6th case of measles linked to Florida elementary school outbreak

The number of measles cases linked to an elementary school outbreak in South Florida has risen to six.

The outbreak at Manatee Bay Elementary School in Weston -- 20 miles west of Fort Lauderdale and located in Broward County -- was first reported on Friday with the initial patient being a third-grade student without a history of travel, according to the Florida Department of Health (DOH).

On Tuesday, Broward County Public Schools was notified of one additional confirmed measles case at the elementary school, bringing the total number of confirmed cases to six, according to a statement from John Sullivan, chief communications and legislative affairs officer for Broward County Public Schools.

"We expect to receive further guidance from the Florida Department of Health tomorrow and will continue to keep the school and its families updated with the latest information," Sullivan said.

It's unclear what grade the other infected students are in as well as other identifying information about them including age, sex and race/ethnicity.

"The District is maintaining close coordination with the Health Department to address this ongoing situation," Sullivan said in a statement to ABC News.

"Over the weekend, the District took further preventive measures by conducting a deep cleaning of the school premises and replacing its air filters," the statement continued.

Sullivan added that the school's principal is "actively communicating with families, ensuring they are kept up to date with the latest information."

Measles was declared eliminated in the U.S. in 2000, meaning the disease "is no longer constantly present in this country." The dip in routine childhood vaccinations in recent years -- as well as travelers bringing measles into the country -- has resulted in outbreaks.

It's unclear if the students who contracted measles are unvaccinated. The current two-dose measles, mumps and rubella (MMR) vaccine recommended by the Centers for Disease Control and Prevention is 93% effective after one dose and 97% effective after two doses.

"It is very likely that this outbreak is among unvaccinated students, given that nearly 90% of measles cases in past outbreaks were among those not vaccinated," said Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital and an ABC News contributor. "This pattern aligns with historical data showing that measles primarily spreads among unvaccinated populations."

The first measles vaccine, a single-dose vaccine, was introduced in the U.S. in 1963. In the prior decade, there were 3 to 4 million cases annually, which led to 48,000 hospitalizations and 400 to 500 deaths.

While two doses of the MMR vaccine are required to attend public schools in Florida, parents are allowed to seek exemptions for religious reasons, according to the National Conference of State Legislatures.

In Florida, at least 90.6% of kindergartners were fully vaccinated with the MMR vaccine for the 2022-23 school year, according a November 2023 CDC report. However, at least 4.5% of children were exempted from one or more vaccines.

The overwhelming majority of cases in outbreaks are typically the unvaccinated. Nearly 90% of the 1,249 measles cases in 2019, which was the greatest number of cases reported since 1992, were people who were unvaccinated.

"DOH-Broward is continuously working with all partners, including Broward County Public Schools and local hospitals, to identify contacts that are at risk of transmission. Health care providers in the area have been notified," according to a weekend alert from the Florida DOH in Broward County.

Brownstein said it is very possible that the number of cases could rise because measles spreads rapidly among those who are not immune.

"An outbreak like this is very concerning because measles is a highly infectious disease that can lead to serious health complications, especially in children and immunocompromised individuals," he said. "It indicates potential gaps in herd immunity, which are vital to preventing the spread of such diseases."

Health officials said if anyone suspects or notices symptoms, to contact their health care provider to receive instructions on how to seek medical care without exposing others and to not visit the health department or a doctor's office without contacting officials ahead of time.

The Florida DOH did not immediately reply to ABC News' request for comment.

Weston is the most recent city in the U.S. to face a measles outbreak over the last few months.

Since December 2023, there have been eight confirmed cases in Philadelphia among unvaccinated individuals. Cases have also been identified in Delaware, New Jersey and Washington state, according to local reports.

ABC News' Youri Benadjaoud contributed to this report.