How Long Does It Take for the COVID Booster to Be Effective? Here's What Experts Say

It's not possible to know the exact moment when your booster vaccine becomes fully effective, but research provides some insight about how long it might take.

Health officials are encouraging 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.

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According to the CDC, people are generally considered fully vaccinated two weeks after their second Pfizer or Moderna mRNA dose or two weeks after their single-dose Johnson & Johnson 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.

According to the CDC, any fully vaccinated individual older than 16 can get an mRNA booster shot. People who received the Pfizer or Moderna vaccine can receive the booster six months after completing their second dose, and those who initially received the Johnson & Johnson vaccine can get a booster two months after the single-dose vaccine.

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 Pfizer COVID-19 Vaccine Works

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.

Johnson & Johnson announced that when people receive its booster two months after the first shot, antibody levels rise by four to six times. Meanwhile, antibody levels are increased 37-fold by Moderna boosters and 25-fold by Pfizer boosters, per reporting from NBC. Research also suggests that mixing and matching vaccines offers just as much—or even more—protection as getting boosted with the same vaccine as you initially did.

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 that tracked Pfizer vaccine trial participants for 100 days after their boosters 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 had a much lower incidence of experiencing 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.

While the Pfizer booster showed promising effects, Penaloza-MacMaster said that both mRNA vaccines should start working in similar timeframes, meaning similar results could be expected of the Moderna booster.

The Johnson & Johnson report said that when its booster was given six months after the single shot, antibody levels increased nine-fold one week after the booster. Those antibody levels continued to climb to 12-fold higher a month after the booster. This study was done among people who were boosted six months after their shot as opposed to the recommended two months. (More on getting the vaccine ASAP vs waiting in a bit.)

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. That said, Penaloza-MacMaster did not recommend waiting to get a booster in hopes of getting better protection. Now is the best time to get your booster shot if you qualify. "In the middle of a pandemic, we want to get the higher amount of protection right now rather than waiting," Penaloza-MacMaster said.

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