How Long Does It Take for the COVID-19 Booster To Be Effective?

Health officials encourage the public to get vaccinated against the SARS-CoV-2 virus, which causes COVID-19. In addition to an initial course of vaccines, the Centers for Disease Control and Prevention (CDC) provides guidelines for when to get the booster dose.

According to the CDC, people are generally considered vaccinated after their primary series. The primary series may be two doses of the Pfizer or Moderna mRNA vaccine, a single-dose of the Johnson & Johnson vaccine, or two doses of the Novavax vaccine.

But to build back up any protection that might have waned since your initial vaccination series, it's recommended that everyone who's eligible get a booster. A review published in 2021 in The New England Journal of Medicine found that doing so decreases the likelihood you'll get COVID-19 or become severely ill if you do contract it.

As of October 2022, there are two boosters available: one from Pfizer and one from Moderna. According to the CDC, adults ages 18 years and older can get either booster at least two months after the last dose of the primary series—and it doesn't matter which vaccine you got in your primary series.

For children, it's a little different. The CDC says children aged 5 to 11 years can receive the Pfizer vaccine as a primary series and as the booster, at least two months after the last primary series dose. Children aged 6 and above can receive the Moderna vaccines as a primary series and are eligible for either booster. After age 12, they can get either booster, regardless of which vaccine they got in the primary series.

While getting boosted is important in stopping the spread of COVID-19, what's not as clear is how soon after that shot someone is considered "fully boosted." Here's what you need to know about how quickly your booster shot will start to work, according to experts.

How the COVID-19 Booster Works

When you get any vaccine, your immune system mounts an antibody response, which should work to fight off future infection. Say you got your last COVID-19 vaccine several months ago. As time goes on, your immune protection can weaken, and a booster shot re-exposes your immune system, so it makes more antibody-producing cells.

A key factor in this process is a type of white blood cell called memory B cells, which remain in your body "waiting" to recognize and fight off the same pathogen.

"Once you incorporate another jab, your memory B cells can sense the proteins made by the virus; then they start making more antibodies," Pablo Penaloza-MacMaster, PhD, assistant professor of microbiology-immunology at Chicago's Northwestern University Feinberg School of Medicine, told Health.

By the time of your booster shot, your memory B cells have already encountered the viral proteins—either once or twice depending on which vaccine series you initially received. As a result, Penaloza-MacMaster said the cells can create more and better antibodies against COVID-19—which means you'll be more protected if you're exposed to the virus. Penaloza-MacMaster said it's also possible the booster may even offer more cross-protection against different variants.

How Soon the Booster Works

It's not possible to know the exact moment when your booster vaccine becomes fully effective. It's unlikely you'd have extra protection the day after you get your booster because it usually takes days or weeks for the memory cells to produce more antibodies.

"The only way we could make an estimate is extrapolating how the immune response behaves with other viruses," Penaloza-MacMaster said. "We know between the first and second week you have a massive increase in protection, but there haven't been experiments looking at hours or days."

Infectious disease expert Amesh A. Adalja, MD, a senior scholar at the Johns Hopkins Center for Health Security, agreed that most people will experience some positive effects from the booster within a week, but the full effect is believed to kick in two weeks after the booster. "In general, that's how the immune system responds and how long it takes to arrive at peak level of protection," Dr. Adalia said.

CDC data from 2021 on Pfizer vaccine trial participants was collected for 100 days after their boosters. The results after data analysis suggest that protective effects of the shot may start as soon as seven days after getting boosted. In the trial, people who received Pfizer boosters were less likely to experience symptomatic COVID-19 infections between a week to two months after getting a booster compared to people with just two shots who had received a placebo booster.

Factors that Decrease Booster Effectiveness

It's important to keep in mind that a few things can interfere with how effective a booster is, and how much protection a vaccine offers at all. For example, Dr. Adalja said elderly people usually respond less effectively to vaccines, and people on certain immune-suppressing medications may not get the full benefit. "But that's true of any vaccine," Dr. Adaja said.

There's one other factor that can impact the effectiveness of a booster: time between doses. According to Penaloza-MacMaster, the longer the interval between your original vaccine series and the booster dose, the better antibodies your memory cells can create.

If you happen to have waited a few extra months before getting your booster, now's the perfect time to get it. But don't postpone getting a booster in hopes of getting better protection later, Penaloza-MacMaster said. If you haven't already gotten a booster, getting one now if you qualify is best.