People who have been fully vaccinated for COVID-19 yet still get infected with the delta strain could transmit the infection to unvaccinated people, Centers for Disease Control and Prevention Director Rochelle Walensky said Tuesday in justifying renewed recommendations for mask-wearing.

“In rare occasions, some vaccinated people infected with the delta variant after vaccination may be contagious and spread the virus to others,” she told reporters on Tuesday. “This new science is worrisome and unfortunately warrants an update to our recommendations.”