Here's why doctors think some people may benefit from an additional dose.

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The US Centers for Disease Control and Prevention (CDC) on Friday endorsed a recommendation from its vaccine safety panel that moderately to severely immunocompromised people get an extra dose of the Pfizer-BioNTech or Moderna COVID-19 vaccines.

CDC Director Rochelle Walensky, MD, announced the agency's decision on Friday, just hours after the Advisory Committee on Immunization Practices (ACIP) voted unanimously to recommend the additional doses.

ACIP reviewed data on the safety and benefit versus risk of extra doses after the US Food and Drug Administration (FDA) late Thursday authorized use of a third shot of the COVID-19 mRNA vaccine for people with severely weakened immune systems. The FDA specified that the extra dose is indicated for organ transplant patients and individuals with equally immune-compromising conditions.

The CDC is updating its guidance for the general public and health care providers on how those additional doses would be implemented in practice. For example, how would you even go about getting an additional dose? Would you need a doctor to sign off?

The intent is to "allow for some flexibility for providers to assess their patients' immunosuppression," Amanda Cohn, MD, of the CDC's National Center for Immunization and Respiratory Diseases, said during the ACIP meeting. However, there won't be any requirement that people get a prescription or recommendation from a health care provider. It's likely that people seeking an additional dose at the pharmacy would be asked to attest that they are moderately or severely immune compromised, Dr. Cohn said.

CDC's recommendation includes organ and stem cell transplant recipients, people with advanced or untreated HIV, people being treated for cancer, and individuals taking medicines (such as high-dose corticosteroids) that weaken their immune systems. CDC's website provides additional details on who needs an additional COVID-19 vaccine.

Not included are people with chronic medical conditions, such as diabetes or heart disease.

In line with the FDA action, an extra Pfizer dose may be given to moderately or severely immunocompromised people 12 and older or Moderna dose for adults 18 and older following the initial two-dose series of those shots.

To be clear, the new guidance does not apply to people who initially received the Johnson & Johnson/Janssen vaccine. FDA's expanded emergency use authorizations did not include an additional dose for J&J recipients due to a lack of data, panelists explained.

"However, we absolutely understand that this is an important issue that needs to be addressed; we're working to address that population," said Dr. Cohn.

One other point: This is an "additional dose," in CDC parlance, not a "booster." So what's the difference? ACIP says an additional dose is one given following a vaccine series when a person's initial immune response is likely to be insufficient. It defines a booster shot as a dose given after a person's immune response has likely waned over time.

The news comes as the very contagious Delta variant rips across the nation, straining hospitals in hot spots like Florida, Texas, Arkansas, and Louisiana.

CDC Booster Vaccine , From above set of sterile syringes and bottles of medications arranged on blue background
Credit: Getty Images

About 3% of Americans have weakened immune systems for a variety of reasons, from a history of cancer to use of medications like steroids, according to the New York Times.

Use of a third COVID-19 shot for people who are severely immunocompromised has already happened in other countries, including Israel, which has offered an additional shot of the Pfizer-BioNTech vaccine to people over the age of 60.

Nearly 59% of Americans over the age of 12 are vaccinated against COVID-19, according to data from the CDC. A little more than 69% of this population has received just one dose of the vaccine.

But why do immunocompromised people need a third shot, and what does it mean for everyone else? Infectious disease experts break it down.

Why is a third shot of the COVID-19 vaccine needed for people who are severely immunocompromised?

Research has suggested that people who are severely immunocompromised, like those who have had organ transplants, don't have a strong immune response to the standard recommended two doses of the mRNA COVID-19 vaccines.

"When someone is immunocompromised, it refers to the fact that they do not have the same immune response as somebody with an ordinary immune system," infectious disease expert Amesh A. Adalja, MD, a senior scholar at the Johns Hopkins Center for Health Security, tells Health. "That suppression can also suppress the immune response to a vaccine."

It's not shocking that this would happen with COVID-19 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, tells Health. "Historical data with other vaccines identifies many people who are immunocompromised who do not respond well to vaccines," he says. "The expectation was that they might not respond well to the COVID vaccine, and the data that is evolving demonstrates that concern is correct."

A research letter published in JAMA in May noted that just 17% of solid organ transplant recipients had a good antibody response after receiving one dose of an mRNA COVID-19 vaccine (i.e. a Moderna or Pfizer-BioNTech vaccine). The researchers analyzed the response of 658 transplant recipients and found that 15% had "measurable" antibody responses after doses one and two. The researchers also found that 301 (or 46%) had no antibody response after dose one or two, and 259 had no antibody response after dose one, but had a response after dose two.

"These data suggest that a substantial proportion of transplant recipients likely remain at risk for COVID-19 after two doses of mRNA vaccine," the researchers concluded.

How much can a third shot help?

It's not entirely clear, but it's likely to do something, William Schaffner, MD, an infectious disease specialist and professor at the Vanderbilt University School of Medicine, tells Health. "There are many different ways someone can be immunocompromised and you cannot predict with confidence that a third dose is indeed going to help," he says. "But it looks to be pretty benign on the safety side, and small studies we have do indicate that some people who are immunocompromised do get an improved response with that third dose."

One study published in the New England Journal of Medicine earlier this week, for example, found that a third dose of the Moderna COVID-19 vaccine created a better immune response in organ transplant recipients than those who didn't get a third shot.

But ultimately, Dr. Schaffner says, "everyone has an individual immune response to the vaccine."

Will this eventually expand to other people?

The FDA's action does not apply to people who are not immune compromised, and experts don't expect recommendations that everyone get a third dose of the COVID-19 vaccine anytime soon. "In spite of these company presidents going in the media saying we need third doses by December, none of us have heard of anything yet that indicates we're getting degradation of protection," John Sellick, DO, an infectious disease expert and professor of medicine at the University at Buffalo/SUNY, tells Health.

But, Dr. Sellick says, it may eventually get to a point where federal health officials add so many people to the "immunocompromised" group that things will change. "Once you start adding too many groups on, it eventually gets to the point where it's just easier to say, 'everyone get a third dose,'" Sellick 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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