The Centers for Disease Control and Prevention on Monday strongly recommended in newly issued guidelines that all passengers and workers on planes, trains, buses and other public transportation wear masks to control the spread of the novel coronavirus.

The guidance was issued following pressure from the airline industry and amid surging cases of the coronavirus and strong evidence on the effectiveness of masks in curbing transmission, according to CDC officials.

The recommendations fall short of what transportation industry leaders and unions had sought, and come long after evidence in favor of mask-wearing was well established.

The CDC had previously drafted an order under the agency’s quarantine powers that would have required all passengers and employees to wear masks on all forms of public transportation, according to a CDC official who spoke on the condition of anonymity to discuss internal deliberations. Such orders typically carry penalties. The order was blocked by the White House, the official said. It was first reported by the New York Times.

Monday’s recommendation followed a request from Vice President Pence to CDC Director Robert Redfield, CDC officials said. Although the agency already recommends the use of masks generally, the new language is more strongly worded and gives the airline industry more cover to press for mask-wearing, one CDC official said.

In a statement Monday, the agency said “transmission of the virus through travel has led to — and continues to lead to — interstate and international spread of the virus.” It added: “Local transmission can grow quickly into interstate and international transmission when infected persons travel on public conveyances without wearing a mask and with others who are not wearing masks.”

Traveling on airplanes, ships, ferries, trains, subways, buses, taxis, ride-shares and in locations such as airports, train stations and bus or ferry terminals increases the risk of getting and spreading covid-19, the disease caused by the virus. That can happen because people are brought into close contact with others, often for prolonged periods, and because they can be exposed to frequently touched surfaces, the CDC guidance says.

“Face masks help prevent people who have COVID-19, including those who are pre-symptomatic or asymptomatic, from spreading the virus to others,” the CDC guidelines say. “Masks are most likely to reduce the spread of COVID-19 when they are widely used by people in public settings.”

The recommendations from the CDC come at a time when infection rates are rising in the United States and President Trump’s favored pandemic adviser, Scott Atlas, has railed against masks, falsely claiming they are ineffective. On Saturday, Atlas wrote on Twitter that masks do not work, prompting the social media site to remove the tweet for violating its safety rules for spreading misinformation. Several medical and public-health experts flagged the tweet as dangerous misinformation.

Numerous studies have shown the effectiveness of masks in preventing transmission. Before a mandatory mask regimen was put in place this spring at Mass General Brigham, the largest health-care system in Massachusetts, covid-19 cases were increasing exponentially among medical workers, according to a research letter published in JAMA. After the mask rule was imposed, cases steadily declined.

Trump has cast doubt on the efficacy of masks for months. Infectious-disease experts have criticized what they have described as flagrant flouting of basic public-health guidance at the White House, such as the failure to wear masks and socially distance.

Trump and at least 34 White House aides and other contacts tested positive for the virus, according to an Oct. 7 internal administration document. Some of them are suspected of having become infected at the White House Rose Garden event celebrating the Supreme Court nomination of Judge Amy Coney Barrett.

Ensuring every passenger on a plane or bus is properly wearing a mask has proved to be a major challenge. The new CDC guidance received mixed reactions from the transportation industry.

Sara Nelson, international president of the Association of Flight Attendants-CWA, applauded the CDC for being “as strong as possible” with the guidance, despite the previous failure of the agency to implement the stronger action.

But she expressed frustration that a debate continues over whether people should wear masks when in public.

“We have flight attendants from other countries wearing space suits on the plane and we’re still talking about whether everyone should be wearing masks?” Nelson said. “We asked for this on April 23 and within seven days the airlines had policies. Why? They understood very quickly that if there’s any chance for air travel, you’ve got to put procedures in place that give the public confidence that it’s safe to fly.”

While airlines threaten to ban passengers who fail to follow corporate mask mandates and transit agencies urge riders to follow local mask rules, operators’ instructions are sometimes ignored, unions representing transportation workers said.

Some said the CDC’s new recommendation falls short of what harried transportation employees need.

The guidance is “a good step in the right direction,” said Larry Willis, president of the Transportation Trades Department of the AFL-CIO.

“But at the end of the day, these are still just recommendations. … To really give teeth to these requirements, there actually has to be a federal mandate,” he said, casting blame at the top of the Trump administration.

“The White House has made its hostility to common sense CDC guidelines, including their mask requirement, a central part of their message” in the pandemic, Willis said.

The Transportation Trades Department had petitioned the Department of Transportation to institute a mask requirement, but a senior official there said the department “embraces the notion that there should be no more regulations than necessary.”

“This is too little, too late and downright shameful,” John A. Costa, international president of the Amalgamated Transit Union, said in a statement. After at least 219,000 deaths, including 90 of the union’s members, the “inadequate” federal recommendation also “jeopardizes the safety of operators by asking them to kick people off who aren’t wearing masks,” Costa said. “This can only lead to potentially dangerous confrontations.”

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