU was significantly lower in waves 5 and 6 (9.4%) when compared with waves 1 through 4 (18.1%).

Deaths dropped as pandemic progressed

Finally, adult deaths in hospitalized patients dropped during each wave of the pandemic. During waves 1 and 2, 16% of adults hospitalized with COVID-19 died in the hospital, which dropped to 10.4% in waves 3 and 4, and 7.0% during waves 5 and 6.

These findings are likely due to multiple factors, including higher COVID-19 vaccination coverage during later waves, the emergence of a potentially less virulent SARS-CoV-2 variant, prior natural immunity, and improvements in COVID-19 management over time

Only 31 pediatric deaths were recorded during the study period, and were seen at the same rate at each wave of the pandemic.

“These findings are likely due to multiple factors, including higher COVID-19 vaccination coverage during later waves, the emergence of a potentially less virulent SARS-CoV-2 variant, prior natural immunity, and improvements in COVID-19 management over time,” the authors said.

Of note, the authors said there was more hospital-acquired COVID-19 (diagnosed 7 days or more into the hospital stay) in waves 5 and 6 than earlier waves. They said there were multiple reasons for this, including higher transmissibility of the Omicron strain, and a potential decrease in adherence to infection control practices.