Whether you're planning on receiving a booster shot when they're available, or will be getting an additional dose due to your immunocompromised status, here's what you need to know.

When the COVID-19 mRNA vaccines were first released, there was a lot of talk about potential side effects—especially after people had their second dose. The Centers for Disease Control and Prevention (CDC) even addressed this, noting at the time that side effects from the second dose may be more intense than ones people experienced after their first shot. Those side effects, the CDC said, are normal signs that your body is building protection.

Fast forward about eight months and the conversation is ramping up again: Booster doses of the COVID-19 mRNA vaccines are slated to open up to all Americans, starting on September 20 (pending green lights from the CDC and FDA), and some immunocompromised residents are getting an additional dose of the vaccine even sooner (or they've already gotten it).

These third doses of the vaccine have people wondering: Will these COVID-19 booster or additional doses come with side effects too—and will they be worse or better than the second or first doses? Here's what you need to know about any symptoms you might feel after a booster or additional shot of an mRNA COVID-19 vaccine, according to experts.

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Credit: Getty Images

First, where do things stand with COVID-19 booster shots specifically?

The US Department of Health and Human Services (HHS) and medical experts announced in a joint statement on August 18 that, in accordance with recommendations from the Biden Administration, all Americans who received a two-dose mRNA COVID-19 vaccine series should get booster shots eight months after their second dose—but that guidance isn't in effect just yet.

There are a few hurdles that still stand in the way of full approval for the additional doses to be used as booster shots—both the Food and Drug Administration (FDA) and the CDC's Advisory Committee on Immunization Practices (ACIP) need to do their own independent evaluations and reviews of the evidence suggesting that booster shots are beneficial against waning COVID-19 protection and provide increased protection against the Delta variant. Keep in mind, too, that the HHS announcement was only for mRNA vaccines, not the single-dose Johnson & Johnson (Janssen) vaccine.

"Booster shots for all Americans is not a forgone conclusion," infectious disease expert Amesh A. Adalja, MD, a senior scholar at the Johns Hopkins Center for Health Security, tells Health. "This was announced by the White House—it's usually the CDC and FDA first—and it has understandably caused a lot of pushback from the scientific community."

There is still a group of people who are currently able to get an additional dose of an mRNA COVID-19 vaccine right now: individuals who are moderately to severely immunocompromised. These additional doses are different from booster doses, per the CDC: "Sometimes people who are moderately to severely immunocompromised do not build enough (or any) protection when they first get a vaccination," the agency says. That additional dose can then help immunocompromised people have a better chance of mounting some sort of immune response to the virus. A "booster dose," on the other hand, is a dose given to someone who was able to build protection after their initial vaccination, but then that protection decreased over time (aka, "waning immunity"), the CDC says.

As of right now, those moderately to severely immunocompromised people are the only ones able to get an additional dose of an mRNA COVID-19 vaccine after their first two doses. The CDC says this additional dose should happen at least 28 days after the person's second dose of the vaccine.

What side effects can you expect after an additional or booster dose of the COVID-19 vaccine?

Whether you're getting an additional dose of an mRNA COVID vaccine due to your immunocompromised status, or if you're receiving an additional dose as a booster shot, you can probably expect to have a similar (or possibly better) response to how you reacted after having the two-dose series of the vaccine, William Schaffner, MD, an infectious disease specialist and professor at the Vanderbilt University School of Medicine, tells Health.

"There is no tendency to have more side effects with a third dose than either the first or the second," he says. "What's clear is that it should not be worse."

Dr. Adalja agrees: "The side effects are probably going to be very similar to the second dose for most people," he says. And, if you didn't feel so hot after your second dose, keep this in mind: Your third dose likely won't be as bad. "You may have less of a reaction because there is a lot of space between the second and third doses [versus] first and second doses," Dr. Adalja says. "Your immune system may be dampened somewhat."

The CDC says online that reactions reported after a third dose of the mRNA vaccine were "similar" to those of the two-dose series. The most common side effects, the CDC says, have been fatigue and pain at the injection site.

Pfizer has offered up a little more detail in its application for an emergency use authorization (EUA) for a third dose. That application says that side effects after the third dose were similar to those after a second dose of the vaccine, including:

  • Redness and swelling around the injection site
  • Tiredness
  • Headache
  • Muscle and joint pain
  • Chills

Pfizer also said in a press release about its EUA application for a booster dose that side effects were "similar to or better than" people had after their second dose of the vaccine.

How can you plan for your additional or booster dose?

There's been a lot of expert advice about planning to take it easy after your second dose of the COVID-19 vaccine, and doctors say it's probably not a terrible idea for your third dose, just to be safe.

"It's sensible to take it easy afterward," Dr. Schaffner says. "Don't plan to do anything strenuous the day after your shot."

Dr. Adalja says you can expect to have a similar reaction to your second dose of the mRNA vaccine. "However rough it was, use it as your baseline to see if you need to take any special precautions," he says.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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