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Clinical trials on the Pfizer-BioNTech and Moderna vaccines both showed the first dose provided some protection against serious illness from COVID-19, but the majority of patients in both trials received a second dose on schedule, leaving very limited data on how long the protection from the first dose lasts.
Dr. Supriya Sharma, Health Canada’s chief medical adviser, said last week determining the effectiveness of the vaccines after the first dose is based on a sort of “back-of-the-envelope” calculation and the methods differ depending on when you start measuring.
It can take up to two weeks after a vaccine for the body’s full immune response to be in place.
Reviews of the Pfizer-BioNTech vaccine, which looked at how many people acquired and had symptoms of COVID-19 from the day after the first injection, found the first dose prevented illness in about half the patients monitored. But when the data was measured only starting two weeks after the first injection, effectiveness at preventing symptoms of COVID-19 was about 92 per cent.
For Moderna, the first dose was effective at preventing serious illness in 80 per cent of patients when the measurement starts the day after the dose is given, and also 92 per cent if starting to measure after two weeks have passed.
The problem with that is that for Pfizer, the majority of patients in the study were given the second dose on day 21, leaving just one week to measure the first dose’s effectiveness, and for Moderna, on day 28, leaving just two weeks to measure.
“You’re looking at really short time periods, right, when you’re counting cases (of COVID-19), said Sharma in an interview with The Canadian Press. ”The studies weren’t set up to do that. So, you know, it’s kind of a back-of-the-envelope calculation.“
There is also almost no data to understand how long that first dose remained effective without getting a second one because more than 90 per cent of patients in both vaccine trials got that second dose.
NACI’s recommendations say if the second dose is delayed those patients must be followed so more evidence of the impact can be attained.
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