Messenger RNA vaccines were highly effective at preventing people from suffering the most severe outcomes from COVID-19 even as infectious new coronavirus variants spread, a U.S. study found.

Receiving two or three doses of a vaccine made by either Moderna Inc. or partners Pfizer Inc. and BioNTech SE led to a 90% reduction in the risk of needing to be put on a ventilator or death because of COVID-19, according to a study published by the Centers for Disease Control and Prevention on Friday.

Both the Pfizer and Moderna shots are approved by federal regulators as an initial two-shot primary series for most people. Regulators have cleared third-dose booster shots on an emergency basis, and the drugmakers have asked that clearance for additional doses be considered. Those who support the idea of more booster shots cite the potential for a new variant to cause another wave of widespread infections.