A measles outbreak has infected nine people in Florida including children and an adult after the Centers for Disease Control and Prevention (CDC) confirmed that there have been 35 cases of the highly transmissible virus in more than 15 US states this year.

The most recent case of measles involves an adult aged 20 to 24 who lives in Polk County, Central Florida, the Florida Health Department said. Other cases in Broward County involved eight young people, including a child under age 5.

Most of the cases among children in the Sunshine State have been linked to Manatee Bay Elementary School in Weston, near Fort Lauderdale. Health officials warned that it was likely that the outbreak would spread from schoolchildren to others.

While Florida has the largest number of measles cases in the country, the state's Surgeon General Dr Joseph Ladapo has sparked controversy over how he has handled the issue as he has offered advice that contradicts other medical guidance.

Doctors suggest that children who haven't been vaccinated for measles should be isolated for 21 days after being exposed at school.

Last week, Ladapo wrote to parents at Manatee Bay Elementary School that it was up to them to decide whether they wanted to allow their children to keep attending school or quarantine.

The letter read: "Because of the high likelihood of infection, it is normally recommended that children stay home until the end of the infectious period.

"Due to the high immunity rate in the community, as well as the burden on families and educational cost of healthy children missing school, DOH [Department of Health] is deferring to parents or guardians to make decisions about school attendance."

The CDC advises that any child who is unvaccinated and becomes exposed to measles should stay home for three weeks.

Under Florida law, students must be vaccinated for measles and other contagious diseases. They can get an exemption for medical reasons or if parents believe it violates their religious tenets and practices.

Measles are highly transmissible and easily spread through the air and on surfaces, especially in a confined space like a school. It is prevalent in late winter and early spring. If someone coughs or sneezes, the measles virus can stay in the air for up to two hours.

In his letter, Florida's surgeon general stopped short of encouraging parents to get their children vaccinated against the virus. He added that his recommendations may change, and any children with symptoms of measles shouldn't go to school.

Yet, the CDC advises that all children from ages 12 to 15 months old and students at post-high school educational institutions, adults or those traveling internationally should be inoculated against the virus.

"CDC recommends that people get MMR vaccine to protect against measles, mumps, and rubella," it said. "Children should get two doses of MMR vaccine, starting with the first dose at 12 to 15 months of age, and the second dose at 4 through 6 years of age. Teens and adults should also be up to date on their MMR vaccination."

Among unvaccinated people, 90 percent can become infected, the CDC said. Around 97 percent of those who are fully vaccinated are protected against infection.

The virus can be particularly severe among pregnant women or children under age 5. It can lead to hospitalization or pneumonia — the most common cause of death from measles in young children. At least 1 in 5 unvaccinated people who contract measles will be hospitalized.

There were just 58 cases of measles in 2023, according to CDC data. Of the 35 measles cases already confirmed this year, nine were found in Pennsylvania in January, including eight in Philadelphia. Last week, Michigan's Health Department reported its first case of measles since 2019.

The outbreak had also spread to Arizona, California, Georgia, Indiana, Louisiana, Maryland, Minnesota, Missouri, New Jersey, New York City, Ohio, Pennsylvania, Virginia and Washington as of Feb 22, according to the CDC. Most have been linked to international travel.

Worldwide, the number of cases confirmed has increased due to low vaccination rates generated, in part, over fears after the COVID-19 pandemic.

In January, the CDC urged doctors to "stay alert" for measles after a slew of cases among the unvaccinated.

Florida's surgeon general, Ladapo, was appointed by Republican Governor Ron DeSantis in 2021. He was formerly a clinical researcher at the University of California, Los Angeles.

Ladapo is opposed to COVID-19 vaccines and wrote in a January letter that the vaccine should be suspended because "freedom trumps public health, freedom trumps protection".

The Food and Drug Administration warned that his stance could "put people at risk of death or serious illness".