“The deterrent effect is huge,” he said.

CDC officials said the agency had been contacted about the individuals by health officials in New York, California, Illinois, Oklahoma and Washington.

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The government’s travel ban authority often gets little discussion “because it is a politically charged and politically visible request,” said Lawrence Gostin, a professor of global health policy at Georgetown University.

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Though less restrictive than isolation or quarantine, the public health measure “is seen as a government using its power over the people and the states, which is kind of toxic in America right now,” said Gostin. “There is nothing unethical or wrong about it. It’s just plain common sense that if you have an actively infectious individual, they should not get on an airplane.”

Health officials emphasize that vaccination is the best and most effective way to protect against measles, and that the majority of people with infectious, communicable diseases, like measles, listen to doctors’ advice not to travel.

Officials in Rockland County, N.Y. and New York City, the epicenter of measles outbreaks since last fall, say they have advised several infected individuals against traveling.

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Earlier this spring, Rockland health officials, who have had 238 measles cases since last October, consulted with CDC about placing two infectious individuals on the list to prevent them from flying to Israel for the Passover holiday, a county spokesman said.

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“It served as an effective deterrent,” said spokesman John Lyon. “They did not travel."

In New York City, which has 523 cases in the nation’s largest outbreak, the health department advised two individuals “who were not immune to measles” and had been exposed to the virus, against flying during the disease’s 21-day incubation period.

“We have worked with passengers to minimize the inconvenience of travel disruptions in order to protect the health of New Yorkers and other travelers,” spokesman Patrick Gallahue said in statement this week. “People have been very cooperative.”

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Both those governments have already taken more controversial and restrictive public health measures to stem the outbreaks. New York City closed schools that refused to keep unvaccinated children home and issued mandatory vaccination orders for people living in several Brooklyn neighborhoods with a potential $1,000 fine; Rockland County issued an emergency order banning anyone diagnosed with measles or exposed to a person with measles from gathering in public places for up to 21 days, or face a fine of $2,000 a day.

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The United States is experiencing a record number of measles cases this year – 880 cases have been reported in 24 states, according to data updated Monday by the CDC. That number is the largest since 1994.

The outbreaks are occurring because vaccination coverage globally and domestically is faltering, fueled in part by an increasingly organized anti-vaccine movement. Global travel is playing an enormous role in spreading one of the most infectious pathogens from one location to the next.

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The majority of U.S. measles cases originated from unvaccinated U.S. residents returning from places where large outbreaks are occurring, including Israel, Ukraine and the Philippines.

Rockland County Executive Ed Day said his county’s outbreak began with seven travelers coming from countries with big measles outbreaks. On Monday, he wrote President Trump asking the White House to issue an executive order, or ask Congress to pass a law, requiring visitors to present “certification of appropriate immunization.”

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The current international health regulations require proof of routine immunization certificates only for yellow fever, Cetron said. "Changes to this policy would require a significant amount of international cooperation,” he added.

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It would be “chaos” and unwieldy and probably a violation of international health regulations, Gostin said, for the United States to single out proof of measles vaccination.

The Do Not Board list was developed in 2007 after an Atlanta man with drug-resistant tuberculosis caused a health scare after he flew to Europe for his wedding and honeymoon after health officials unsuccessfully advised against overseas travel. Although no other passengers were believed to have been infected, the episode led to the creation of the list, which has been used primarily for people with tuberculosis. In 2014, when the United States had 667 measles cases, two people with measles were placed on the list and were kept from travel.

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The risk of catching measles on a plane is relatively low since 80 to 85 percent of U.S. travelers are immunized, Cetron said. Nonetheless, the record number of measles cases this year has already led to 62 airplane-related investigations of contacts of people with measles who were on flights. (The CDC counts each leg of a flight as one investigation). That’s a big increase over previous years: in 2017, there were a total of 15 of these labor-intensive investigations, and in 2018, there were 81.

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Placing someone on the list requires the CDC to determine that that person is infectious, or likely to be infectious with a serious communicable illness, and occurs only after all avenues have been exhausted, Cetron said. In addition, health officials work with airlines to eliminate change fees.

“If all those things are not enough to convince somebody, then the last thing we do is contact the Department of Homeland Security, give them the appropriate identifying information, and someone gets put on the list,” Cetron said. "And if they were to go to the airport, they’re not issued a boarding card.”

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Some health departments have taken steps to try to secure refunds for those who voluntarily agreed to change their plans.

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In suburban Detroit, which had 41 cases spread by one man who traveled there from Brooklyn, for instance, health department officials wrote letters to airlines asking that individuals who followed their advice get their money back, said Russell Faust, medical director of Oakland County, Mich.’s health department.

Correction: An earlier version of this story listed Texas as one of the states that consulted with the CDC about the list. The CDC provided that information in error. It was Oklahoma, not Texas, that consulted with CDC.