The Centers for Disease Control and Prevention (CDC) has authorized booster shots for all Americans ages 12 and up.

Children 12 and older are eligible for the Pfizer-BioNTech booster, while adults can also choose the Moderna or Johnson & Johnson booster.

Public health officials are recommending that fully immunized people get boosted both to bolster vaccine effectiveness, which wanes over time, and to combat the new and extremely infectious omicron variant.

In December 2021, the South African insurance company Discovery Health released data showing that people who were fully immunized with the Pfizer vaccine but not yet boosted had just 33 percent protection against COVID-19 infection during the omicron wave, with 70 percent protection against complications severe enough to require hospitalization.

Public health officials in the United States have been urging Americans to get a booster shot — typically, a third dose for people who received the two-shot Pfizer or Moderna mRNA vaccine, or a second dose for those immunized with the one-shot Johnson & Johnson vaccine.

“Although we don’t have all of the answers on the omicron variant, initial data suggests that COVID-19 boosters help broaden and strengthen the protection against omicron and other variants,” said CDC director Rochelle P. Walensky, MD, MPH, in a statement.

Why Exactly Do We Need COVID-19 Boosters?

Studies published in mid-January 2022 show that booster shots do an excellent job of preventing severe illness, hospitalization, and death due to COVID-19, even with omicron in the mix.

One CDC investigation analyzed data from 220,000 COVID-19-related emergency room and urgent care visits between August 2021 and January 2022. During the period when the delta variant was predominant (August through mid-December), a booster dose of the Pfizer or Moderna vaccine was 94 percent effective at keeping people out of emergency rooms and urgent-care clinics. From mid-December through January, when omicron became the predominant U.S. variant, booster effectiveness dropped but was still high: 82 percent.

The investigators also analyzed almost 88,000 COVID-19-related hospitalizations during that time and found that during the delta months, a Pfizer or Moderna booster was 94 percent effective at preventing hospitalization; during the omicron period, that number was 90 percent.

A second CDC report based on data from 25 U.S. jurisdictions between April 4 and December 25 (when the delta strain was predominant, and then later as omicron was gaining ground) indicated that protection against infection and death was highest among people who were vaccinated and boosted, compared with those who were only vaccinated or were unvaccinated. This was particularly true for individuals ages 50 and older. The study indicated that adults who were unvaccinated had a five times higher risk of infection than adults who were fully vaccinated and boosted.

A third paper, published in JAMA, looked at more than 13,000 omicron and 10,000 delta infections. The study suggested that people who had received three doses of the Pfizer or Moderna vaccine were more protected against omicron than those who had received just two doses or were unvaccinated. The study authors warned, however, that vaccines overall offered less defense against omicron.

When Can You Get a Booster Shot?

For people who received the Pfizer or Moderna vaccine, the CDC now recommends a booster as soon as five months after getting the second shot.

But the J&J crowd has to wait just two months after getting their initial jab. The one-dose J&J vaccine has been shown to be less protective than the two-dose Pfizer and Moderna vaccines, so health authorities want this group to get boosted as soon as possible.

What Is in the Booster Shots?

A booster is identical to the original vaccine, although in the case of Moderna the dosage is half that of the original two shots.

Like the original vaccines, the booster stimulates the immune system to produce antibodies that kill the SARS-CoV-2 virus that causes COVID-19.

“The booster is the same [basic formulation] as what you got the first time,” says Aarti Raja, PhD, an associate professor of virology, immunology, and microbiology at Nova Southeastern University in Fort Lauderdale, Florida.

Do You Get More Protection if You ‘Mix’ Vaccines?

Federal health authorities say that vaccine recipients can “mix and match” when it comes to boosters.

Some experts have suggested that J&J recipients might benefit from getting either a Pfizer or Moderna booster in order to maximize protection.

Plus, the CDC now recommends the Pfizer and Moderna boosters over the J&J shots in most situations because of a link between the J&J shot and very rare incidences of blood clots.

Researchers have been exploring the effectiveness of mixing vaccines. A study led by researchers at Oxford University in England, published in The Lancet in January 2022, found that following up first doses of the Oxford-AstraZeneca or Pfizer-BioNTech vaccines with second doses of the Moderna or Novavax jabs generated robust immune responses against COVID-19.

