With new information becoming available each day on the COVID-19 vaccine and vaccine boosters, an internal medicine and infectious disease specialist shares the most important facts you should know about the COVID-19 booster shots.
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According to the Centers for Disease Control and Prevention, 192.2 million people in the United States have been fully vaccinated against COVID-19 and another 17.7 million people have received a booster dose of the COVID-19 vaccine as of October 31, 2021. In addition to those who have already received boosters, millions more are now eligible to receive a vaccine booster shot.

With all of the information about COVID-19 vaccines and booster shots now available, it can be difficult to make sense of all of the data. The sheer volume of information available on these topics is overwhelming. And some of the guidelines are fairly vague, leading to confusion about eligibility, timing, types of vaccines and more.

To provide clarity, internal medicine and infectious disease expert Danny Branstetter, M.D., M.S.N., shares his expertise on what the COVID-19 boosters are meant to do, who is eligible, why they are important and when to get one. Additionally, he helps answer some of the most common questions many still have about the COVID-19 pandemic, addresses considerations for pregnant women and shares valuable insight into the daily habits we can include to improve our immune response.

What You Should Know About COVID-19 Vaccines

Most everyone knows someone who has been impacted by COVID-19. As of October 31, 2021, in the United States alone, there have been 46 million reported cases of COVID-19 and 743,410 deaths since the pandemic began, according to CDC data.

The good news is that we now have vaccines that are widely available that can help "break" the pandemic. Branstetter says. "The goal of getting as many people vaccinated as possible is to reach what we call 'herd immunity' for our communities." While this doesn't mean that COVID-19 would be completely gone, it would break the pandemic by reducing the high numbers of infections in our communities.

Branstetter wants to be very clear, however, about what herd immunity does not do. He shares that herd immunity alone does not prevent us from getting COVID-19 or from having complications from it. What it does do is that it prevents large numbers of symptomatic infections in our communities at any given time.

This information emphasizes the importance of the vaccines and ultimately the booster shots as well. Branstetter warns, "Even if we reach herd immunity, if you are unvaccinated, you still can get infected and still have complications related to COVID-19."

Dr. Branstetter's Takeaway: "If you are eligible and you can get vaccinated, to protect yourself and stay as healthy as possible to take care of your families and live productive lives, get vaccinated and don't rely on herd immunity."

What's the Difference Between COVID-19 Vaccines and Boosters?

The CDC defines vaccines as: "A preparation that is used to stimulate the body's immune response against diseases."

With this definition in mind, it is important to clarify what we should and should not expect vaccines and boosters to do.

While COVID-19 vaccines and boosters have proven effective in preventing severe symptomatic illness, they do not keep us from being exposed and may not keep us from becoming infected. Branstetter explains, "Vaccines prime our immune system to respond in a robust manner so we hopefully never develop a symptom."

The role of the booster shot is to "boost" the immune response of an individual, as their immunity may wane over time after receiving the primary series. Booster shots are administered in order to help prevent symptomatic disease, hospitalization or death.

Dr. Branstetter's Takeaway: "The biggest thing that vaccines (and boosters) do is prime our immune system to respond as robustly as possible to prevent us from needing to be hospitalized and prevent us from dying from COVID-19."

Who Can Get a COVID-19 Booster Shot?

When considering eligibility for additional doses of the COVID-19 vaccine, it's important to clarify the difference between the recommendation for a third shot in the primary series versus the recommendation for a booster shot.

Here Branstetter provides insight into these two different types of additional vaccine doses:

Third Shot in COVID-19 Primary Vaccine Series

It is estimated by the CDC that 3% of the U.S. population is moderately to severely immunocompromised due to being actively treated for cancer, having organ or stem cell transplant, having a moderate or severe immunodeficiency syndrome, having advanced or untreated HIV or taking immunosuppressive drugs or corticosteroids. Studies found that individuals in this group may not build immunity after vaccination at the same level as people who are non-immunocompromised.

In mid-August, the CDC started recommending that this group of moderate to severely immunocompromised individuals receive a third dose of the Pfizer or Moderna COVID-19 vaccination as part of their primary series. These additional doses are to be administered at least 28 days after their second shot. These third shots in the primary series are not the same as booster shots.

