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COVID-19 variants of concern, particularly Delta, more harmful than original strain: Canadian study


COVID-19 variants of concern (VOCs), particularly the Delta variant, cause more severe illness than the native strain of the virus, increasing the risk hospitalization, admission to the intensive care unit (ICU) and death, according to a large Canadian study released Tuesday.

The study found that people infected with VOCs were significantly younger and less likely to have comorbidities than those with non-VOC infections, and, even after adjusting for age, sex, comorbidities and other factors, the increased risk of adverse outcomes persisted.

Using case data from Ontario, the country’s most populated province, the study included 212,326 cases of COVID-19 reported between Feb. 7 and Jun. 26, 2021. Variants of concern included those with the N501Y mutation such as Alpha, Beta and Gamma as well as Delta, which have replaced the original SARS-CoV-2 strain.

Of the total cases, 22.4 per cent were non-VOC, 76.7 per cent were infections with N501Y mutations, and just 2.8 per cent were probable Delta.

The study found that with Alpha, Beta and Gamma VOC infections, risks of hospitalization were 52 per cent higher than non-VOC cases, with the risk of admission to ICU 89 per cent higher and death 51 per cent higher than the original virus.

But, with Delta infections, the risks jumped to 108 per cent, 235 per cent and 133 per cent higher respectively, despite the small number of infections included in the study.

“Once we adjusted for confounding factors such as age, vaccination status, comorbidity and temporal trends, elevated per-infection risk, including risk of death, remained markedly higher with VOCs, and with the Delta variant in particular,” co-authors David Fisman and Ashley Tuite, of the University of Toronto’s Dalla Lana School of Public Health, wrote in the study.

“Given the relatively small number of Delta infections in our study, it is remarkable that we detected a clear and significant elevated risk of uncommon, delayed outcomes, such as death.”

The study authors note that an important limitation to the analysis is the possibility that early Delta variant infections were misclassified as non-VOC, “owing to the absence of routine screening for characteristic Delta mutations, with likely underestimation of the prevalence of the Delta variant before May 2021.”

But the study, published in the Canadian Medical Association Journal (CMAJ), echoes findings from England, Scotland and Singapore-bases studies indicating the Delta variant increases risk of emergency department visits, hospitalization and severe outcomes.


In a related editorial, Dr. Kirsten Patrick, interim editor-in-chief of the CMAJ, says it is vitally important that Canadian leaders use “all available tools” to stop the spread of COVID-19 as the country approaches its second pandemic winter.

"Canada is battling a different pandemic from the one it faced in early 2020. The virus has become smarter and more dangerous, which means that we need to be smarter too,” Patrick wrote in a media release.

“Canada's governments can keep people safe by enacting policies that wisely combine all the measures that have been shown to be effective.”

Patrick’s call to action includes extending vaccine mandates and prioritizing the approval of safe vaccines for children, pointing to the study’s evidence that suggests vaccinations blunt the severity of variant infections, reducing the risk of severe disease and death in both partially and fully vaccinated people.

"The effects reported here represent a substantial degree of protection against death conferred by vaccines (about 80–90 per cent), even when they fail to prevent infection,” reads the study.

“Such direct protective effects may help reduce the health impacts of ongoing SARS-CoV-2 transmission in Ontario, even if herd immunity proves elusive, given the high reproduction numbers of VOCs.”

Fisman, coauthor of the peer-reviewed study, has served on advisory boards related to influenza and SARS-CoV-2 vaccines for Seqirus, Pfizer, AstraZeneca and Sanofi-Pasteur.​ Top Stories

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