Officials in Hong Kong as well as in Britain, Norway and other countries have recommended a single dose of the Pfizer-BioNTech vaccine for children ages 12 and older — providing partial protection from the virus, but without the potential harms occasionally observed after two doses. On Wednesday, Sweden and Denmark joined the ranks, announcing that adolescents should get only one jab of the Moderna vaccine.

Health officials in those countries are particularly worried about increasing data suggesting that myocarditis, an inflammation of the heart, may be more common among adolescents and young adults after vaccination than had been thought.

The risk remains very small, and significant only after the second dose of an mRNA vaccine. But the numbers have changed the risk-benefit calculus in countries where new infections are mostly lower than in the United States.