How long does it take for the COVID-19 vaccines to become effective?
TORONTO -- As Canada gears up to receive up to 1.9 million more vaccines from suppliers this week – including the single-shot Johnson & Johnson – and the percentage of Canadians getting jabs continues to climb, CTVNews.ca offers an explainer on the differences between the vaccines approved by Health Canada and are available or will soon become available.
Pfizer-BioNTech, Canada’s first authorized vaccine, is effective in preventing both COVID-19 symptoms and severe outcomes caused by COVID-19, such as severe systemic illness, respiratory failure, shock, acute renal, hepatic or neurologic dysfunction, admission to an intensive care unit, or death, according to the Government of Canada.
The data shows that after the first dose and before the second dose, the estimated efficacy of the Pfizer-BioNTech vaccine is initially 52 per cent effective, but then becomes 92 per cent effective 14-21 days after the first dose.
While the manufacturer has the second dose listed to be taken 21 days later, the National Advisory Committee on Immunization (NACI) has recommended the second dose for all vaccines to be administered up to four months after the first dose to allow more Canadians to receive the vaccine. The NACI said that by having more individuals vaccinated with their first dose, this will decrease the chances of community transmission.
One week after the second dose is administered, the Pfizer-BioNTech vaccine will be 95 per cent effective in preventing symptomatic COVID-19 and severe outcomes related to it.
Moderna was authorized by Health Canada in late December 2020 and offers protection against symptoms and severe outcomes of COVID-19. The vaccine has an 80 per cent efficacy rate after the first dose and increases to a 92 per cent efficacy rate 14 days later. Two weeks after the second dose is administered, the vaccine will be 94 per cent effective.
For AstraZeneca, the vaccine is effective in protecting individuals from contracting the virus as well as preventing symptoms related to COVID-19 such as fever, coughing, shortness of breath, and anosmia or the complete loss of smell. The vaccine is also effective in preventing hospitalization and death due to the virus.
AstraZeneca will be 76 per cent effective in protecting individuals from COVID-19 and its related symptoms 22-90 days after the first dose.
When the second dose is administered 12 weeks or more later, the vaccine then becomes 82 per cent effective in preventing COVID-19, with immunity building over time.
Johnson & Johnson
Johnson & Johnson is Canada’s first single-dose vaccine that has been authorized. The vaccine is 66 per cent effective in preventing COVDI-19 two weeks after the vaccine is administered, and immunity will develop over time.
This article previously cited a 63 per cent efficacy rate for the second dose of AstraZeneca when taken two weeks after the first dose, which may have been interpreted as a drop in efficacy from the first dose. However, confidence intervals in the range of data from studies vary. As additional analyses have found around an 82% efficacy rate after a second dose, when given 12 weeks or more following the first, we are no longer citing the 63 per cent number under AstraZeneca.