A Blog by Jonathan Low


Feb 9, 2021

What Happens To People Who Miss Their 2nd Covid Vaccine Dose?

People who receive two doses will enjoy greater protection than those who receive only one dose. But one is better than none. JL

Ed Cara reports in Gizmodo:

There will be individual variation [for someone who only takes one dose]. One possibility is that they might not have full protective immunity. Trials of the Pfizer or Moderna vaccines found 70% efficacy in avoiding serious illness after one dose, 25% less than having both doses. Recent data suggest that if you have had covid, a single dose of the vaccine is sufficient to stimulate antibody levels to what are thought to be a protective level. don’t panic if it’s delayed, even by several weeks. "One is certainly better than none."

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. 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?

In sum, for now, get the second dose on time, or as soon as possible after the dose is missed. But don’t panic if it’s delayed, even by several weeks.

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.

Dr. David Alland

Professor and Chief of the Division of Infectious Disease at Rutgers New Jersey Medical School

So first of all, you know, we don’t have any long-term data on any of these vaccines in terms of how long they are going to last. Typically, the second shot for vaccines are used both to increase immunity after the first shot but also to increase the duration of immunity. So we have some data on the first question, but not the second question. We know that the level of protection goes up with the second shot, so there’s clearly a benefit there. But we don’t know how long this protection is going to last. And it’s likely that your immunity lasts longer with the second shot.

That said, I think people should be happy if they get one shot. And certainly, if there’s a need to resupply, or it takes time, then having a delay for a second shot is probably not too bad a thing. It probably doesn’t make a big difference if the delay goes from four weeks to six weeks or eight weeks, for example. I think if you waited a year, you would not get the same boost, and you might not get the same protection.

I do want to say though: If you have to choose between getting your shots the “right” way or the “wrong” way, I would probably still try to get it the right way. Because right now is the time where we need the most protection— when we’re really at a peak of transmission. We’re starting to see it fall now; hopefully, that will continue. But with these new variants coming out, it’s really critical that we do everything we can to stop the spread now. So for personal reasons, for public health reasons, I think we want to do what we can to get what we know is outstanding protection with these two available vaccines, as opposed to uncertain protection if we wait longer.


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