Recent studies indicate that Canada’s decision to extend the interval between the first and second doses of COVID-19 vaccines may actually increase resistance to the virus. It also detects mixing brands and dosage types providing better protection.
The decision by Canadian officials to immunize as many people as possible with any available dose of the COVID-19 vaccine, then to extend the time until a second dose, appears to pay off.
Recent data compiled by the British Columbia Centers for Disease Control and the Quebec National Institute of Public Health also shows the strategy of using the first available vaccine for the second dose, even if not the same brand as the first, actually increased effectiveness and lifespan. has been saved .
Pfizer, Moderna and AstraZeneca all recommend 21-28 days between the two shots. Canadian experience shows that the security becomes even stronger after a gap of six weeks.
For the Pfizer vaccine, this effectiveness increased from 82% to 93% after an interval of three to four weeks when a booster, or secondary dose, was given after four months.
The study also found that two doses of the AstraZeneca vaccine provided less protection than the mRNA vaccines from Pfizer and Moderna. However, those who received mRNA as a booster dose had the same protection as if they had the same two, even though their first dose was AstraZeneca.
All three vaccines were found to be more than 90% effective in keeping recipients out of hospital for COVID-19.
During the pandemic, Dr. Bonnie Henry, the provincial health officer for the Canadian province of British Columbia, encourages the first dose to be administered as soon as possible – and not to worry if the second dose is from a different vaccine.
Overall, she said, the Canadian experience can provide insight for the rest of the world.
“We don’t want countries to have to withdraw doses or wait for manufacturers to be able to give people full protection when they’re seeing outbreaks in other countries — and we saw it in India, for example,” she said. So globally it’s really important that we be able to use whatever vaccines are available to give people good protection. “
Joan Robinson, a pediatric infectious disease physician and professor at the University of Alberta and Stollery Children’s Hospital in Edmonton, said increasing the time between doses may be good for longer periods in areas with stable or low coronavirus levels.
However, Robinson said, there is a downside for the short term, especially in areas with high concentrations of COVID-19 cases.
“So the delay between your first and second dose of doses, you have a much higher chance of getting COVID than if you got this second dose earlier,” she said. “Certainly with the delta version, one gets the impression that a single dose may be less effective.”
The findings of researchers in British Columbia and Quebec, which are thousands of kilometers apart, are nearly identical.
This most recent Canadian data has not been widely published or peer-reviewed, but the researchers released the information to make it available globally as soon as possible.