ATLANTA, Jan. 14 (UPI) -- In a report on a pertussis outbreak at a child care center in Florida in 2013, the Centers for Disease Control and Prevention suggests a combination of incomplete immunization schedules and waning effectiveness of the current version of the vaccine is at fault.
The new CDC report details an investigation of 39 cases of pertussis, or whooping cough, at a preschool in Tallahassee, which the agency said is the first time sustained transmission of the infection among vaccinated children has been reported in the United States.
A 2015 study found most of the recent pertussis outbreaks in recent years -- 50,000 cases were reported in 2012, the most since 1955 -- were due to vaccinated people who did not know they were infected because they showed no symptoms.
The pertussis vaccine includes shots at 2, 4 and 6 months of age, with booster shots at between 15 and 18 months old and between 4 and 6 years old, in order to be fully vaccinated. This means children could also be on schedule but not fully immunized.
The report, and theory about waning efficacy of the vaccine, comes with other outbreaks in the United States, including cases currently being investigated in Kentucky, Colorado, Illinois and Wisconsin, raising questions about why pertussis appears to be having a widespread resurgence.
The CDC study, published in the journal of Emerging Infectious Diseases, reviewed the spread of pertussis to 39 people at a day school in Tallahassee during a five-month period from September 2013 to January 2014.
Of the students at the school, only five had not received the full series of pertussis vaccinations, yet attack rates in one classroom for all students who were up to date on their shots was nearly 50 percent.
Aside from students who were not fully vaccinated, researchers said the newer version of the vaccine, which came into use in the 1990s, uses proteins from pertussis bacteria, rather than inactivated cells, in order to bypass side effects of the original version used since the 1950s.
The difference is ascribed to a possibly weaker vaccine that includes the apparent waning in efficacy of immunization after a few years.
"The general consensus has been, however, that immunity does last for at least 2 years," Dr. Catherine Troisi, an infectious disease epidemiologist at the University of Texas, told MedPage Today. "This study provides some evidence that, at least in some cases, this may not be true."