What Happens If You Skip a Second COVID-19 Vaccine Dose?

For a majority of Americans, a full COVID-19 vaccination means sitting in a doctor’s office or clinic for a series of two different shots, developed by teams behind Pfizer and Moderna’s leading vaccine products. Current guidelines set by officials at the Food and Drug Administration dictate that patients must wait at least 21 days for a second Pfizer dose; those who receive Moderna shots, at least 28 days (outside of the U.S., AstraZeneca recipients wait at least 56 days for a second dose). These second shots are commonly called “booster” shots by professionals, as they’re designed to “boost” your immunity, much like the series of shots used to protect you against mumps or measles, as an example.

Because of high demand, sitting down for two shots is a lot easier said than done for some — long lines at public clinics are a big deterrent for those taking time off work. Some may be hesitant to return for a second shot after a very sore arm or a string of side effects following their first shot. And others may simply be unable to make their predetermined second dose appointment due to extenuating circumstances, and re-booking can be just as tricky as securing a first-time appointment.

Could receiving just one vaccine be good enough to keep you healthy and safe, you may wonder? If you’ve had to miss your second appointment, is it too late to reschedule for your second vaccine? And what happens if you’re only able to access a vaccine that is made by a different manufacturer?

The short answer to most questions: It’s very important for you to finish your COVID-19 vaccine series to reap the most benefits, explains Nicholas Kman, M.D., an emergency medicine physician at The Ohio State University’s Wexner Medical Center. Your CDC-issued vaccine card will often include details for when you should return for a second shot, which ensures as much immunity against life-threatening cases of COVID-19 as possible. But even if you can’t make that date, it’s still in your best interest to receive a second dose — and not to try to get a different dose or “start over” when it comes to vaccines.

Why is a second booster dose important?

In order for your Pfizer or Moderna mRNA vaccine to be as effective as it can be, you’ll need to sit for both doses. Research published in the BMJ indicates that Pfizer’s vaccine is only 52% effective at preventing the spread of SARS-CoV-2 (the virus that leads to a COVID-19 diagnosis) after one dose, whereas Moderna’s vaccine could be over 80% effective in the span of the first two weeks, per BBC reports and the World Health Organization. Both vaccines are well over 90% effective at preventing life-threatening cases of COVID-19 after a brief period following two full doses, according to the CDC.

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Data does indeed suggest that immunity develops in some form after your first dose, but the same data suggests that vaccinated individuals are fully immunized within just 2 weeks of the second dose. “Antibodies can increase 10 times after the second dose,” Dr. Kman explains. “This second shot is especially important in those with compromised immune systems who are considered high risk for severe complications or symptoms.”

What kind of protection does only one shot provide against COVID-19?

There has been limited research into how a single shot of a two-series vaccination can impact your immunity against COVID-19 in the long run, but early data suggests that there is a significant source of immunity with just one shot. Dr. Kman explains that a single dose of both vaccines proved 80% effective in preventing COVID-19 infections in a pool of 3,950 study participants in six different states over a 13-week period. “Following a single dose of either vaccine, the participants’ risk of infection with SARS-CoV-2 was reduced by 80% two or more weeks after vaccination,” he says. “It’s important to note that all the vaccines — including Johnson & Johnson’s — are 100% effective in preventing death from COVID-19, which is probably the most important endpoint.”

Receiving only one shot won’t lead to any adverse side effects; it won’t cause any residual side effects on your immune system. Why are two shots so essential, then? Anthony Fauci, M.D., the director of the U.S. National Institute of Allergy and Infectious Diseases, has recently hinted that a single dose may not provide immunity as long as a full vaccine series could. “When you look at the level of protection after one dose, you can say it’s 80%, but it’s somewhat of a tenuous 80%,” Dr. Fauci told members of the press last week. “When you leave it at one dose, the question is, how long does [immunity] last?”

And because new variants of the SARS-CoV-2 virus are proving to be an issue even for fully vaccinated adults, Dr. Fauci adds that a single shot may offer even less protection against new COVID-19 strains. “You’re in a tenuous zone if you don’t have the full impact,” he added.

Can I receive my second dose of a COVID-19 vaccine later than usual?

If you’ve missed your appointment for a second dose of your vaccine, good news: It is not “too late” to get a second one for better immunity. Sachin Nagrani, M.D., the medical director of healthcare services provider Heal, tells Good Housekeeping that you can complete your vaccine series up to 42 days after your first dose without any impact on its effectiveness.

All experts we consulted agreed: Even if you are outside of the 42-day range, you should still schedule a booster dose for best results. Due to a lack of data at this time, Dr. Nagrani says it’s unclear if the effectiveness of the vaccine is different when someone receives a booster dose long after a 42-day interval. “It’s reasonable to suspect that the vaccine remains effective if the series are spaced out farther, but data to bear this out isn’t available at this time,” he says.

Sometimes, not keeping your second dose appointment is unavoidable — especially for those who may be permanently moving to another state. Dr. Kman says the answer isn’t to try and get your second dose earlier than a recommended timeframe, as this can impact your immune system’s response to the vaccine. “However, if you do receive your second shot of the vaccine earlier or later than recommended, you do not have to restart the vaccine series,” Dr. Kman adds.

You should also reschedule your second COVID-19 vaccine dose (even beyond the 42-day mark!) if you are feeling sick on the day of your appointment, or your healthcare provider instructs you to do so.

Can I get my second dose from another manufacturer?

Whether it’s because you had painful side effects on the first dose, or because you’re having trouble accessing a second dose appointment for a certain vaccine, you may be wondering if you can switch from Pfizer to Moderna and vice versa. Current CDC guidelines maintain that you should receive the same vaccine in both doses, even if that means sacrificing the timeline for a second dose.

Why is that so important, you might ask? “Getting a different vaccine for your second dose might be theoretically possible, but certainly not studied or recommended,” Dr. Kman says. “It would be better to wait to get the same manufacturer given that you do have good protection at the two-week point after the first shot.”

In very rare cases, Dr. Nagrani says Johnson and Johnson’s Janssen COVID-19 vaccine (which is designed to be administered only once) could be substituted for a second-dose mRNA vaccine. Current CDC guidelines indicate that this strategy will only be recommended by healthcare providers to patients with severe extenuating circumstances for requesting a different vaccine. It seems that getting another type of vaccine altogether is better than considering a booster dose from another manufacturer in the end.

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