Ontario public health officials will offer the province’s remaining inventory of AstraZeneca COVID-19 vaccine to those awaiting their second dose of the shot, saying new studies show the risk of adverse effects is low enough to proceed.
Chief Medical Officer of Health Dr. David Williams says those who received their first AstraZeneca dose between March 10 and 19 can book a second shot as soon as this week.
Others who wish to wait until 12 weeks from their first dose will be allowed to do so, Williams said.
But it appears the first cohort of AstraZeneca vaccine recipients are being asked to take the shot early simply to keep the province’s remaining doses from expiring.
The province stopped administering AstraZeneca as a first dose ten days ago.
Up until May 15, Public Health Ontario said it was aware of 14 instances of people developing blood clots after receiving the AstraZeneca vaccine, known as Vaccine-Induced Immune Thrombotic Thrombocytopenia (VITT).
They also say that overall, the AstraZeneca vaccine develops serious adverse reactions at three to four times the rate of the Moderna and Pfizer-BioNTech vaccines.
Global data now indicates a risk of fatality of around 1 in 600,000.
As of yesterday, the province had received 1.1 million doses of AstraZeneca and was expected to receive a further 254,000 doses this week through the global COVAX cooperative vaccine sharing scheme.
About 55,000 doses from the province’s earlier deliveries remain unused, and must be used by May 31.
Chief Coroner Dr. Dirk Huyer says the people who received first doses from March 10-19 will be offered shots approximately 10 weeks after instead of 12, in order to use up those doses before they expire.
The doctors stressed that all available information suggests it is still safe to receive a second dose of the AstraZeneca coronavirus vaccine at 10 rather than 12 or 16 weeks.
“We think it's fine to use the 10 to 12 weeks,” Huyer said. “Many countries like UK were down to eight weeks, so we're trying to get a combination of utilization of the timing and getting the maximal effect out there to do with the cohorts and different groups we've got, we'd still like to go for that 12 week period if we can to make sure people get the maximum peak effect from the vaccination.”
The use of the additional 254,000 doses arriving this week is still being determined. Williams said they would need a further 750,000 AstraZeneca shots to allow everyone in the province who received a first shot to get a second.
Officials say they are still considering whether to mix AstraZeneca doses with Moderna or Pfizer shots, which use an entirely different delivery mechanism known as messenger RNA, and National Advisory Council on Immunization is still studying the issue.
A relatively small study from Spain published earlier this week suggested mixing doses was safe and effective.
The province will not offer AstraZeneca shots to anyone else as a first dose, except in very limited circumstances where an allergist confirms a likely allergic reaction from both of the mRNA vaccines in use in Ontario.
