At a press conference on Monday Dr. David Williams, Ontario's chief medical officer of health, stressed that it is very rare that someone who did not have an adverse reaction to their first dose of the AstraZeneca vaccine would have one after their second shot. But he indicated that some may prefer to have the Moderna or Pfizer vaccine as their second dose.
"Right now we don’t know the actual vaccine effectiveness level or the immunological level of the two mixed," Dr. Williams said. "We’re hoping to have an answer to that soon with some studies early in June."
Ontario's chief medical officer of health added that individuals get a "prime effect" from the vaccine when the doses are administered 12 weeks apart, so the province is trying to get individuals who received their first AstraZeneca shot to get their second dose 12 weeks later, or beyond.
"At the same time, I’m not willing to wait and give expired vaccine at all, under any means, to anyone in Ontario. That would be totally wrong," Dr. Williams said.
He added that if the timeline between the two doses is too short the overall immune response "may be less than maximum," recognizing that some individual may be alright with that risk and would want their second dose sooner.
Another factor Dr. Williams said is being considered is where these doses are located and if they can be administered without moving them around too much.
Could more outdoor activities be coming?
As the weather gets warmer, Dr. Williams said that as Ontario continues to administer COVID-19 vaccines, hopefully outdoor restrictions can be loosened "fairly soon," getting "more optimistic" about outdoor activities.
"It’s a matter of can some of them be done and still maintain the adequate public health measures to avoid congregate or close activity," he said. "I think some of them have a could do that."
Dr. Williams added that there are specific concerns around individual actions before and after activities like golf, rather than the actual sport itself.