Experts Warn Against Delaying Your Second COVID-19 Vaccine Dose

The Pfizer-BioNTech COVID-19 vaccine is a two-dose series; the second shot must be administered three weeks after the first to build full protection against the virus. However, a recent study suggests that delaying the second dose by 12 weeks boosted antibody responses more than threefold for people aged 80 and older.

The recent study, posted on the preprint server MedRxiv, is not the first to look at whether it's possible to extend the interval between the two COVID-19 vaccine doses. Back in February, researchers found that administering the second dose of the Oxford-AstraZeneca vaccine three months after the first—instead of six weeks or less after—increased the number of antibodies that a person's body produced.

As more research suggests that it's safe—and possibly even beneficial—to intentionally delay the second dose of the COVID vaccine, you might be wondering if you should consider it. But experts recommend against it.

Why Would Delaying a Vaccine Dose Be Beneficial?

Seeing an increased antibody response after delaying the second dose of a vaccine is not a new discovery. Margaret Liu, MD, board chair of the International Society for Vaccines, tells Verywell that it "has been observed for a number of vaccine candidates, and even in clinical trials of other licensed vaccines for other diseases, that extending the period between the prime and the boost results in higher antibody titers."

The response probably happens because the body's B cells and/or T cells continue to develop or mature in their response. "Not only does the quality of antibody change, but when the booster dose is given, the quantity of responding antibody increases compared to shorter booster periods," Liu says.

Another example is the seven-valent pneumococcal conjugate (PCV7) vaccine, which prevents invasive pneumococcal diseases (IPD) such as sepsis, bacteremia, and meningitis. A 2013 study showed that delaying the booster dose from 11 months to 24 months resulted in a significant increase in antibodies.

Liu says that “the bottom line is that this is not a new finding for many types of vaccines."

How Are Multiple-Dose Vaccines Scheduled?

According to Liu, when designing protocols for preclinical and clinical experiments, vaccinologists don’t just consider a vaccine's immune response within the body—they also think about how “user-friendly” it would be.

“For example, even if a 9-month boost was found to be optimal, that would be hard to have people remember to come back," Liu says. "Whereas they would more easily remember to come back in 3 to 4 weeks."

What's the Best Dosing Schedule for COVID Shots?

That said, the COVID vaccines are still relatively new, which means that we haven't quite nailed down what the ideal dosing schedule is.

“For many, we just don't have the data to know when the optimal time to give the booster is, only that a booster helps increase the potency and duration of immunity,” F. Perry Wilson, MD, a physician at Yale Medicine and researcher at the Yale School of Medicine, tells Verywell. “There may be a truly optimal time for a booster that we could figure out with intense study, but for many vaccines, the efficacy is good enough the way we are doing it now that there is little interest in reinventing the wheel.”

In clinical trials, researchers determined that the second doses of the Pfizer-BioNTech and Moderna COVID-19 vaccines needed to be given three to four weeks after the first dose (depending on the vaccine).

According to the Centers for Disease Control and Prevention (CDC), if a delay is unavoidable, the second shot should be administered up to 6 weeks after the first.

“Dosing schedules are often based on a ‘best guess’—and can certainly be refined as new data comes out,” Wilson says. However, he adds that we are unlikely to see a substantial shift to a longer interval in the current vaccine rollout.

Last week, the United Kingdom announced that it will be reducing the 12-week COVID-19 dosing interval to eight weeks with the intention of increasing vulnerable populations’ protection from the B1.617.2 variant first identified in India.

Don't Delay Your Second Vaccine Dose

While there is a chance that longer intervals between the first and second vaccine doses can result in higher antibodies and better protection, experts say that you should get your second dose of a COVID vaccine as soon as it is available to you.

“The response using the current dosing interval is potent and robust,” Wilson says. “And when people are between dose 1 and dose 2, they are still at risk—albeit at a lower risk than if they didn't get vaccinated at all—so we want to minimize that as much as possible.”

Liu adds that if COVID-19 was a low-risk disease, people might feel safer waiting longer between doses. However, she points out that the present situation with COVID is complex “because we don't fully understand the mutants and the risks of new mutants arising and spreading, nor do people everywhere follow masking or social distancing guidelines to the same extent."

Sticking to the Schedule

Why do COVID vaccines have the dosing schedules that they do? Liu says that vaccine makers likely did their initial COVID-19 vaccine clinical trials with three to four-week intervals between doses for several reasons:

• To provide people with higher antibodies as quickly as possible because of the severity of the pandemic
• To vaccinate more people in a shorter span, since people were resisting masking and failing to take the pandemic seriously
• To make it easier to remember when to come back for the second dose

The Risks of Delayed Doses

"The longer we delay the second dose, the more likely people will lose track, miss appointments, and not get that second dose at all," Wilson says.

CDC data from earlier this year indicated that more than five million Americans have reportedly missed their second doses of the Pfizer-BioNTech or Moderna vaccines. People who are not yet fully vaccinated are more at risk for getting COVID-19 and could get infected in between doses.

“The protection after the second dose even using the short interval is quite good,” Wilson says. “There just really isn't much bang for your buck for waiting longer.”

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.