COVID-19 vaccines still offer strong protection against severe illness and death, but Moderna and Pfizer are testing combination shots as a possible new kind of booster.
The vaccines now available in the U.S. were made to fight the original version of the virus.
Variants are chipping away at some of their benefits, particularly their effectiveness against mild infection.
The newer vaccine versions being tested are mixes — the original vaccine plus protection against the super-contagious omicron mutant.
Other companies are pursuing nasal vaccines that might one day better prevent milder infections.
The hunt for improvements comes amid concern that “booster fatigue” may dampen public confidence in the successful shots.
