Should you get the new COVID-19 booster shot? Here's everything you need to know.

Two newly updated COVID-19 booster shots are now available in the U.S.: one made by Moderna and one made by Pfizer and BioNTech. The Moderna booster is available for people ages 18 and up, and the Pfizer-BioNTech booster can be given to those ages 12 and up. Individuals qualify for a booster if they meet these age requirements and it’s been at least two months since they completed their primary vaccine series or received their last booster shot.

Here's everything you need to know about the updated boosters, including what variants they guard against, who's been recommended to get the shots, and when.

Related: Quick guide: Most widely used COVID-19 vaccines and how they work 

What's different about the new boosters?

The new boosters guard against both the original SARS-CoV-2 strain targeted by the old vaccines and two "flavors" of the omicron variant, known as BA.4 and BA.5, Live Science previously reported. When the new boosters were authorized at the beginning of September 2022, these two omicron subvariants were responsible for the majority of new COVID-19 cases in the U.S.: BA.5 accounted for about 90% of all cases, and BA.4 made up most of the rest, STAT reported. Health officials expect both subvariants to continue circulating during the fall and winter, according to the U.S. Food and Drug Administration (FDA).

Specifically, the boosters contain genetic molecules called mRNA, which contain instructions to build spike proteins, the pointed structures that the coronavirus uses to infiltrate cells. Once inside the body, the boosters instruct cells to build the spike protein of the original SARS-CoV-2 variant and the spike proteins of BA.4 and BA.5, which are identical to one another. 

Because they contain instructions for two spike proteins, the updated boosters are considered "bivalent," while the previous vaccines are "monovalent." With the authorization of the bivalent boosters, the monovalent vaccines are no longer authorized as boosters for people ages 12 and older, per the FDA. The updated boosters are already being distributed nationwide, although for now, the availability may vary between vaccination sites; this availability is expected to ramp up in the coming days and weeks, CNBC reported.

(Children ages 5 to 11 can still get a monovalent booster, according to the Disease Control and Prevention's (CDC) Advisory Committee on Immunization Practices (ACIP).)  

How protective are the updated boosters?

Scientists don't know how much protection the updated boosters offer, as the shots have not been formally tested in people. This is similar to the annual flu shot, whose level of effectiveness only becomes clear as flu season progresses. 

But based on the animal studies and recent clinical trials of similar boosters described in the final section of this article, the updated boosters are expected to grant better protection against circulating omicron subvariants than the original COVID-19 vaccines. The boosters will likely be most protective against severe infections that could lead to hospital admission and death.

"They can help restore protection that has waned since previous vaccination and were designed to provide broader protection against newer variants," CDC director Dr. Rochelle Walensky said in a CDC statement. 

Who should get a booster?

The CDC recommends that all individuals ages 12 and older receive an updated booster shot, provided that it's been at least two months since they completed their primary vaccination series or received their most recent booster dose with a monovalent vaccine. The updated Moderna booster can be given to people ages 18 and older, and the updated Pfizer-BioNTech booster can be given to people ages 12 and older.

"In the coming weeks, CDC also expects to recommend updated COVID-19 boosters for other pediatric groups, per the discussion and evaluation of the data by ACIP on Sept. 1, 2022," the CDC statement notes; ACIP is the CDC’s Advisory Committee on Immunization Practices. "When data are available and FDA authorizes these other types of COVID-19 boosters, CDC will quickly move to help make them available in the United States."

Boosters will likely offer the most benefit to people ages 60 and older, people with weakened immune systems and those with chronic conditions that raise their risk of contracting a severe COVID-19 infection, experts told The New York Times. However, even in younger people and those without chronic conditions, boosters should lower the risk of COVID-19 infection and of developing long COVID — the prolonged, debilitating symptoms that can arise after a COVID-19 infection, Shane Crotty, a virologist at the La Jolla Institute for Immunology, told the Times. 

And at a community level, boosters could reduce transmission in the coming months and help to blunt potential surges in the fall and winter, experts told NPR. 

When is the best time to get a booster?

The best time to get a booster will vary from person to person.

In general, the FDA allows people to get a booster as long as it's been two months since their last shot. However, immunologists generally recommend that people wait roughly four to six months after their last shot or their most recent COVID-19 infection, in order to maximize their protective immune response, The New York Times reported. 

Similarly, the CDC's ACIP suggests that those who recently caught COVID-19 should consider waiting three months before getting an updated booster; the three-month countdown starts from the point of symptom onset or a person's first positive test, if their infection was asymptomatic. People who currently have COVID-19 should at least wait until their symptoms resolve and they meet the criteria to exit isolation before getting any COVID-19 vaccine, updated booster or otherwise, the committee adds.

Some people may consider timing their booster dose so they reach maximum protection during the holiday season. This could be somewhat risky, because it's difficult to predict if and when a surge might occur, Dr. Robert Wachter, a professor and chair of the department of medicine at the University of California, San Francisco, told NPR.

"You are basically accepting a period of vulnerability that you don't need to have," Wachter said. "And as I weigh all that, my thinking is I'd rather not do that." It's also key to note that, as with most vaccines, the immune system will take a few weeks to ramp up its defenses after a boost. 

Considering that a potential winter surge would coincide with flu season, the CDC advisory committee has also urged healthcare providers to offer people both flu and COVID-19 vaccines at the same visit. "With both influenza and SARS-CoV-2 circulating, getting both vaccines is important for prevention of severe disease, hospitalization, and death," the committee stated.

(The CDC now offers an online tool to help determine which COVID-19 vaccine you should get and when; to use the tool, click the button that reads "Find Out When to Get a Booster.")

When and how were the boosters authorized?

The FDA issued an "emergency use authorization" for the updated boosters on Aug. 31, 2022. On Sept. 1, the CDC’s advisory committee recommended use of the shots and Dr. Walensky promptly endorsed that recommendation, thus clearing the boosters for widespread use.

Similar to the annual influenza vaccine, the updated boosters were authorized for use without first being tested in formal human trials, Live Science previously reported. The FDA's and CDC's decisions were instead based on the extensive safety and efficacy data gathered on the original Moderna and Pfizer-BioNTech COVID-19 vaccines, first rolled out in late 2020. 

In addition, the agencies evaluated data from two recent clinical trials of similar boosters designed to target BA.1, an omicron subvariant that's no longer circulating. And finally, the newly authorized boosters were tested in mice to ensure that the shots triggered an immune response in the animals. In Moderna's mouse studies, scientists also infected the rodents with BA.5 and found that the new booster more effectively guarded against infection in the lungs than the original Moderna shots, The New York Times reported.      

"The FDA has extensive experience with strain changes for annual influenza vaccines. We are confident in the evidence supporting these authorizations," Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said in the FDA statement. "The public can be assured that a great deal of care has been taken by the FDA to ensure that these bivalent COVID-19 vaccines meet our rigorous safety, effectiveness and manufacturing quality standards for emergency use authorization."

Originally published on Live Science.