The US launches a one-of-a-kind trial to tell us whether mixing vaccine booster shots is a good idea

• The US is testing whether mix-and-match booster vaccines for COVID-19 work and are safe.
• An NIAID-backed trial will test giving a dose of Moderna's COVID-19 shot to fully vaccinated people.
• The people may have previously received vaccines from Moderna, Pfizer, or Johnson & Johnson.

US scientists could soon work out whether it's a good idea to mix vaccines to boost the body's immune response to the coronavirus.

The National Institute of Allergy and Infectious Diseases said on Tuesday that a trial would give fully vaccinated Americans a booster dose of Moderna's COVID-19 vaccine to see whether it works and is safe.

Participants will have had any of the three COVID-19 vaccines available in the US, from Pfizer and BioNTech, Moderna, or Johnson & Johnson, the institute, which is funding the research, said.

About 150 volunteers would take part, it said. They'd be split in three according to which vaccine they'd received, then each group would be split again into two age categories, 18- to 55-year-olds and people 56 and older.

A final group of people who haven't been vaccinated yet would receive three doses of Moderna's vaccine during the trial, it said.

The first-of-its-kind trial is designed in a way that other vaccines could later be added to the mix, the institute said.

The results, expected later this year, "are intended to inform public health policy decisions on the potential use of mixed vaccine schedules should booster doses be indicated," Dr. Anthony Fauci, the institute's director and President Joe Biden's chief medical advisor, said in the press release.

The US needs to "prepare for the possibility of needing booster shots to counter waning immunity and to keep pace with an evolving virus," Fauci said.

Moderna has reported early results from trials of COVID-19 vaccine booster shots

It wasn't clear from the press release why Moderna's vaccine was chosen to be the booster dose, but Moderna is furthest along in booster-shot trials.

Both Pfizer and AstraZeneca are working on booster shots but haven't formally reported on their trial results.

Moderna said at the start of May that early trial results indicated that a third dose of its vaccine boosted antibodies, part of the immune response, in participants. The trial included a vaccine that had been tweaked to tackle the variant first found in South Africa. Moderna said all its booster vaccines were generally well tolerated, with similar side effects as after a second dose of the vaccine.

Moderna CEO Stéphane Bancel said in April that he hoped to provide booster shots to the US later this year.