A leaked CDC document shows new data on the Delta variant’s spread. Here are key things to know.

Vaccination alone won’t stop the rise of new variants and in fact could push the evolution of strains that evade their protection, researchers warned Friday. 

They said people need to wear masks and take other steps to prevent spread until almost everyone in a population has been vaccinated.

Their findings, published in Nature Scientific Reports, support an unpopular decision by the US Centers for Disease Control and Prevention to advise even fully vaccinated people to start wearing masks again in areas of sustained or high transmission.

“When most people are vaccinated, the vaccine-resistant strain has an advantage over the original strain,” Simon Rella of the Institute of Science and Technology Austria, who worked on the study, told reporters.

But if so-called non-pharmaceutical interventions are maintained – such as mask use and social distancing – the virus is less likely to spread and change. “There is a chance to remove the vaccine resistant mutations from the population,” Rella said.

The findings suggest that policymakers should resist the temptation to lift restrictions to celebrate or reward vaccination efforts.

This is likely to be especially true with a more transmissible variant such as the Delta variant, said Fyodor Kondrashov, also of the Institute of Science and Technology Austria.

“Generally, the more people are infected, the more the chances for vaccine resistance to emerge. So the more Delta is infectious, the more reason for concern,” Kondrashov told reporters. “By having a situation where you vaccinate everybody, a vaccine resistant mutant actually gains a selective advantage.”

People should not complain, he said. “The individual who already vaccinated and putting on a mask should not think this is pointless but should think that there is a vaccine resistant strain running around,” he said.