Why One Dose of a Two-Dose Vaccine Isn’t Enough

Why One Dose of a Two-Dose Vaccine Isn’t Enough
Photo: Yuganov Konstantin, Shutterstock

When you get your first dose of a two-dose COVID-19 vaccine, evidence indicates your protection begins about two weeks later — before you get the second dose. That’s good news, but it doesn’t mean you can skip the second dose.

The latest study to confirm this timeline came out this week, showing that the healthcare and essential workers in the study were 80% protected two weeks after their first dose, with effectiveness rising to 90% two weeks after the second dose.

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Other studies over the past few months have come up with similar results. In one study in Israel, the first dose of the Pfizer vaccine was 46% effective at preventing infections between days 14 and 20, with an even higher efficacy if you’re looking at symptoms, hospitalisation, or severe illness rather than just positive COVID tests. A week after the second dose, though, effectiveness against infection was up to 92%.

Business Insider has rounded up several studies here, finding that both Pfizer and Moderna vaccines seem to be 80% effective after the first dose. But it’s important to note these studies ask what happens between the two doses, not whether it’s ok to only get one.

Clinical trials have not tested what happens when you skip or delay the second dose

Despite this lack of evidence, Canadian scientists have argued that the first dose provides enough protection that second doses should be delayed to 12 weeks instead of 3-4 weeks. Others have countered that a 12-week delay was never tested in clinical trials, so we don’t know if spacing the shots further apart will result in the same effectiveness in the long run.

Anecdotally, I’ve heard people speculate whether it might be ok to skip the second dose altogether. Since the immune response is often stronger after the second dose, that might seem like an attractive proposition. But, again: we don’t know if a single dose of a two-dose vaccine actually works, and so expert advice remains firm: get both doses.

“I can’t emphasise enough how important it is to get the second dose,” an infectious disease specialist told Yahoo! Life.

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“If you don’t get your second shot you’re not fully protected and you’re not fully protecting others at the same time,” South Carolina’s deputy of public health said, after noticing an uptick in residents skipping their second doses.

“Get a vaccine when it’s available to you, and make sure you don’t skip the second dose,” an infectious disease specialist wrote on the Ohio State University website. He points out that trials in animals did compare a one-dose to a two-dose regimen, and that the double dose became the standard because it was more effective.

So if you got the first dose of a two-dose vaccine, experts agree that you should get the second.

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