The Biden administration announced Wednesday that it will extend mask requirements for public transportation by 15 days nationwide to monitor the rise in COVID-19 cases.

The Centers for Disease Control and Prevention said it would extend the order, which was due to expire on April 18, until May 3 to allow more time to study the BA.2 omicron subvariant that is now responsible for the vast majority of cases US

“In order to assess the potential impact of increased cases on serious illness, including hospitalizations and deaths, and on health care system capacity, the CDC order will remain in effect at this time,” the agency said in a statement.

Last month, the Transportation Security Administration, which enforces rules for planes, buses, trains and transportation hubs, extended the requirement, saying the CDC has been looking to introduce a more flexible mask policy to replace the nationwide requirement.

Mask-wearing is the most visible vestige of the government’s restrictions to control the pandemic, and possibly the most controversial. The surge in verbal abuse and sometimes violence on planes has largely been attributed to the mask-wearing controversy.

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Critics have seized on the fact that states have lifted requirements for masks in restaurants, stores and other indoor settings, but COVID-19 cases have fallen sharply since the omicron variant peaked in mid-January.

Cases have risen slightly in recent weeks, driven by the BA.2 strain, from around 25,000 to more than 30,000 daily confirmed cases across the country. These numbers are underestimated because many people are now testing positive at home, and those test results are not reported to public health agencies.

Severe illness and death often lag the infection by several weeks. The CDC is waiting for signs on whether the increase in cases is associated with an increase in adverse outcomes before announcing a mask policy with fewer travel restrictions.