A massive Canadian study has measured the virulence of the SARS-CoV-2 Delta variant compared to earlier strains of the virus. The research found, compared to the original strain, Delta is considerably more deadly and leads to greater numbers of hospitalizations.
Over the course of 2021 the emergence of several new SARS-CoV-2 variants of concern (VOCs) entirely altered the path of the pandemic. While it quickly became clear many of these new variants were much more transmissible than the original strain of the virus, it wasn't been clear whether these variants generate more severe disease.
This new study, from a pair of public health researchers at the University of Toronto, investigated records from over 200,000 Canadian COVID-19 cases. Virulence was measured by rates of hospitalization, intensive care unit (ICU) admission and death.
Cases with the original strain of SARS-CoV-2 were compared to cases with early variants (Alpha, Beta and Gamma) and cases with the now-dominant Delta variant. Looking at those three initial variants the researchers found they increased risk of hospitalization by 52 percent compared to the original strain. The three initial variants also increased risk of ICU admission by 89 percent and death by 51 percent.
But looking at the data from Delta cases the researchers detected another big increase in virulence from the prior variants. Compared to the original SARS-CoV-2 strain, the Delta variant increased hospitalization risk by 108 percent, ICU admission risk by 235 percent and death by 133 percent.
In real-world terms what these numbers mean is that if one in 100 people were to die from the original strain of SARS-CoV-2, then 2.33 in 100 will die from the Delta variant.
Study authors David Fisman and Ashley Tuite also found new SARS-CoV-2 variants led to higher rates of infections in younger people. However, the good news is vaccination was significantly effective at reducing the increased risk of severe disease and death from the variants, including Delta.
"The effects reported here represent a substantial degree of protection against death conferred by vaccines (about 80%–90%), even when they fail to prevent infection,” Fisman and Tuite write in the new 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 and Tuite propose the emergence of these variants has extended the duration of the pandemic. And their research suggests increased transmissibility of the Delta variant is just one way this problematic VOC has altered the course of the pandemic.
"The emergence of novel SARS-CoV-2 VOCs has slowed progress against the pandemic in 3 distinct ways, namely by increasing transmissibility and the virus' reproduction number, by increasing immune escape and diminishing vaccine effectiveness, and by increasing the virulence of SARS-CoV-2 infection," write Fisman and Tuite.
Kirsten Patrick, Interim Editor-in-Chief of the Canadian Medical Association Journal (CMAJ) says Canada, much like the rest of the world, is facing a very different pandemic than the one it encountered early in 2020. In an editorial commenting on Fisman and Tuite’s research, Patrick argues it will take a whole host of measures to get through this pandemic. Despite vaccines still proving effective at preventing severe disease and death the increased virulence of the Delta strain demands measures such as improved ventilation, robust contact tracing, rapid antigen testing and novel therapies.
"Canada is battling a different pandemic from the one it faced in early 2020,” Patrick writes. “The virus has become smarter and more dangerous, which means that we need to be smarter too. Canada's governments can keep people safe by enacting policies that wisely combine all the measures that have been shown to be effective.”
The new study was published in the Canadian Medical Association Journal (CMAJ).
Source: CMAJ
