The Biden administration is extending a mask mandate for airplanes and transit for 15 days, the Centers for Disease Control and Prevention said Wednesday.

The mandate was set to expire after April 18, following a one-month extension announced in March. Airlines have required masks on planes since early in the Covid pandemic in 2020, but the Biden administration made them mandatory in early 2021.

The CDC said it is monitoring the spread of omicron, including the BA.2 subvariant.

“Since early April, there have been increases in the 7-day moving average of cases in the U.S. In order to assess the potential impact the rise of cases has on severe disease, including hospitalizations and deaths, and health care system capacity, the CDC Order will remain in place at this time,” the CDC said in a statement.

It said the mandate will stay in effect through May 3.

Airlines have repeatedly asked the administration to drop that requirement as well as other Covid restrictions such as predeparture testing for all international arrivals, including citizens.

The CDC last month said it would work with other government agencies to determine “when, and under what circumstances, masks should be required in the public transportation corridor.”

The extensions to the mask mandate have become shorter, however, as the administration weighs the risks of Covid spread. A decline in cases from a peak this winter helped fuel a surge in travel demand that is so strong, it is helping airlines cover much of their soaring fuel bills.