The National Advisory Committee on Immunization has doubled down on its recommendation of a four-month gap between doses of COVID-19 vaccines in an update released Wednesday (April 7).
In the update, NACI said that delaying the second dose until everyone has received their first dose would allow 75 per cent of Canadians to get it by mid-June.
“Second doses should be offered as soon as possible after all eligible populations have been offered first doses, with priority given to those at highest risk of severe illness and death from COVID-19 disease,” NACI said. “With Canada’s expected vaccine supply, the interval between the first and second dose is expected to be less than four months.”
NACI said that provinces could choose to shorten the time between doses in certain populations based on local cases, vaccine supply and new evidence. The committee first made the four-month gap recommendation in March, citing vaccine availability.
Evidence for delaying the second dose came from a variety of real life scenarios, both in Canada and abroad, although NACI said many of the studies had not yet been peer reviewed.
“Effectiveness data from Canada and the UK demonstrates protection from the mRNA vaccines based on analyses extending to about 8 weeks from vaccination,” NACI said.
“Experience with other multi-dose vaccines after a single dose suggests protection could last for six months or longer in adolescents and adults (e.g., hepatitis A and human papillomavirus vaccines).”
NACI said that as a “general vaccination principle,” interrupting an immunization schedule doesn’t require redoing the first dose.
“Furthermore, a longer interval between the priming and boosting doses allows maturation of the memory B cells, resulting in a higher and more durable response.”
NACI said that current evidence shows 92 per cent efficacy for the Pfizer and Moderna mRNA vaccines and 76% efficacy for the AstraZeneca viral vector vaccine.
“Effectiveness has been documented for up to two months after the first dose of the mRNA vaccines, and the AstraZeneca clinical trial publication modelled one-dose efficacy up to 90 days after vaccination,” NACI said.
Data from long-term care residents in B.C. showed vaccine effectiveness of 87 per cent between 21 and 54 days after the first dose, while other data provided by the B.C. Centre for Disease Control showed effectiveness of 80 per cent from 21 to 43 days after the first dose in long-term care residents. Similar immunization intervals in health-care workers showed similar results.
NACI said that data from the AstraZeneca clinical trial showed maximum efficacy when the interval between doses was extended to over 12 weeks. Efficacy at less than six weeks after the first dose was at about 55 per cent, while efficacy 12 weeks after the first dose was at 81 per cent.
B.C. has used Pfizer, and to a less extent Moderna, in its age-based vaccination program, as well as for health-care workers. AstraZeneca was initially being used for frontline workers, but after concerns over effectiveness in older adults and blood clots in younger people, B.C. moved to using it to vaccinate 55 to 65 year old via community pharmacies.
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