(CNN) State and local health officials have consulted with the US Centers for Disease Control and Prevention in recent months about the possibility of preventing individuals from flying to prevent measles transmission.

This year, there have been such discussions about eight individuals in five states, as first reported by the Washington Post on Thursday.

The eight individuals were either confirmed to be infected, believed to have a high probability of having measles or at high risk due to not being immune to the highly contagious virus and suspected of being in close contact with someone who has measles, CDC spokeswoman Caitlin Shockey told CNN.

She characterized the conversations as "pre-discussions" and said that state and local health departments contacted the CDC to say "we have people that we may have to place on the do not board list."

The "do not board" list is a tool the federal government can use to prevent an infectious person from flying while sick, explained Dr. Martin Cetron, director of the CDC's Division of Global Migration and Quarantine.

This measure has been used since 2007 for patients who have tuberculosis and in 2014 was used twice for measles.

After trying to dissuade a passenger who is sick and contagious from traveling on a plane, state or local health officials may contact the CDC to request assistance. The agency then has to verify that attempts were made to prevent the individual from traveling and that the person has flight reservations. The CDC speaks to the airline to arrange for any fees related to canceling the trip to be waved.

Then, the CDC works with the Department of Homeland Security to place the person on a public health do not board list, which tells the airline not to issue a boarding pass, Cetron told CNN. He said that it's a persuasive option. "Ninety-something percent of the time, when we inform people we want them to change their mind and we can tell the airline not to issue a boarding pass, they change their mind."

In the instances that have come up this year due to measles, all of the individuals decided not to travel without the CDC contacting them, Shockey said.

The states that contacted the CDC about the possible need for this intervention were California , which has reported 47 cases of measles this year; Illinois , which has reported eight cases of measles this year; Oklahoma , which has had one case; Washington , which has reported 79 cases; and New York, which has reported more than 630 cases.

"The NYC Health Department has worked with two individuals who were not immune to measles to prevent them from flying during the 21 days after they were exposed to measles virus, while they were at risk for getting sick," Patrick Gallahue, spokesman for the New York City Department of Mental Health and Hygiene, wrote in an email. "No one has been placed on the 'Do Not Board' list during this outbreak, and we have worked with passengers to minimize the inconvenience of travel disruptions in order to protect the health of New Yorkers and other travelers. People have been very cooperative."

The Rockland County, New York, Department of Health requested that two individuals be added to the federal do not board list, according to John Lyon, director of strategic communications for the county executive. "These requests could only be made for people with confirmed cases of measles. The two individuals we requested be added to the list had been planning to travel to Israel for the Passover holiday. Adding them to the list served as an effective deterrent; they did not travel."

Melaney Arnold, a public information officer for the Illinois Department of Health, confirmed that it contacted the CDC about an individual. "For the most recent case of measles diagnosed in Illinois, the Illinois Department of Public Health did contact CDC as a precautionary measure," she said. "The individual did have plans to travel this past week while still infectious, but CDC was able to assist with changing the date of the flight until after the individual was no longer infectious. The individual was cooperative and a federal Do Not Board order was not needed. The individual was isolated after being diagnosed."

Julie Graham, public information officer for the Washington State Department of Health, said it has "not needed to resort to a discussion of putting people on a 'do not fly' list," because people with measles have been cooperative and amenable to changing their plans to avoid exposing others to the illness.

Warning passengers

Cetron also said that there has been a significant increase in the number of investigations due to passengers flying with measles, for example after someone has returned from a trip and then is diagnosed with the virus but was contagious while they traveled. The CDC works with the airline to inform at-risk passengers of the possible exposure. A person with measles is contagious for four days before developing the telltale symptom of a rash.

On average, there have been about 10 investigations per year for the decade they've been conducted. However, in 2017, there were 15; in 2018, there were 81; and this year, as of May 18, there have been 62.

"You can project over 200 or 240 [investigations] this year. That's a lot more people flying with measles," Cetron said.

He noted that the increase in investigations coincides with the increase in measles cases. On Monday, the CDC reported 880 cases this year, the highest number of cases in a single year since the virus was declared eliminated in the United States in 2000 and the highest number of cases in a year since 1994.

Of the 62 measles investigations on flights this year, "21 were from domestic flights, 14 inbound international, 5 outbound IHR (outbound flights from the US to another country reported to CDC by a foreign agency) and 22 international IHRs (international to international flights with a US resident reported to CDC from a foreign agency)," Shockey said, adding that "the difference on the last two is that Outbound IHRs are from a US airport to a Foreign Airport. International IHRs are flights from an international airport to another international airport with a US resident on board."

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Cetron noted, "The problem is, most of the time when we do contact investigations, the time window for measles is narrow. ... We are hearing about it after the fact, so [our] ability to know about cases before they travel is infrequent."

There was one known secondary case of measles this year, meaning a passenger became infected while traveling from another infected traveler, he said. "We can't always say it was the cabin environment for sure."

Cetron said the best way to for anyone to protect themselves and others from measles while traveling -- or any time -- is to get the MMR vaccine.