Pfizer says its vaccine continues to be effective against COVID-19 up to six months later.

Pfizer and its German partner, BioNTech, announced updated results Thursday from their ongoing late-stage study of more than 44,000 volunteers.

The companies said the vaccine was 91% effective against symptomatic disease and was even more protective in preventing severe disease. Of 927 confirmed COVID-19 cases detected through March 13, 77 were among people who received the vaccine and 850 were among people who got dummy shots.

There were no serious safety concerns and the vaccine also appeared to work against a variant first detected in South Africa, the companies said.

The U.K. and U.S. gave the emergency green light to roll out Pfizer’s vaccine late last year followed by many other countries. The vaccine is authorized for ages 16 and up.

This week, the companies said the vaccine is safe and strongly protective in kids as young as 12, based on a study of 2,260 U.S. volunteers.