By December 2021, nearly 204 million Americans—more than 72% of the population—had been vaccinated against COVID-19. More than 58 million had received their COVID-19 booster shot since August 2021.

But your body needs time to build immunity after receiving a vaccine. This article shares information about your immunity window after vaccination, steps you can take the keep from getting sick after you get your shot, and why COVID-19 boosters are necessary to stay protected.

How Vaccine Immunity Works

Vaccines work by “teaching” your immune system how to fight a specific infection, like COVID-19 or the flu. The vaccine safely imitates the infection, and your body responds just as it would if the infection were real. The immune system responds by producing antibodies and cells called lymphocytes that help you fight off the virus or bacteria.

Most importantly, your immune system will remember this response. If by chance you’re exposed to the real infection, your body can more effectively fight it off because it has done it before.

But this “test drive” takes time. According to WorldClinic Chief Medical Officer William Lang, MD, MHA, the body doesn’t have immunity immediately after getting a vaccine.

“It takes the body time to make adequate antibodies for any vaccine,” he told Verywell.

How Long Does it Take to Achieve Immunity?

When can you consider yourself fully vaccinated? It depends on which vaccine you get.

In general, you’re fully vaccinated 14 days—or two weeks—after receiving your full series of shots. If you get a single-shot vaccine (Johnson & Johnson) you’re fully vaccinated two weeks later. If you get a two-shot series (Pfizer-BioNTech or Moderna), you’re fully vaccinated two weeks after your second shot.

Why do some COVID-19 vaccines take two shots to be fully effective?

“The first time primes your body [to the virus]," Lang said. “The second time tells the body that we’re serious about this and you really need to make immunity to it.”

Do I Need a Booster Shot?

Though some vaccines provide life-long or long-term immunity against a disease (such as polio), others are not as long-lasting. The duration of immunity provided by a vaccine depends on how easily a virus can change its genetic code (genome).

As the COVID-19 virus replicates and spreads, it duplicates its RNA. During this process, changes (mutations) can occur. This can result in variants, which are different versions of the same virus.

There are many variants of the COVID-19 virus, but two had emerged as variants of concern by late 2021: Delta and Omicron. Because variants have a different genetic code and can act differently than original viruses, sometimes immunity can wane. Your immune system may need a boost to make sure you’re fully protected.

Experts recommend a COVID-19 booster for people ages 16 and older who have have been fully vaccinated. For those who received the two-dose vaccine from Pfizer-BioNTech or Moderna, boosters are recommended at least six months after receiving the second shot. People who received the single-shot vaccine (Johnson & Johnson) should consider a booster at least two months later.

Steps to Take after Vaccination

According to William Moss, MD, MPH, executive director of the International Vaccine Access Center at John Hopkins in Baltimore, it’s crucial that people continue wearing masks even after getting their shots.

Even after vaccination, a large percentage of the population may not be protected because the vaccine isn’t 100% effective against COVID-19.

“If 95% efficacy holds up, 5% of people who get the vaccine will not be protected after receiving vaccination,” Moss told Verywell. “That sounds like a small percentage, but when you’re vaccinating millions of people, that’s a large number of people.” 

Understanding the Immunity Window

Pfizer-BioNTech and Moderna vaccines were shown to be 82% effective after the first dose. But patients shouldn’t let their guard down just because they’ve started the vaccination process because full immunity isn’t achieved until two weeks after the final shot of the series.

Moss explains that there is a likelihood that anyone can become infected with COVID-19 within that time period. However, it will depend on whether people continue to follow COVID-19 safety guidelines within the immunity window. 

“How likely they are to contract COVID-19 between the two doses is a function of how intense the transmission is in that particular area,” Moss says. “The likelihood of anyone acquiring infection and COVID-19 within a three week period—that’s not uncommon because the pandemic is out of control.” 

Summary

Three vaccines—from Johnson & Johnson, Moderna, and Pfizer-BioNTech—are available to prevent serious COVID-19 illness. Immunity is achieved two weeks after completing the full vaccination series. During this immunity window, it’s important to continue practicing COVID-19 protocols such as wearing a face covering and maintaining social distancing.

Vaccines may be less effective against some COVID-19 variants, and booster shots may be required to maintain immunity. If you received the two-shot series from Pfizer-BioNTech or Moderna, boosters are recommended after six months; for the single-dose series from Johnson & Johnson, a booster should be considered after two months.

“We’re in the midst of a terrible pandemic. Anything we can do as individuals and as a community to get us through these very hard times is going to benefit us all,” Moss says. “It’s going to decrease the burden on the healthcare system, help us get our economy back, and open up businesses that are so crucial to our society.”