The FDA just authorized booster shots of Moderna's and Johnson & Johnson's COVID-19 vaccines and is letting users mix and match shots

• The FDA authorized booster shots of the Moderna and J&J vaccines on Wednesday.
• The agency also authorized people to get a different vaccine brand as a booster shot.

The Food and Drug Administration authorized on Wednesday booster shots for Moderna's and Johnson & Johnson's COVID-19 vaccines.

The FDA said that it had authorized booster doses of the Moderna COVID-19 vaccine for patients aged 65 and older as well as younger people at high risk of severe disease.

For the J&J vaccine, the FDA authorized a booster dose for anyone who has recieved the vaccine.

A third shot of Moderna for at-risk adults

The FDA's decision comes after an advisory panel unanimously voted to recommend the Moderna booster shot on October 14, saying that data shows that the booster shot leads to a strong immune response and similar side effects as the first two doses.

"We remain committed to staying ahead of the virus and following the evolving epidemiology of SARS-CoV-2," Moderna CEO Stéphane Bancel said in a statement after the committee's vote. "We look forward to making our booster available to people in the U.S. to help protect themselves against this ongoing public health emergency."

The booster shot will be half the strength of the first two Moderna vaccines, and should be given at least six months after the second dose of the vaccine.

Moderna is still studying the third booster dose, and the data that it has presented to the FDA is limited.

The data includes a small sample of 149 volunteers who got the booster shot in a phase 2 trial and had their antibody levels measured and compared to antibody levels after the second dose. The research showed that the antibody levels from the booster shot reached the same level as after the second dose, but not all of the patients saw a 4-fold increase in antibodies, one of the FDA's criteria.

"The data itself is not strong," said FDA committee member Dr. Patrick Moore on October 14. Moore, a microbiology and molecular genetics professor at the University of Pittsburgh said he voted to recommend the vaccine anyway based on a "gut feeling."

The booster shots appear to be just as safe as the initial two doses of the vaccine, with most people experiencing mild to moderate side effects. This is true even though the booster shot dose is half the strength of the original two doses. Both of the original doses were 100 micrograms, while the third shot is 50 micrograms.

The most common booster shot side effects include pain at the injection site, fatigue, headache, and muscle pain.

A second J&J shot for everyone

The FDA also authorized a second dose of the Johnson & Johnson booster shot for everyone who has gotten that vaccine.

Unlike the mRNA-based shots, which are given as two doses, the J&J vaccine was initially authorized as a single-dose shot. Also unlike the mRNA shots, the FDA recommended that all 15 million people in the US who got a J&J shot be eligible for a booster shot, regardless of age or risk of severe disease.

People can get a booster shot as soon as two months after their first J&J vaccine dose.

J&J submitted data to the FDA from a study that included just 17 volunteers who got a second dose after six months. While that data appeared to meet the FDA's goals for what a booster should do, later on the FDA's review stated the results are limited by the small sample size and were also impacted by a subpar lab test J&J used.

J&J also has data from a much larger trial of 16,000 volunteers who got a booster shot after only two months. While this data hasn't been independently verified by FDA scientists, it does show more evidence of the benefit of a booster shot.

"We now have generated evidence that a booster shot further increases protection against COVID-19 and is expected to extend the duration of protection significantly," Dr. Paul Stoffels, J&J's chief scientific officer, said in a September news release.

FDA also endorses mix & match for booster shots

The FDA also authorized a "mix-and-match" approach for Americans seeking booster shots, allowing Americans to receive a different brand of COVID-19 vaccine as a booster than the one they initially received. These vaccines are also called "heterologous" booster shots.

Recent results from a new study found that an mRNA booster shot after the J&J shot might lead to a better immune response compared to a second J&J booster.

This "mix-and-match" trial enrolled 458 people who had been vaccinated with either Pfizer's, Moderna's, or J&J's vaccine, then offered a boost of one of those three brands. J&J recipients saw a significantly larger jump in virus-fighting antibody levels with a Moderna or Pfizer booster compared to a second J&J shot.

"What the study shows is that regardless of what an individual received originally, getting boosted with one of the three vaccines that we evaluated, the one from Moderna, the one from Janssen, the one from Pfizer, led to good antibody responses in each of the groups," lead study author Dr. Robert Atmar from Baylor College of Medicine told Insider at the time.