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COVID-19 VACCINE Q+A

Here are the most recent frequently asked questions on the intervals between the first and second doses of the vaccine.

Q: Why has the interval between vaccine doses been extended?

A: Each vaccine dose reduces the likelihood of severe illness and saves lives, and also increases the protection that our health care system needs, which is why Provincial Health Officer Dr. Bonnie Henry has directed that the interval between first and second doses be further extended. Available vaccine is being directed to outbreak sites around the province. This is a temporary approach that will protect as many people as possible from this virus during a time of limited vaccine supply.

Both the Pfizer-BioNTech and Moderna vaccines require a second dose to maximize their effectiveness and give longer lasting protection. However, the protection after even a single dose is over 90 per cent, and this protection lasts for at least six weeks. For both approved vaccines, this dosing schedule is within the approved range and has been endorsed by Canada’s National Advisory Committee for Immunization as well as by immunization experts at the World Health Organization.

Q: In Pfizer’s Phase 3 trial of its COVID-19 vaccine, efficacy between the first and second doses was observed to be 52 per cent. Why are health officials in Canada and around the world now saying it’s more than 90 per cent after the first dose?

A: The phase three trial compared results for people who received two full doses, 21 days apart, against results for people who received a placebo. Although the study was not designed to assess the efficacy of a single-dose regimen, in the 21-day interval between the first and second doses the observed efficacy was 52 per cent.

However, this early protection doesn't take effect until at least day 12 — until then, there was no difference between the two groups. Taking this timing into account, when protection is assessed from 14 days after the first dose, the efficacy of the vaccine jumps up to >90 per cent, because it is not being diluted by the relatively high number of infections before the vaccine begins to have an effect.

Q: How will I know when it’s time for my second dose, and where I can go to get it?

A: Public Health is tracking who has received vaccine, which kind was administered, and when people will be invited to receive their second dose, but you can help by making a note of your own dates and details, and by being ready when it’s time for your second vaccination.

This page last updated Jan 29, 2021 6:59pm PST