The US Centers for Disease Control and Prevention (CDC) may soon recommend wearing two masks — one over the other — to enhance protection against the coronavirus, according to Dr Anthony Fauci, the nation's top infectious disease expert.

Fauci and the CDC discussed on Monday the possibility, but the agency doesn't yet have the data to make any formal recommendation, he said Tuesday in a livestreamed interview with The Washington Post. Still, "it makes common sense" to increase protection, Fauci said.

"The CDC makes recommendations based on data that they accumulate because it's a science-based organization, the CDC," Fauci said Wednesday during a White House COVID-19 briefing. "Right now, they're looking at these different options of mask-wearing."

Although the CDC hasn't yet outlined specific guidelines regarding wearing two masks to prevent the spread of the virus, Fauci said he often double masks.

"If you put two on … [and] you're looking for enhancing the physical barrier, it makes common sense that it certainly can't hurt and might help," he said.

There are some concerns that the practice could cause more harm if one isn't worn correctly.

Michael Osterholm, an infectious disease expert who was an adviser to President Joe Biden's transition team, said problems can occur with masks that have an "already compromised fit or filtration capacity", allowing respiratory droplets to escape out of holes.

"If you add on another mask, you may actually make it tougher for the air to move through the two-cloth area, and then at that point it causes more air to actually leak around the sides, which actually enhances your ability to get infected," he told NBC on Sunday.

Meanwhile, Iowa Democrats decided to wear jeans on the state House floor all week to protest the lack of a mask mandate as they ban members from wearing jeans.

"I told the chief clerk, jeans aren't hurting anybody but all the people wandering around without masks on, they are," Democratic Representative Beth Wessel-Kroeschell, said Wednesday.

Neither the Iowa House nor Senate allow members or staff on the floor of the chambers to wear jeans or T-shirts, while the House rules encourage but don't require mask-wearing.

Wessel-Kroeschell wore jeans Tuesday during a House debate. Iowa's Republican House Speaker Pat Grassley refused to recognize her when she tried to speak during the debate, saying she had violated House rules. She was allowed to vote.

A COVID-19 vaccine manufactured by British-Swedish firm AstraZeneca appears to provide strong protection three months after just one dose.

The study supports a strategy of prolonging the interval between the first and second dose so more first doses can be delivered to more people.

Fauci lauded the British researchers for responding to their data but said that the US won't delay second doses and will continue to recommend that Pfizer booster shots be given about 21 days after the initial shot, Moderna boosters about 28 days after.

"We also are going very much by the data and the science that has emanated out of very large clinical trials," Fauci said Wednesday during the White House COVID-19 briefing. "We feel strongly that we will go by the science, which has dictated for us the optimal way to get the 94 to 95 percent response, which is, in fact, durable for the period of time that we've been following it."

At the same time, Fauci also warned that the coronavirus could spread further if people hold or attend Super Bowl parties on Sunday when the game is on TV.

He said during TV interviews on Wednesday that now isn't the time to invite people over to watch the game because of the possibility that they are infected with the coronavirus and could sicken others.

"Every time we have something like this, there always is a spike," Fauci told NBC's Today. "Super Bowl is a big deal in the United States. Enjoy the game, watch it on television, but do it with the immediate members of your family, the people in your household."

The National Football League has invited 7,500 vaccinated healthcare workers to the game in Tampa, Florida. They will join about 17,500 fans at Raymond James Stadium, which will fill the venue to about a third of its capacity to facilitate social distancing.