An outbreak of whooping cough, or pertussis, at a Florida preschool in which nearly all the students had been fully vaccinated against the disease, raises new concerns about the vaccine's effectiveness, a new report suggests.

During a 5-month period between September 2013 and January 2014, 26 preschoolers, two staff members and 11 family members of the students or staff at the facility in Leon County came down with whooping cough, according to a report of the outbreak published today (Jan. 13) in the journal Emerging Infectious Diseases.

Only five of 117 students attending the preschool had not received all of the shots required by their age. This is the first time a "sustained transmission of pertussis in a vaccinated group of 1- to 5-year-old children has been reported in the United States," the report said.

It was surprising that this outbreak occurred among a highly vaccinated preschool population, said five epidemiologists who are staff members at the Florida Department of Health in Tallahassee —writing to Live Science in a joint email. "This age group is generally thought to be protected against whooping cough through vaccination," they said.

The prolonged length of time (five months) over which the cases occurred at the preschool was another surprising aspect of the outbreak, the epidemiological staff said. [5 Dangerous Vaccine Myths]

Whooping cough is a highly contagious bacterial infection of the respiratory tract that affects the nose, throat and lungs. At first, the disease may seem like a cold, and people tend to develop a runny nose, mild cough and low fever. But a week or two later, an infected person may develop fits of rapid coughs followed by a loud "whooping" sound. The coughing fits can cause vomiting and exhaustion, and severe cases need hospitalization.

Vaccination against the disease involves a series of five shots given to young children at 2, 4 and 6 months of age, somewhere between 15 and 18 months, and a fifth dose between 4 and 6 years old. (However, the vaccine's protection against the disease can wear off as kids get older, so a booster dose of the vaccine is typically given between ages 11 and 18.) Pregnant women also now receive the whooping cough vaccine during their third trimester to provide protection to infants, the population at highest risk for pertussis complications.

People who get the whooping cough vaccine and still come down with the illness are more likely to have a mild case, compared with those who never got the vaccine, the epidemiological staff said.

Investigating an outbreak

The outbreak began in September 2013, when the Florida Department of Health in Leon County became aware of a case of pertussis in a 1-year-old preschool student who was exempt from recieving the vaccine. The student's 3-year-old sibling had symptoms of the illness first, but did not go to the preschool. Two months later, another whooping cough case was reported in the same county in a 1-month-old infant. The infected baby's sibling and mother had ties to the preschool as a student and substitute teacher.

The local health department launched an investigation into these two pertussis reports connected to the preschool that eventually involved state health officials and the federal Centers for Disease Control and Prevention. They examined all students' vaccination status and distributed a questionnaire to students and staff members to evaluate how widespread potential symptoms of the illness were in their households.

Of the 33 children in the outbreak with pertussis, which included both students and siblings of the preschoolers, 28 of them had received three or more pertussis vaccinations, and 23 had received four or more vaccinations, the investigators found. They also estimated the vaccine effectiveness rate among all the preschool students to be 45 percent.

The highest rates for whooping cough in this outbreak were in the preschool classrooms with 3-year-old students, the epidemiology staff told Live Science. This supports the concept of "waning immunity," or the idea that the vaccine's protection declines over time, they said.

The 3-year-old group has not yet received the vaccine dose that is recommended at 4-6 years of age. Waning immunity has also been seen with the pertussis vaccine in 7- to 10-year-old children, the epidemiologists said.

Another factor that contributed to the spread of the outbreak was that ill people were not properly diagnosed as having pertussis and promptly treated for it, the staff said. This caused more people to be exposed to the disease and infected by it. Many local physicians were hesitant to diagnosis patients with pertussis and did not test for the disease, although they were aware of an outbreak in the community, the staff reported.

That hesitancy may have resulted, in part, from the outbreak occurring during the cold and cough season, when respiratory viruses are most commonly acquired, the epidemiological staff said. Doctors may also have assumed their vaccinated patients were protected against the disease. [9 Weird Ways Kids Can Get Hurt]

The lack of diagnosing cases may have also stemmed from the fact that whooping cough is much less common than other respiratory diseases, and the illness can have a wide range of presentations — sometimes the infection appears as a persistent cough and sometimes the classic "whooping" sound is absent, especially in older children and adults, the epidemiological staff explained.

Further monitoring of the performance of the pertussis vaccine in preschool-age children is needed to figure out if this Florida outbreak was an isolated incident or possibly an emerging epidemiological trend, the case concluded.

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