MIAMI — How quickly could measles actually spread in South Florida?
An outbreak of the highly contagious disease in the Miami-Fort Lauderdale-West Palm Beach metro area could see more than 100 people, or possibly thousands, infected within nearly nine months, depending on how many people lack immunity, either because they weren’t vaccinated or haven’t been previously infected.
This is according to a computerized model built by Mark Roberts and his team at the University of Pittsburgh and Karen Liller, Ph.D., of the University of South Florida in Tampa.
The online simulator, which was rolled out in 2019, uses a variety of data, including 2017-2018 school vaccination rates for seventh grade students in Florida. The model demonstrates what could happen in “one out of 20 outbreaks” — a worst-case scenario — if a student with measles were to attend school in a Florida metropolitan area and no vaccination programs, quarantine and isolation procedures were implemented, according to Roberts and Liller.
What the measles simulator shows
In the 2017-2018 school year, Broward and Palm Beach counties had a vaccination rate of school-required shots of nearly 97% and 95%, respectively, among seventh graders in public and private schools, meeting or surpassing the 95% threshold recommended by the World Health Organization for herd immunity, state data shows. Miami-Dade County’s vaccination rate among seventh graders in public and private schools was just under the threshold, at 94%.
However, each school has its own vaccination rate. And while some schools might have met the recommended 95% vaccination threshold, others have fewer vaccinated students, meaning a higher measles spread risk.
To develop the model of the Miami-Fort Lauderdale-West Palm Beach metro area, researchers used U.S. Census population data and the “average vaccination rate of seventh graders in each county,” with rates for public and private schools calculated separately. They also used size and location of every school and business in the counties.
The results?
A child with measles in South Florida could lead to an outbreak of 111 cases among adults and children in the county within a span of nearly nine months in a worst-case scenario, the model predicts.
While there would be some cases among students whose parents refused to vaccinate them, about 86% of infections would be among “bystanders.” This includes unvaccinated adults, as well as adults and kids who cannot get vaccinated because of a medical condition or who did get the vaccine but did not build enough immunity.
“One out of 20 times it would be worse than that, and 19 out of 20 times it would be less than that,” said Roberts.
Measles quickly spreads
Measles is a highly contagious disease that spreads through respiratory droplets released by coughing and sneezing. The virus can live for up to two hours in the air after an infected person leaves an area, according to the U.S. Centers for Disease Control and Prevention. While infection is rare for the vaccinated, about 90% of people without immunity will be infected if exposed to the disease, the CDC says.
Two doses of the measles, mumps and rubella vaccine, or MMR, are about 97% effective in protecting someone against measles, though about 3% of people who get vaccinated do not build enough immunity, the public health experts said.
The model also shows what would happen if the vaccination rate in all South Florida schools decreased by 10%. Under this simulation, cases would balloon, with the model bathing the counties in red measles dots. The simulator estimates there could be 54,476 cases of measles recorded in nearly nine months in a worst-case scenario, with 77% of the cases in people who don’t have immunity.
“The purpose isn’t, was never to scare people … the purpose was to educate and to show you this could happen. It’s a plausible outcome,” said Liller. “Will this happen for sure? No, no one knows.”
But given how Florida’s vaccination rate among children is decreasing, “you inch a little bit closer to how bad this could really be.”