For a short time, back when the initial COVID-19 vaccine supply was alarmingly low, some health experts floated the idea of using existing vaccine doses that had already been set aside for second injections. The theory, at the time, was that it would be more strategic and effective to vaccinate a larger group of people with a single dose than to employ a two-dose schedule on a much smaller population size.

In a letter published in The New England Journal of Medicine last month, Doctors Danuta M. Skowronski and Gaston De Serres opined: “With such a highly protective first dose, the benefits derived from a scarce supply of vaccine could be maximized by deferring second doses until all priority group members are offered at least one dose.”

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Thankfully, vaccine supply from Pfizer and Moderna managed to ramp up significantly over the past few weeks, thus rendering the above idea moot. Still, the idea raises an interesting and important question: what happens if you miss your second coronavirus vaccine shot or need to delay it? While both Pfizer and Moderna recommend getting the second shot three to four weeks after the first, it’s inevitable that some people simply won’t be able to make it to their second shot on time.

According to data from both Pfizer and Moderna, missing your second coronavirus shot will still provide you with some protection against a COVID-19 infection, albeit to a lesser extent. Whereas a two-dose administration of their respective vaccines was shown to be 95% effective at preventing a COVID infection, Pfizer’s vaccine was found to be 52% effective after a single dose. Moderna’s vaccine, meanwhile, was shown to be more than 70% effective at preventing a COVID infection after the first dose.

So while the data here is encouraging, the severe symptoms that can sometimes manifest with a COVID infection should motivate everyone to complete their vaccine regimen as prescribed.

The Ohio State University Wexner Medical Center writes:

The first dose primes the body by telling it to produce antibodies, and the second dose increases it, or essentially completes the antibody training. The goal is to get the second dose as close as possible to the recommended interval, as that timing was studied for each of these two vaccines and led to the very high levels of effectiveness.

Even if you can’t make it to your second dose on time, the CDC indicates that second vaccine doses from Pfizer and Moderna can be administered up to 42 days after the first. Studies about the efficacy of these vaccines beyond the 42-day window haven’t been done yet.

It’s also worth mentioning that maximum immunity against a COVID infection doesn’t occur immediately after receiving the second dose. Rather, it takes about two weeks for full protection to kick in, which is to say that anyone who has been vaccinated should still strictly follow coronavirus safety measures for at least two weeks following their second dose.

Incidentally, one reason why some people have been requesting Johnson & Johnson’s coronavirus vaccine — at clinics where a choice is an option — is because it only requires a single shot. Still, it seems that if given the chance, most people have indicated a preference for either Pfizer or Moderna’s vaccine due to its 95% efficacy rate.

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A life long Mac user and Apple enthusiast, Yoni Heisler has been writing about Apple and the tech industry at large for over 6 years. His writing has appeared in Edible Apple, Network World, MacLife, Macworld UK, and most recently, TUAW. When not writing about and analyzing the latest happenings with Apple, Yoni enjoys catching Improv shows in Chicago, playing soccer, and cultivating new TV show addictions, the most recent examples being The Walking Dead and Broad City.