Concerned with an uptick in coronavirus cases, the US Centers for Disease Control and Prevention has extended the mask mandate on public transportation.

On April 5, the 14-day average of new coronavirus cases in the United States was 28,550. On April 12, it increased to 31,567, according to The New York Times.

Although hospitalization and death rates are trending down, the CDC announced on Wednesday that masks will be required on planes, trains, buses, ferries and other public transportation until May 3. The current mandate was scheduled to expire on April 18.

Citing the rapid spread of an Omicron subvariant, BA.2, which makes up more than 85 percent of coronavirus cases in the US, the CDC said it needed more time to monitor a recent uptick mostly in the Northeast region.

"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 statement said.

In Philadelphia, cases have increased more than 50 percent in the past two weeks. The city's health officials announced on Monday that beginning on April 18, masks are required for public indoor places.

The bump in cases may be a sign of a larger outbreak to come, Philadelphia Health Commissioner Dr Cheryl Bettigole told Fox's local station. "This looks like we may be at the start of a new COVID wave like Europe just saw."

Two Washington DC schools — American University and George Washington University — announced a return of mask mandates on Tuesday after campus cases rose less than a week after mandates were lifted. Both schools have seen cases rising after spring break. The mask requirement will remain for the semester.

The CDC decision came as a blow to the airline industry. A group of airlines had recently issued a letter to lobby the government to lift the mask mandate on planes along with the negative test result for people flying from abroad. They argued that public health benefits are diminishing, and the costs from the mandate are significant.

Dr Anthony Fauci, the chief medical adviser to President Joe Biden, last week told All Things Considered at NPR: "I think we're going to be seeing an uptick of cases that we are already seeing in certain states."

Fauci said the US is going to see a turnaround as people get out more and into the inside venues without masks. "That's going to be certainly resulting in infections, even in people who are vaccinated."

He also predicted that the US is likely to follow the pattern of the UK where cases are rising sharply with the new BA.2 variant. However, with the degree of vaccination, he hopes "we will not see an increase in severity in the sense of a concomitant increase significantly in the number of hospitalizations".

Fauci also said that a second booster shot might be needed come fall. "I'm saying this merely as extrapolations. No one knows for certain what will be required. We will have to just look at the data and make decisions," he said.