(CNN) Washington, DC is coping with a cluster of high-profile Covid-19 cases after a series of public events exposed high-level officials.
House Speaker Nancy Pelosi, Sens. Susan Collins and Raphael Warnock, and Rep. Peter DeFazio have all this week announced that they tested positive.
Health experts say the outbreak may be rooted, in part, in outdated and confusing guidelines from the US Centers for Disease Control and Prevention that help people assess their risk of getting the virus that causes Covid-19 or passing it on to others.
On Thursday, after announcing that she had been in close contact with someone who had tested positive for Covid-19, Vice President Kamala Harris presided over the Senate confirmation of Ketanji Brown Jackson without wearing a mask, though CDC guidelines advise masking around other people for at least 10 days after exposure to the virus.
The same day, at a press event for World Health Day, Xavier Becerra, secretary of the US Department of Health and Human Services, explained that he and World Health Organization Director-General Tedros Adhanom Ghebreysesus would both be wearing masks at the event -- except while speaking -- "because each of us has been close to someone who tested positive recently."
Health experts said Friday that Americans are relying on CDC guidance that's overdue for an update.
Since the early days of the pandemic, the CDC has defined someone who's a "close contact" -- and is therefore at risk of contracting and spreading the virus -- as someone who has spent a cumulative total of at least 15 minutes within 6 feet of another person who has lab-confirmed Covid-19 or who's been told they have Covid-19 by a doctor.
With newer, more contagious variants such as BA.2 on the loose, Kimberly Prather, an aerosol scientist at the Scripps Institute of Oceanography, said the rule needs rethinking.
"Fifteen minutes and 6 feet was not really useful in the first place," she said. "We know people get infected in less time and longer distanced."
Prather thinks the rule for close contact should have been based on anyone sharing the air in a room for a certain number of minutes.
Distance, specifically the distance of 6 feet, has been in the infection equation since in the late 1800s, when a scientist named Carl Flugge figured out that infections could be transmitted by respiratory droplets through the air. He recommended separating people to prevent infections. Scientists tested it using glass plates and came up with a distance of 6 feet.
In the 1930s, another scientist, William F. Wells, figured out that although some droplets that come from the mouth or nose are large and fall to the ground quickly -- within 3 to 6 feet -- sick people can also emit smaller virus-filled aerosols that float in the air for minutes or even hours. Those can also be infectious.
Since March 2020, when 52 members of a choir in Skagit County, Washington, got Covid-19 after attending practice with just one person who was sick, health officials have known that the virus that causes Covid-19 can be transmitted by smaller aerosols, making distance less important than ventilation and time.
Yet the CDC continues to factor 6 feet into its risk equations.
In response to a question from CNN, a CDC spokesperson said Thursday that the agency was not planning to change the close contact definition "at this time."
"If you were part of an event where there's multiple infections, you will have been exposed. I don't care if it's 6 feet or 15," said Dr. Carlos Del Rio, an infectious disease specialist at Emory University.
If you're exposed but up to date on your vaccinations, Del Rio said, you should watch for symptoms and wear a mask for 10 days, which is what the CDC says, too.
"If I was in that room with Pelosi and others where they got infected, I would consider myself a close contact because I was there," he said. It's not known exactly where Pelosi was infected, but she was among lawmakers who appeared maskless with President Biden at a signing ceremony on Wednesday. According to CDC guidelines, Pelosi was not considered to be a close contact of the President, the White House said in a statement.
That's closer to the way some other countries have defined exposure.
Until February, when the UK began to roll back its pandemic restrictions, health authorities there defined a close contact more broadly. Their definition included anyone who:
Linsey Marr, a professor of civil and environmental engineering at Virginia Tech University who studies aerosols, said the CDC needs to take another look at its contact precautions.
"I do think they should update it, because I think it's based on outdated thinking about transmission," she said.
Marr said the CDC probably made the cutoffs of 6 feet and 15 minutes to try to make the best use of limited public health resources such as contract tracing.
"It's based more on convenience than on science at this point," she said.
Marr said that all superspreading events have four things in common: lots of talking, shouting or singing; long exposure times; poor ventilation; and no masks.
"If you have that type of situation, then I would say everyone in the room is potentially exposed," she said.