The U.S. National Institutes of Health is currently conducting a trial in which adult volunteers who have been fully vaccinated against COVID-19 are receiving booster doses of different COVID-19 vaccines to determine their effectiveness and safety.

Preliminary data (that has not yet been peer-reviewed) published online on MedRxiv in October indicated that antibody levels shot up 76-fold within 15 days in individuals who followed a Johnson & Johnson inoculation with a Moderna booster; a Pfizer booster kicked up antibody levels in this group 35 times. For the J&J recipients who stuck with a J&J second jab, antibodies only climbed fourfold.

What Underlying Medical Conditions Add to the Urgency for a Booster?

Per the CDC, certain underlying medical conditions raise the risk that a person who develops COVID-19 will experience serious complications, making the need for a booster especially vital.

These underlying conditions include:

Cancer
Chronic kidney disease
Chronic liver disease
Chronic lung diseases, including asthma (moderate to severe), COPD (chronic obstructive pulmonary disease), and cystic fibrosis
Dementia or other neurological conditions
Diabetes (type 1 or type 2)
Down syndrome
Heart conditions
HIV infection
Immunocompromised state, including a weakened immune system caused by prolonged use of corticosteroids or other immune-weakening medicines (Note: Some individuals with autoimmune diseases such as rheumatoid arthritis who take medications that suppress their immune systems may qualify.)
Mental health conditions, including depression and schizophrenia
Overweight and obesity
Pregnancy
Sickle cell disease or thalassemia
Smoking, current or former
Solid organ or blood-stem-cell transplant
Stroke or cerebrovascular disease, which affects blood flow to the brain
Substance use disorders
Tuberculosis

What About People Who Are Moderately to Severely Immunocompromised?

People with a moderately to highly compromised immune system — for instance, because they are receiving certain types of cancer treatment, are on immunosuppressant medications following a solid organ transplant, or have advanced or untreated HIV — fall into a separate booster category, says the CDC.

Patients who received either a Pfizer or Moderna vaccine should talk to their doctor about getting a third vaccine dose as soon as 28 days after completing their initial vaccine series.

“These are people who had a less than optimal response to the initial vaccines, and they’re saying that a third dose in that population is required in order to give them the protection that most of the rest of the population got from those initial two shots,” says Albert Rizzo, MD, the chief medical officer for the American Lung Association.

These individuals should get a fourth shot five months or so later, says the CDC.

Are Boosters Safe?

Evidence so far suggests that booster shots should be safe, just as the first shots have posed very few serious health issues.

“There should be no question whether a body can handle a booster,” says Dr. Raja. “We’ve had boosters for plenty of vaccines out there, including chickenpox, tetanus, diphtheria, mumps, measles, and rubella. So, boosters shouldn’t harm anyone.”

How Long Will Booster Protection Last? Will We Need to Get Booster Shots Again?

U.S. health authorities say that they can’t answer this question yet because COVID-19 and the vaccines are still too new. Still, other countries are moving ahead with administering second boosters. Israel started giving out additional doses of mRNA vaccines in mid-January to healthcare workers and everyone 60 and older. On January 12, Denmark began offering a second booster to the highly vulnerable.

Given the unknowns about omicron, vaccine makers are already in the process of producing boosters specifically formulated to protect against this new variant. Pfizer CEO Albert Bourla said in January that an omicron vaccine will be ready in March, and the company is already manufacturing doses.

“It could be that the coronavirus is similar to the flu, where each year there may be mutations that occur that require people to be vaccinated to get protection,” says Dr. Rizzo. “But it also may turn out that as we get these boosters, we may start to see that the virus becomes less lethal and less likely to mutate. And therefore it may ultimately become very much a background and not something that requires regular intervention with a vaccine.”

How and Where Do You Get a Booster?

President Biden has stressed that getting a booster is free and easy. “Just show your vaccination card and you’ll get a booster,” he said in remarks from the White House. “No other ID. No insurance. No state residency requirement.”

Raja adds that people will not be required to return to the location where they received their original shots.

The federal government website Vaccines.gov allows you to enter your zip code and find a nearby location that administers all three types of vaccines and boosters.