American children ages 6 months through 4 years recently became eligible to receive coronavirus vaccinations, after federal regulators and health officials cleared vaccines from Moderna and Pfizer-BioNTech for very young children.

U.S. health authorities are facing a critical decision: whether to offer new COVID-19 booster shots this fall that are modified to better match recent changes of the shape-shifting coronavirus.

When the Food and Drug Administration (FDA) meets at the end of June to discuss whether the vaccine should be modified before the likely rollout of more booster shots in the fall, it will have to make decisions without knowing how the virus will continue to change.