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What Could Happen If You Miss Your Second Covid-19 Vaccine Dose?

Dr. Rebecca Wurtz

Infectious disease physician and population health expert at the University of Minnesota

So what could happen if you miss your second dose of the Pfizer or Moderna vaccines? What is the right/best schedule? I wouldn’t miss the second dose willfully—not, say because you had to run an errand to the hardware store. But if a second dose isn’t available when it’s time or there’s a snow storm or some other event which prevents it, then I would get the next dose when it’s possible, ideally within days or a few weeks of the previously scheduled dose. Until then, you have less protection—although still some—from infection and serious illness and you may shed virus if you do get infected, in contrast to better protection and less shedding if you are completely immunized. And it’s almost certain that you will get significant protection even if you get the 2nd dose weeks late. Alternate dosing schedules are being studied

I’d make a distinction between the initial dosing schedule (2 doses given a few weeks apart for the Pfizer and Moderna vaccines) and a “booster” dose, which is usually given at a much later date to boost the waning immune response to the initial vaccine series. Tetanus booster doses are given every 10 years after the initial vaccine series in childhood. The protection afforded by the authorized covid vaccines as the initial immune response wanes is unknown but is being studied. Booster doses may become routine.

Recent data suggest that if you have had covid, a single dose of the Pfizer or Moderna vaccine (acting in essence as a booster dose) is sufficient to stimulate antibody levels to what are thought to be a protective level. This had been shown in individuals with a history of highly symptomatic, test-positive cases of covid. Clearly, many months have passed between illness and vaccination and yet people still get a strong antibody boost. No one knows—yet—what kind of immune response is provoked by a single dose of vaccine in people who previously had asymptomatic (possibly undiagnosed) covid. Do those people have lower doses of antibody at baseline, and therefore less of a response to a single dose?