As of October 31, 2021, the CDC has not yet released recommendations on booster shots for this group of moderately to severely immunocompromised individuals, however Branstetter anticipates that the recommendation for a booster shot for this group may be coming in the near future.

A hand holding a COVID-19 vaccine bottle on a designed background
Credit: Getty Images / Kilito Chan

COVID-19 Vaccine Booster Shot

Booster shots are not part of the primary vaccine series, but are given at a later date to "boost" the immune response in higher-risk individuals.

Branstetter explains that the purpose of the booster shot is to add an additional layer of protection to prevent symptomatic disease in groups that are known to be at higher risk for complications related to COVID-19 infection.

On October 21, 2021, the CDC began to recommend booster shots of the Pfizer or Moderna COVID-19 vaccines for people who are:

  • 65 and older
  • Over 18 and live in long-term care settings
  • Over 18 and have underlying medical conditions
  • Over 18 and work or live in high-risk settings

These booster shots for the Pfizer or Moderna vaccine should be given six months or longer after the second dose of the primary series.

When asked to help define "high-risk" for those who may be unsure of whether they fall into that category, Branstetter considers the key words in this determination to be "public facing." He shares that for this group, we are talking about "people who interact with a variety of the general population and may be in situations where there is close contact and adherence to prevention measures such as distancing, hand-washing and face-mask wearing are compromised."

Additionally, the CDC recommends for all individuals who are 18 and over and received the Johnson & Johnson COVID-19 vaccine to receive a booster shot at two months or more after their first shot.

Dr. Branstetter's Takeaway: There is a difference between a third shot in the primary series and a booster shot. The purpose of the booster shot is to "increase the protection for those who may have that symptomatic disease and need to be hospitalized, even after receiving a primary series."

Types of COVID-19 Booster Shots

In the United States, we have ample vaccine supply, and we now have options to choose which booster shot we can get.

The CDC has determined, and Branstetter confers, that it is safe to "mix and match" vaccines when getting a booster. For example, if you initially received two doses of Pfizer, you can choose to either get a booster dose of the Pfizer vaccine or you can switch and get a dose of the Moderna vaccine or the Johnson & Johnson vaccine as your booster. In addition to being safe to mix and match, the immune response appears to be equal regardless of which brand you choose as your booster.

One question that Branstetter says he often gets from individuals who initially received the Johnson & Johnson vaccine is, "Do I need to do an entire series of one of the other brands (i.e., two shots), or do I just need one booster shot?"

His response: "If you are going to boost, regardless of whether you initially received Johnson & Johnson, Pfizer or Moderna, you do it with a single boosting dose. You don't do an entire 'series.'" A single boosting dose, i.e., one booster shot of any of the brands available, is what is recommended.

In addition, Branstetter points out that this mix-and-match approach gives us flexibility and adaptability to better help those individuals who may not have been able to complete their initial vaccine series due to a tolerance issue with the first dose. Those who may have had a negative reaction to the first dose of either of the mRNA vaccines can now get greater protection by boosting with the Johnson & Johnson vaccine.

Dr. Branstetter's Takeaway: "If you are eligible for a booster, don't worry about the brand, just get one."

When Should You Get a COVID-19 Booster Shot?

Given all that we now know about the protective benefits of the booster vaccines, it may be tempting to want to rush out and get a booster shot as soon as you are eligible, regardless of timing. Don't. The minimum wait period recommended for each of the vaccines is crucial in creating the best immune response.

Branstetter breaks this down by the two different types of vaccines we currently have available in the United States:

Johnson & Johnson

Branstetter explains that the Johnson & Johnson vaccines provide great protection from severe illness and death in a single shot. What has also been discovered is that individuals can receive even greater protection against severe illness and death and protection from symptomatic disease by boosting the Johnson & Johnson vaccine at two months after the primary series. For those living overseas, this is also true for the AstraZeneca vaccine.

"The data is very clear, if you are past that two-month window, you can get a significant benefit, and probably better benefit if it's longer than two months, of immune protection by boosting with the Johnson & Johnson again."

"The important key here is the minimum amount of time. If you are less than two months (from your primary dose) for Johnson & Johnson, you're only a month out, it is too early to boost. You really need to wait that minimum amount of time."

