Some People Can Now Get a 4th Dose of Their COVID Vaccine—Here’s Who the CDC Says Is Eligible

The CDC updated its booster guidance, and the new information affects adults who are moderately or severely immunocompromised.

The Centers for Disease Control and Prevention (CDC) has updated its guidance on who can get a COVID-19 booster shot. Now, there are some people in the US who are eligible to receive four doses of the COVID vaccine.

On October 25, the CDC added to its COVID vaccination guidance to say that adults who are moderately or severely immunocompromised who’ve received two doses of an mRNA vaccine (either Moderna or Pfizer-BioNTech) and a third additional shot can also receive a booster shot. “In such situations, people who are moderately and severely immunocompromised may receive a total of four COVID-19 vaccine doses,” the CDC says.

That booster shot should come at least six months after the third or additional dose of the mRNA vaccine, according to the CDC. And it can be Pfizer-BioNTech, Moderna, or Janssen—the booster is not limited to the vaccine brand that your first three doses were or an mRNA vaccine.

Here’s what you need to know about the latest guidance on a fourth dose, including what makes the booster shot different from an additional dose and who is actually eligible for it.


What’s the difference between an additional dose and a booster shot?

Back in August, the CDC cleared the way for adults with moderately to severely compromised immune systems who had completed the full two-dose series of an mRNA vaccine to receive an additional shot. The guidance for that third or additional dose was that it should be the same type of vaccine as the first two doses and given at least 28 days after the second dose.

The CDC made the recommendation for the additional dose “to improve immunocompromised people’s response to their initial vaccine series.” As the CDC reports, these people “are especially vulnerable to COVID-19, and may not build the same level of immunity to [a two-dose] vaccine series compared to people who are not immunocompromised.”

Now, the CDC is recommending that this population also receive a booster shot at least six months after their additional dose.

What makes the booster shot different from the additional dose is that an additional dose has to be the same mRNA vaccine as the first two doses and aims to improve immunocompromised people’s response to their initial vaccine series, whereas a booster shot can be a different COVID vaccine and is administered when a person has completed their vaccine series and protection against the virus has decreased over time.

There might be a difference in the actual dosage too, per the CDC. An additional dose of Moderna, for example, is the same dosage as the first two doses. The booster, however, is a half dose. Pfizer’s primary, additional, and booster doses all have the same dosage, according to the CDC.

Who can get this fourth dose?

The CDC’s latest guidance on a booster shot after a two-dose vaccination series and an additional dose is only for people aged 18 or older who are moderately or severely immunocompromised. Per the CDC, you might be considered moderately or severely immunocompromised if you:

  • Are receiving active cancer treatment for tumors or cancers of the blood
  • Have received an organ transplant and are taking medicine to suppress the immune system
  • Have received a stem cell transplant within the last two years or are taking medicine to suppress the immune system
  • Have moderate or severe primary immunodeficiency (such as DiGeorge syndrome or Wiskott-Aldrich syndrome)
  • Have advanced or untreated HIV infection
  • Are taking high-dose corticosteroids or other drugs that may suppress your immune response

The CDC reports that about 3% of the entire US adult population is considered moderately or severely immunocompromised. The fact that this group has less of a reaction to the COVID vaccine, prompting the need for four doses, is not entirely unexpected, as Health previously reported. “Historical data with other vaccines identifies many people who are immunocompromised who do not respond well to vaccines,” Stanley Weiss, MD, professor of medicine at the Rutgers New Jersey Medical School and professor of biostatistics and epidemiology at the Rutgers School of Public Health told Health. “The expectation was that they might not respond well to the COVID vaccine, and the data that is evolving demonstrates that concern is correct.”

If you believe you fall into either of these immunocompromised categories, the CDC recommends checking in with your doctor before getting an additional dose or booster shot.

What about if you’re moderately or severely immunocompromised and received a Johnson & Johnson shot?

If you’re a moderately or severely immunocompromised adult and your original vaccination was the one-dose Janssen, you’re not affected by this latest guidance on a fourth dose. However, the CDC does offer guidance for your booster shots. According to the latest guidance, you should receive a single COVID-19 booster vaccine—of any of the three currently available vaccines—at least two months after receiving your initial Janssen dose. In total, you should receive no more than two doses.

Regardless of which vaccine you received and how many doses you’ve gotten, the CDC points out that “people who have a condition or are taking medications that weaken their immune system may not be fully protected even if they are fully vaccinated. They should continue to take all precautions recommended for unvaccinated people, including wearing a well-fitted mask, until advised otherwise by their healthcare provider.”


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