Recent studies indicate Canada’s decision to extend the interval between the first and second doses of COVID-19 vaccines can actually lead to increased resistance to the virus. It also finds mixing the brand and type of doses gives better protection.
The decision by Canadian authorities to immunize as many people as possible with any available dose of COVID-19 vaccine, then extending the time until administering the second dose, appears to be paying off.
Recent data compiled by the British Columbia Centre for Disease Control and the Quebec National Institute of Public Health also show the strategy of using the first available vaccine for a second dose, even if not the same brand as the first, actually increased effectiveness and saved lives.
Pfizer, Moderna and AstraZeneca all recommend 21-28 days between the two shots. Canada’s experience suggests protection is even stronger after a six-week interval.
For the Pfizer vaccine, this effectiveness went from 82% after a three- to four-week interval, to 93% when the booster, or secondary dose was given after four months.
The study also finds two doses of the AstraZeneca vaccine gave less protection than the mRNA vaccines of Pfizer and Moderna. However, those who received an mRNA as a booster dose have the same protection as if they had two of the same, even if their first dose was AstraZeneca.
All three vaccines were found to be more than 90% effective in keeping recipients out of the hospital for COVID-19.
Throughout the pandemic, Dr. Bonnie Henry, the provincial health officer for the Canadian province of British Columbia, has encouraged first doses to be administered as quickly as possible -- and not to worry whether the second dose is from a different vaccine.
Overall, she said, Canada’s experience could provide insights for the rest of the world.
“We don't want countries to have to hold doses back or wait for manufacturers to be able to give people the full protection they need when they're seeing outbreaks in other countries -- and we saw this in India, for example,” she said. So it is really important globally that we're able to use whatever vaccines are available to support people to have good protection.”
Joan Robinson, a pediatric infectious disease doctor and professor at the University of Alberta and Stollery Children’s Hospital in Edmonton, said increasing the time between the doses can be good for the long term in areas with stable or low coronavirus levels.
However, Robinson said, there is one downside for the short term, especially in areas where there are high concentrations of COVID-19 cases.
“So the delay between the doses during the time between your first and second dose, you're much more likely to get COVID than if you had got this second dose earlier,” she said. “Certainly with the delta variant, one gets the impression that one dose may be less effective.”
The findings of researchers in British Columbia and Quebec, which are thousands of kilometers apart, are almost identical.
This most recent Canadian data have not been widely published or peer reviewed, but researchers released the information early to make it available globally as soon as possible.