Pfizer and Moderna

The recommended minimum wait time between the primary series of the vaccine and the booster shots is equally important for the mRNA vaccines.

For Pfizer and Moderna vaccines, the recommended minimum amount of time between receiving the primary series of vaccines and the booster dose is six months.

"If you are five months after completing your primary series, don't rush out and get a booster. Actually wait," explains Branstetter. "We actually have a better immune response when that period is longer."

Branstetter shares that the timing guidelines—whether to wait six months or eight months for the mRNA booster shots—were widely debated among agencies. "There was clear evidence that the longer you wait, the better "boosting" that you got from your immune response."

Ultimately when weighing the risks and benefits of the timing of the booster shots, the CDC recommended a minimum six-month wait time between the primary vaccine series and the booster shots, because the six-month mark is where studies found a drop off of antibody response.

Dr. Branstetter's Takeaway: "The minimum wait period is important. There is clear evidence: the longer you wait, the better boosting you get from your immune response."

Why COVID-19 Booster Shots Are Important

According to Branstetter, COVID-19 is now endemic, meaning that it is here and it is naturally circulating in the environment all the time. For comparison's sake, in terms of how you think about other illnesses and how they occur, think of it in the same way as you think of the common cold or the flu.

Because it is endemic, we are likely going to get exposed to COVID-19 and may have a symptomatic infection. This means that if you are eligible for the COVID-19 vaccine or for a booster shot, you should get it.

When asked about whether those who were fully vaccinated, but then contracted COVID-19, should go ahead and get a booster shot or if they should wait, Branstetter suggests following the same recommendations spelled out previously.

He shares, "If you qualify for a booster shot and have recovered from COVID-19, go ahead and get that booster shot. The data on that is that it's safe, and that's the important thing. It's best to go ahead and be cautious and protect yourself as much as possible."

Dr. Branstetter's Takeaway: "It's here and it's not going away so, like the flu, we need to protect ourselves as much as possible."

How to Improve Your Immune Response

Branstetter is a huge advocate for vaccines and for the simple health interventions that may help improve your immune response. As he explains, "Vaccines do not put a magical shield around us where we never get infected," so we need to do everything that we can to protect ourselves and improve our health.

Knowing this, it's crucial to recognize the role that each of us plays in our own health outcomes.

The choices that we make each day matter. To improve our body's immune response to COVID-19 and protect against all disease, Branstetter suggests that we follow these four simple guidelines:

  1. Stay up to date on ALL of your vaccinations.
  2. Work with your health care professional to keep chronic diseases under control.
  3. Put practices in place to help achieve and maintain a healthy weight, as obesity is one of the biggest risk factors for complications due to COVID-19, especially in young people.
  4. Stop smoking.

Considerations for Pregnant Women

There has been a lot of misinformation circulated related to pregnancy and fertility and the COVID-19 vaccines. It's important to consult with your doctor ahead of time but Branstetter encourages women who are planning to conceive, who are pregnant or who have just delivered get vaccinated if you haven't already.

He shares that the vaccine "is safe and there is no indication in hundreds of millions of doses worldwide now that there are any complications or adverse reactions related to pregnancy due to the COVID-19 vaccine." In addition, the antibodies created after vaccination are passed on to the newborn for protection.

Branstetter is personally concerned about this population, because "it's two lives that we are dealing with, and it's devastating when we see a young woman have horrible complications."

Unfortunately, Branstetter says that pregnant women often do have more complications with COVID-19 than people in their same age group who are not pregnant.

How to Find a Free Vaccine or Booster Shot 

If you are unsure of where to get a COVID-19 vaccine or booster shot, the CDC offers assistance. To find vaccines in your area, regardless of where you live in the United States, visit vaccines.gov or call 1-800-232-0233 (TTY 888-720-7489).

All COVID-19 vaccines and booster shots are free. Anyone living in the United States is eligible to receive a vaccine free of charge regardless of insurance or immigration status. Visit the CDC resource page for more information.

The situation surrounding COVID-19 continues to change quickly; it's possible that information or data has changed since publication. While EatingWell is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations by using the CDC, WHO and their local public health department as resources.