Answers to Questions About the Latest Booster Recommendations

Kathleen Doheny

October 27, 2021

Editor's note: Find the latest COVID-19 news and guidance in Medscape's Coronavirus Resource Center.

Now that federal regulators have issued their edicts on booster doses for the COVID-19 vaccine, many people have many questions. Here are some answers.

Who Is Eligible for a Booster Dose Now?

As of October 20, the US Food and Drug Administration (FDA) has granted emergency use authorization (EUA) for Moderna, Pfizer-BioNTech, and Johnson & Johnson vaccine booster doses for certain members of the population.

A single booster dose of Moderna or the Pfizer-BioNTech vaccine may be given at least 6 months after completion of the two-shot primary series to:

  • Adults 65 years and older

  • Adults 18 to 64 with a high risk of getting severe COVID-19

  • Adults 18 to 64 with frequent institutional or occupational exposure to SARS-CoV-2, the virus that causes COVID.

A single booster of J&J's vaccine may be given at least 2 months after completion of the primary vaccine, which requires just one shot, to those 18 years and older.

The Centers for Disease Control and Prevention (CDC) also recommends an additional dose with an mRNA vaccine (Moderna or Pfizer) for the 3% of the US population who are moderately to severely immunocompromised, who were found not to have as robust an immune response to the vaccines.

Furthermore, in updated guidance, the CDC says moderately and severely immunocompromised adults 18 years and older who completed the primary vaccine series and got an additional mRNA vaccine may now receive a fourth shot (Moderna, Pfizer, or J&J) at least 6 months after their third dose.

This group includes organ transplant patients, stem cell transplant patients within the past 2 years, those with untreated or advanced HIV infection, those on corticosteroids or other immune-suppression drugs, and those with other conditions that make their immune system deficient.

Who Authorized the Booster Doses, What Drove the Decision?

As time passed after the original injection or injections of the vaccines, reports of reduced immunity appeared. The FDA and CDC officials looked at data submitted from the manufacturers that showed clinical trial participants who got booster doses showed a favorable response, and that no new safety concerns had shown up.

After the FDA granted an EUA to the booster doses, the CDC also reviewed the data and then also recommended the boosters.

In a news release explaining the booster approvals, acting FDA Commissioner Janet Woodcock, MD, said: "The available data suggest waning immunity in some populations who are fully vaccinated."

Why Is the Moderna Booster Half the Original Dose?

Data submitted to the FDA seeking an EUA supported a 0.25 mL dose (half of the original 0.5 mL) as adequate to boost immunity.

Do All Boosters Work Against the More Contagious Delta Variant?

"Yes, all the boosters improve protection against the Delta variant," William Schaffner, MD, professor of medicine and preventive medicine and infectious disease specialist at Vanderbilt University in Nashville, Tennessee, told Medscape Medical News.

When Will Other Age Groups Be Eligible?

The younger groups who got the mRNA vaccines (Pfizer, Moderna) continue to have evidence of strong protection against severe disease that requires hospitalization, Schaffner said.

However, he added that ''this is being watched closely and if waning protection begins to show up, boosters for young adults and adolescents will be recommended at that time."

Can Groups Not Yet Eligible Get It Anyway Now? Should They?

Some people not in the ''recommended" booster groups seeking additional protection may decide to try and get one regardless of the recommendations.

But Schaffner said that ''there is no need for young persons with normal immune systems who received mRNA vaccines (Pfizer, Moderna) to get a booster now unless they are frontline workers (healthcare, teachers, grocery store, etc.) where they have extensive contact with many people. They can discuss that with their doctors."

And, of note, younger people with an illness that compromises their immune system or those on treatment that is immune-suppressive are recommended to get a third dose, he said, and for it to be one of the mRNA vaccines, not the J&J vaccine.

Is it Really OK to Mix and Match Vaccines?

On October 20, the FDA said that getting a booster different than the original vaccine shots was acceptable in eligible individuals. The CDC agreed a day later.

It is a decision supported by the Infectious Diseases Society of America (IDSA). In an October 26 statement, IDSA President Daniel P. McQuillen, MD, noted a lack of data on the best combination of vaccines for long-term protection, but that ''current evidence supports the need for a booster in vulnerable populations, no matter which vaccine an individually initially received."

And for those who got the J&J vaccine, he added, ''data show that they will have a stronger immune response with an mRNA booster."

Will People Be Able to Pick the Booster They Prefer?

"I think the answer is going to be yes," David Hirschwerk, MD, an infectious diseases specialist at Northwell Health in Manhasset, New York, told Medscape Medical News. "In our own health system, we are allowing for that, acknowledging the CDC recommendation [that mixing and matching is appropriate]. I think most systems will work to adopt a system like ours."

What About Getting "Double Boosters"?

Some people say they have gotten two different boosters, spaced apart. Is this wise?

While ''more is better" is a common thought, it is a misconception, Schaffner said.

"More is not better — you want the doses as recommended. If more were better, that is what the official recommendation would be," he said.

Hirschwerk, who's also an associate professor of medicine at the Hofstra/Northwell Zucker School of Medicine, agreed that double boosters are not a good idea: "This is certainly not a CDC recommendation."

Can You Get Your Booster and Flu Shot the Same Day?

Yes, according to the CDC. If possible, the shots should be given in different arms, especially if the other vaccine is likely to cause a local reaction such as soreness at the injection site.

Where Can I Get a Booster Dose? Is It Free?

To find a COVID-19 vaccine, search vaccines at, text your ZIP code to 438829, or call 800-232-0233 for information on locations nearby.

You can also check the website of your local pharmacy for information on appointments or walk-ins. Your state or local health department will also have information on where to get booster doses, which are free.

What if You Unknowingly Have COVID, With No Symptoms, and Get Your Booster?

No problem, Schaffner said. "Vaccines provide higher antibody levels than does the natural infection, so you will benefit. Getting vaccinated during an asymptomatic or mild COVID infection is not harmful."

With the vast numbers of vaccines that have been given to date, ''that scenario has probably happened many times over, and we are not aware of any adverse reactions," Hirschwerk said.

Are Booster Side Effects Typically Worse Than After the Original?

Reactions vary among individuals, and there is no long-term data on booster side effects.

However, a survey conducted in Israel, where people over age 60 years began getting booster shots in late July, found those who got a third dose of the Pfizer vaccine said they had similar or fewer side effects than they did after the second shot. About 4500 people who got the booster from July 30 to August 1 answered the questions, according to Reuters.

That data matches what patients tell him, Hirschwerk said, and also matches his own personal experience after he got his booster.

In the Israeli survey, 88% said they felt similar or better to how they felt after the second dose; 31% said they had a side effect, most often soreness at the injection site. Just 1% said they needed medical treatment due to one or more side effect.

When the FDA reviewed data on the Moderna and Pfizer booster vaccines, it found that swollen lymph nodes in the underarm were reported more often after the booster than the first two injections.

What Do I Take to the Vaccine Booster Appointment or Walk-in?

Your vaccination card with a record of your initial vaccine injection or injections.

Will Kids Need a Booster?

That is yet to be determined, experts said.

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