Toronto, June 6: As more evidence emerges on the efficacy of third doses of the COVID-19 vaccine, experts say it’s high time public health authorities revise the definition of “fully vaccinated” to include a booster dose.
While most SARS-CoV-2 vaccines available in Canada were initially developed and tested to include two doses – with the exception of the Johnson & Johnson vaccine – a primary course should instead consist of three doses, said Colin Furness, an expert in infectious disease epidemiology from the University of Toronto.
A newly released meta-analysisconducted by the Chinese University of Hong Kong earlier this year assesses the efficacy of different vaccine combinations in protecting against COVID-19. Involving 53 studies that include more than 100 million people, the meta-analysis showed that three doses of an mRNA vaccine were highly effective in protecting against COVID-19 infection, whether symptomatic or asymptomatic, at 96 per cent.
Three doses of an mRNA vaccine also had a 95 per cent efficacy rate in reducing COVID-19-related hospitalization. The study concludes that three doses of a COVID-19 vaccine are necessary to protect against infection from the Omicron variant of the virus.
Despite this, the Public Health Agency of Canada’s (PHAC) definition of “fully vaccinated” with regards to COVID-19 vaccines continues to refer to those who have received a primary series of the vaccine. The primary series involves two doses of most vaccines approved for use in Canada (such as mRNA vaccines, as well as the AstraZeneca, Novavax and Medicago vaccines), or one dose of the Johnson & Johnson vaccine.
It’s important to note that as the COVID-19 pandemic has progressed, the SARS-CoV-2 virus has evolved away from what vaccines were initially developed to target, Furness said. This viral evolution has led to a reduction in the efficacy of current vaccines, particularly when it comes to protecting against infection from Omicron, the newest variant of concern. Despite this, three doses of the vaccine continue to offer solid protection, particularly against severe outcomes of COVID-19 such as hospitalization and death, Furness said. As a result, he said he hopes to see a change in the definition of “fully vaccinated.”
“Hypothetically, if we had a vaccine that prevented long COVID, hospitalization and death, but didn’t stop anybody from getting infected, I would call that wildly successful; we’d all get a cold … but no one would get really sick,” Furness said. “There’s no question we don’t have that, but three doses of a COVID-19 vaccine takes us closer to that.
“When we talk about ‘fully vaccinated,’ we should be talking about three [doses].”
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