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AstraZeneca Moderna Pfizer Vaccine

Delayed Second Dose

People who got their first Pfizer and Moderna vaccine are expected to get their second dose 21 and 28 days later. The UK just approved AstraZeneca, of which people got their second dose 28 days later in clinical trials.

However, yesterday the UK made waves by announcing that they are prioritizing first doses. In other words, people won’t get their second dose of AstraZeneca or Pfizer until up to 3 months after their first dose. This is a bold approach… give as many people their first dose as fast as possible (rather than providing two doses to fewer people). Some places in Canada have been doing this and Belgium is considering it too.

Does timing of the second dose matter? This question has sparked quite the scientific debate.

On one hand, this could be a brilliant decision. Especially in a country where transmission is out of control due to a new variant. This decision could result in less deaths. Biologically (and historically) getting a vaccine a day or week late doesn’t matter much. The immune system usually doesn’t need that much precision. And sometimes the longer between doses, the better efficacy. But 3 months later? Not sure.

On the other hand, this could be a regrettable decision. During a pandemic with significant transmission of a disease, getting a vaccine exactly as it had been studied is important because: 1) People are much better off being fully protected. For example, in the Pfizer trial, the level of protection increased dramatically (52% to 95%) after the second dose; 2) The second dose typically provides longer-lasting immunity than the single dose; and, 3) Dr. Paul Bieniasz (Rockefeller biologist that studies the evaluation of viruses) said that this could cause partial antibody resistance among a population that is semi-immunized. “If I were designing a vaccine-resistant [COVID19 virus], I’d do what they are doing in the UK”. This is an experiment in viral evolution during the middle of a pandemic.

So, in short, the vaccine studies weren’t designed for a delayed dose, so we really don’t know the implications. AstraZeneca also hasn’t released their full data report, even though the regulators had a copy this week. So we can’t really parse out the data to make more accurate hypotheses. Word on the street is that some people had their second dose in the AstraZeneca trial much later than 28 days, but I have yet to see that data.

We’ll just have to see how this goes…

Love, YLE

3 replies on “Delayed Second Dose”

I am in the AZ study and am due for my second dose 1/11/21, 28 days after my first. Vaccine trials may turn out to be the only way to get a second dose quickly.

Can you provide your source for the assertion that the UK intends to introduce a three month interval between the 1st and 2nd Pfizer and/or Moderna vaccine application? I can’t find any reference to this being applicable to the Pfizer or Moderna vaccines. There appears to be unverified, limited literature (see attached link) which suggests a 2 to 3 month interval between the 1st and 2nd Oxford vaccine application. https://www.businesstoday.in/current/economy-politics/oxford-covid19-vaccine-95-per-cent-effective-if-given-3-months-apart-says-siis-adar-poonawalla/story/426336.html

“People are much better off being fully protected. For example, in the Pfizer trial, the level of protection increased dramatically (52% to 95%) after the second dose”
Although this quote might technically be true, I think it is misleading. In my review of the graph it looks like the vaccine was 0% effective the first 10 days or so, and then fully effective days 11-21. When you average this out you get the 52% number. It is certainly possible that it would remain fully effective for a period of time (weeks?, months?)without the second dose.

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