The Centers for Disease Control and Prevention is developing guidance that will ease the nationwide mask mandate for public transit next month, according to a U.S. official, but the existing face covering requirement will be extended through April 18.

The requirement, which is enforced by the Transportation Security Administration, had been set to expire on March 18, but was extended by a month to allow the public health agency time to develop new, more targeted policies. The requirement extends to planes, buses, trains and transit hubs.

According to the official, who spoke on the condition of anonymity to discuss the announcement head of time, the CDC is developing a “revised policy framework” for when masks should be required on transit systems based off its newly released “COVID-19 community levels” metric.

As of March 3 more than 90% of the U.S. population is in a location with low or medium COVID-19 Community Levels, where public face-masking is no longer recommended in indoor settings.

The CDC is delaying removing the mask requirement for transit but allowing people to gather maskless in movie theaters and sports arenas.