U.S. health officials have begun a clinical trial with adults fully vaccinated against COVID-19, administering a booster shot of a different vaccine brand to the one they've already received, the National Institutes of Health announced Tuesday.

Why it matters: The study will examine immune responses and the safety of mixing different vaccines. Scientists will also measure the shots' efficacy in staving off emerging variants, according to a statement from the NIH.

• Per a statement from NIAID director Anthony Fauci: "Although the vaccines currently authorized by the U.S. Food and Drug Administration offer strong protection against COVID-19, we need to prepare for the possibility of needing booster shots to counter waning immunity and to keep pace with an evolving virus."
• If booster shots are required, this study could help health officials decide which vaccine combinations are best.

Details: The study will examine 150 volunteers who've received one of the three COVID-19 vaccines authorized for use in the U.S. — Johnson & Johnson's, Moderna's and Pfizer-BioNTech's.

• Each vaccine group will have enrolled some 25 people between the ages of 18 and 55 and about 25 people in the 56 years and older age group.
• "Twelve to 20 weeks following their initial vaccination regimen, participants will receive a single booster dose of the Moderna COVID-19 vaccine as part of the trial," according to the NIH.
• Another aspect of the study will see volunteers who've yet to receive a coronavirus vaccine enrolled. Initially, they'll receive the two-dose Moderna vaccine before receiving a booster dose of a vaccine about 12 to 20 weeks later. 

The big picture: Research indicates that all three vaccines authorized for use in the U.S. protect against variants, to varying degrees.

• The Public Health Agency of Canada updated its coronavirus vaccine guidance on Tuesday to permit Canadians to receive a different dose for their second shot, though it still recommends using the same brand.
• Health officials in the United Kingdom are expected to release initial results this summer of a clinical trial testing the effectiveness of mixing COVID-19 vaccines.

Go deeper: Vaccine boosters could be necessary as soon as September