What happens when you miss your 2nd CoronaVirus shot?

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Over 42 million Americans have so far received a dose of a covid-19 vaccine, as of early February. Importantly, though, most of these people have only received the first of two doses required for the similar Pfizer/BioNTech and Moderna vaccines, currently the only vaccines authorized for use in the U.S.

Both vaccines are highly effective at preventing symptoms from covid-19, at least for people who receive two doses about a month apart. Yet some people, even scientists, have wondered whether a single dose or two doses taken more than a month apart may provide similar benefits, largely based on select data from the clinical trials used to secure their emergency approval. The UK has allowed doctors to delay the second dose of the Pfizer/BioNTech vaccine for up to 12 weeks, but the U.S. has resisted any formal attempts to change the vaccine dosing schedule or to recommend that people only take one dose.

There may still be people who end up getting only one dose or who are unluckily exposed to the coronavirus before they can get their second dose. So we reached out to several scientists and doctors to ask them about what would happen in such a scenario.

Dr. David Lo

Immunologist and a distinguished professor in the Division of Biomedical Sciences at the University of California, Riverside

The second dose can be really important in making sure you have an adequate immune response. Maybe one possible analogy would be to how we learn some complex material in a class. A few lucky people might learn the information straight off with no need for review or reminder booster shots, but most people need to review and repeat to get the information solidly in your head.

There will be individual variation [for someone who only takes one dose], but one possibility is that they might not have full protective immunity. However, even a little bit of immunity from the first dose might yet help reduce the severity of any infection, as it may still give the immune system a bit of a head start.

Dr. Cheryl Healton

Dean of the New York University School of Global Public Health

The trials of the two current vaccines found about 70 percent efficacy in avoiding serious illness after one dose, 25 percent less than having both doses. It remains to be seen whether the vaccines will be equally protective against other variants, but lab studies are hopeful.

It is important to receive both doses, but one is certainly better than none at all. Hopefully the ramp-up will assure timely and complete coverage. One-dose vaccines with easier storage will be a huge improvement.