Ontario is waiting for a recommendation from the National Advisory Committee on Immunization before delaying second doses of COVID-19 vaccines, while British Columbia is pushing ahead with its plan to extend the interval to four months.
A researcher whose work underpinned B.C.’s decision said Tuesday that the change was based on strong data. Dr. Danuta Skowronski, epidemiology lead of influenza and emerging respiratory pathogens at the B.C. Centre for Disease Control, said Pfizer-BioNTech underestimated the efficacy of the first dose of its own vaccine in its submissions to the U.S. Food and Drug Administration.
Skowronski said the company included data from the first two weeks after trial participants received the shot, which is a time when vaccines typically aren’t effective. When she and her colleagues adjusted the data, they made a surprising discovery.
“We took that into account and showed that the vaccine efficacy was not 52 per cent, it was actually 92 per cent. That was actually very similar, that first-dose efficacy, to what the other manufacturer, Moderna, had actually itself correctly analyzed in its own submissions to the FDA,” she said in an interview.
Skowronski said the four-month interval is based on the basic principles of vaccine science. The protection from a first dose of vaccine does not suddenly disappear, it gradually wanes over time, and scientists are typically more concerned about providing a second dose too soon rather than too late, she said.
“I think if the public had a chance to hear and to understand that, they would say, ‘OK, this is not messing around. This is really managing risk in a way that maximizes protection to as many Canadians as possible.’”
She said the province’s decision was backed up by data from Quebec, the United Kingdom and Israel, and comes at a time when B.C. residents are experiencing severe social and economic effects from isolation during the pandemic.
Skowronski added that the B.C. Centre for Disease Control also examined the effects of a single dose on long-term care residents and health-care workers in the province and found that it reduced the risk of the virus by 80 per cent within two to three weeks.
Provincial health officer Dr. Bonnie Henry has said she expects a statement from the national immunization committee aligning with the province’s decision, while Ontario Health Minister Christine Elliott said Tuesday she wanted to wait for such a recommendation.
She said extending the interval between doses would allow the province to get some level of protection to more people.
“This would be a considerable change,” she said.
“With the variants of concern out there, this could make a significant difference for Ontario in reducing hospitalizations and deaths. So, we are anxiously awaiting NACI’s review of this to determine what they have to say in their recommendations.”
Dr. Shelley Deeks, vice-chair of the national committee, said in an email the group is expected to issue a statement on extending the dose interval on Wednesday, but she did not confirm it would align with B.C.’s plan.
Alberto Martin, a University of Toronto immunology professor, said his understanding of a published Pfizer-BioNTech clinical trial was that the first dose provided 60 per cent protection and the second dose provided 95 per cent protection.
He said there is “obviously some concern” about B.C.’s plan because he is not aware of any clinical trial that examined a four-month gap between doses.
“There might be a risk that you’re not going to achieve that 95 per cent protection that the Pfizer-BioNTech vaccine can achieve with two doses separated by a few weeks apart,” he said.
However, he said difficult times — when the vaccine supply is so limited — require drastic measures.
“I think what they’re trying to do is immunize as many people as possible, get partial protection in the population and then hope for the best that the second dose that comes in four months later will boost the immune response enough to achieve that 95 per cent protection,” he said.
“It’s a difficult decision to make. I don’t know whether I’d like to be in that position, but I think it’s understandable why they’re doing this.”