Guidance on the way for growing number of single-dosers

Ottawa's medical officer of health says guidance is in the works to help Ontarians with only a single dose of a COVID-19 vaccine understand the dos and don'ts, but Dr. Vera Etches is reminding everyone to remain vigilant until that happens. 

Most Ontarians won't get their 2nd dose of a COVID-19 vaccine until after Aug. 1

Humber River Hospital's mobile vaccine clinic delivers the first doses to workers at the pharmaceutical company Apotex in Toronto on April 13, 2021. (Carlos Osorio/Reuters)

Ottawa's medical officer of health says guidance is in the works to help Ontarians with only a single dose of a COVID-19 vaccine understand the dos and don'ts, but Dr. Vera Etches is reminding everyone to remain vigilant until that happens. 

"We know people need more guidance," Etches said this week. "I know this is something the province is looking to provide as advice as the immunization coverage goes up."

According to a preprint study released last week by Public Health England, there's a stark difference between a single dose and full vaccination when it comes to protecting against the coronavirus variant now dominant in Ontario.

The study found that a single dose of the Pfizer vaccine is 50 per cent effective, while a double dose is 90 per cent effective. The AstraZeneca-Oxford vaccine was also found to be 50 per cent effective after one dose, and 66 per cent effective after two.

Those findings have prompted calls to reduce the time between doses before loosening restrictions in the U.K., and Etches said Ontario is hoping to do the same "so people may not have to wait 16 weeks for that second dose." 

In the meantime, the advice is to continue observing public health advice on masks and distancing, whether people are fully vaccinated or not.

Do you have one dose of the vaccine? Keep following the rules, scientists say

1 year ago
Duration 1:29
Dr. Peter Jüni, director of Ontario’s science advisory table, says residents with a single dose of the vaccine are still able to contract and transmit COVID-19, making it important to continue physical distancing and wearing a mask.

Dos and don'ts aren't well defined

Dr. Peter Jüni, scientific director of the Ontario COVID-19 Science Advisory Table, says the higher rate of transmission among those with a single dose is a serious concern during this "wobbly phase" of the pandemic, but things are looking up.

"This is about to change," he said. "If daily cases continue to go down more, things will start to get easier."

Jüni said once Ontario starts to dip below 800 new cases a day, certain behaviours such as outdoor dining with other single-dosers could be deemed safe.

Dr. Christopher Labos is a cardiologist and epidemiologist who has studied the research into the effectiveness of a single dose. (CBC)

Montreal epidemiologist and cardiologist Christopher Labos said the dos and don'ts for single-dosers aren't well defined because there aren't many similar examples.

"Part of the problem is that most countries in the world have not had this longer interval between doses the way we have," said Labos.

Comparing Canada and U.S.

Canada's decision to extend the delay between doses to 16 weeks as a strategy to deal with vaccine supply problems has proven successful in getting more first shots into more arms. In the last week, Canada surpassed the United States with more than 50 per cent of the population receiving an initial dose.

However, 40 per cent of Americans are fully vaccinated, compared to just five percent of Canadians, so the pace of reopening south of the border, where vaccine supply has been less of an issue and second doses are being offered after shorter intervals, has been much swifter.

(Our World in Data)

That's not to say single doses aren't effective against COVID-19: According to the Public Health Agency of Canada (PHAC), by the end of April, 2,274 Canadians had contracted COVID-19 after two weeks or more of receiving their first dose of a vaccine, representing just 1.3 per cent of infections since December.

But Labos cautioned against reading too much into those numbers.

"Saying that partially vaccinated people make up a small sliver of new infections is not the same as saying that partial vaccination is effective," he said.


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