A nurse prepares a measles, mumps and rubella vaccine in April in Haverstraw, N.Y. (Johannes Eisele/AFP/Getty Images)

The letter from the District’s top health official was clear: The city must improve vaccination rates if it wants to bolster its odds of fending off a measles outbreak.

LaQuandra S. Nesbitt, director of the D.C. Department of Health, wrote the letter to the city’s private and public school principals last month, urging them to encourage parents of unvaccinated students to take their children to the doctor for a shot of protection.

The vaccination rate, she warned, should be at least 95 percent to decrease chances of an outbreak, and the city is below that threshold. The 113 traditional public school campuses have a vaccination rate of 93 percent, and the rate at the 20 parochial schools is lower, at 87 percent. The vaccination rate is 93 percent at the 124 charter schools and just less than 92 percent at the 52 private schools.

“It’s concerning,” Nesbitt said in an interview.

A national measles outbreak has forced health agencies throughout the country to examine vaccination rates in schools, which require students to be vaccinated against a variety of diseases before they can enroll. In most states, children can opt out of vaccinations if they qualify for medical or religious exemptions.

The ongoing outbreak — with more than 1,000 cases this year, the most since 1992 — has been most pronounced in ultra-Orthodox Jewish communities in New York. But cases have emerged nationally outside the Jewish community, and medical experts fear measles cases could spread further.

The outbreak has been spurred in part by the anti-vaccine movement, which claims there is a link between vaccines and autism, despite repeated studies showing no such correlation.

Lisa Saiman, a pediatric infectious-disease specialist at Columbia University Irving Medical Center and hospital epidemiologist at NewYork-Presbyterian Morgan Stanley Children’s Hospital, said measles is so infectious that if 10 unvaccinated people were in a room and someone with measles walked in, nine people would get the disease. She said the virus that causes measles can linger in the air for two hours after an infected person departs.

That is why schools are susceptible to an outbreak, she said, with children in a confined place for extended periods. In New York, students became infected at school and carried it home to siblings.

“The concern we have is that measles spreads through the air. It has a lot of nasty tricks,” Saiman said. “It’s not just a nuisance disease. People don’t realize how many severe side effects there are from measles.”

The District has the nation’s lowest vaccination rates for diseases when compared with states: 81.3 percent for kindergartners, falling behind its neighboring jurisdictions in Maryland and Virginia, according to data from the U.S. Centers for Disease Control and Prevention. Maryland has one of the highest rates, at 98.6 percent, and Virginia lands at 95.5 percent.

According to data obtained through a public information request, 152 public school children in the District have religious exemptions that allow them to go unvaccinated and 32 have medical exemptions — a small fraction of the 97,500 public school children in the city. Students are not allowed to attend school if they are not vaccinated, although it appears the rules are not regularly enforced. The city does not collect information on the religion of families seeking a waiver on those grounds and does not require families to obtain a letter from a religious institution to claim that waiver.

On Thursday, New York ended religious exemptions, with unvaccinated students given up to a month to show they have started required immunizations.

Nesbitt said she has not seen evidence of particular D.C. populations with a dearth of vaccinations.

City data shows a handful of private schools have very low vaccination rates. Some charter schools also have vaccination rates below 85 percent.

NationHouse, a small, “independent Afrikan-centered school” in Northeast Washington, has a measles vaccination rate of 33 percent, city data shows. Dupont Park Adventist Elementary in Southeast has a rate of 37.5 percent.

An employee at NationHouse declined to comment, and a representative at Dupont Park Adventist did not respond to a request for comment.

Immunization experts cautioned that some of the D.C. data showing low immunization rates could result from poor record-keeping rather than a failure to vaccinate.

Nesbitt said she did not think access to health care was a significant factor in children going unvaccinated. She said nearly all children in the District have health insurance and can obtain vaccines at no cost. Many schools also have health-care resources on campus.

The District has not recorded a measles case this year, but Nesbitt noted that someone could bring the virus to the city and spark an outbreak if not enough people are vaccinated.

“The thing that matters a lot is that the people who cannot be vaccinated are the people who if they get measles would suffer the most,” she said.

In 2018, 96 percent of Virginia sixth-graders in public schools and 94 percent in private schools complied with state requirements for vaccinations, according to data from the Virginia Department of Health.

In the Fairfax County Public Schools system, which educates more than 187,000 students, 2,943 students were exempt from vaccinations as of March, according to county data. Of the more than 80,000 public school children in Loudoun County, 2,462 were exempt as of early May, according to the county’s schools.

This month, Virginia issued an advisory after a measles-infected child was at Dulles International Airport and several other locations in Northern Virginia.

“Overall, when you look at our medical and religious exemption rate, it’s very low,” said Marshall Vogt, an epidemiologist with the Virginia Department of Health. “We do watch that very closely.”

Rabbi Gerald Serotta, executive director of the InterFaith Council of Metropolitan Washington, said in an email he was not aware of any religious leader in the 11 faith communities that belong to the council who opposed vaccination.

He said some small Jewish communities in the region have published guidelines that seek to prevent anyone who has not been vaccinated from attending community functions, including religious services.

DC Minyan, a Jewish congregation in Dupont Circle, requires the same vaccinations mandated by D.C. Public Schools. The congregation does not accept religious exemptions and does not allow anyone who has not been vaccinated to attend services or other events, according to its policy.

In her letter last month, Nesbitt ended with a request to the District’s education leaders. She asked them to contact the parents of unvaccinated children and urge them to visit the doctor.

“Your role,” she wrote, “is one of the most important in ensuring the protection of all of our students.”