Chickenpox Outbreak in a Highly Vaccinated School Population. Tugwell B, Lee LE, Gillette H, Lorber EM, Hedberg K, and Cieslak PR. Pediatrics 2004;113(3):455-59
How effective is the chickenpox vaccine? What are the risk factors associated with “breakthrough” varicella (usually mild chickenpox in vaccinated persons)?
This study investigated a chickenpox outbreak in a well immunized Oregon elementary school in October 2001. It sought to determine the number of vaccinated and unvaccinated students, how effective the vaccine was in those who were vaccinated and why the vaccine might have failed (risk factors) in some cases.
Researchers reviewed varicella vaccination records and history of prior chickenpox, and calculated vaccine effectiveness. They also evaluated whether age, gender, age at vaccination, and time since vaccination increased the risk of developing breakthrough varicella.
Twenty-one cases of chickenpox occurred in 9 of 16 classrooms. Researchers calculated vaccine effectiveness for the 159 students who were in the affected classrooms and who had no prior history of chickenpox. Of these students, 96% had been vaccinated before the outbreak. In total, 18 of 152 (12%) vaccinated students developed chickenpox compared with 3 of 7 (43%) in unvaccinated students. Vaccine effectiveness was 72%.
In this study, the major risk factor for breakthrough chickenpox in immunized children was vaccination more than 5 years before the outbreak. Early age at vaccination (less than 15 months of age), age at the start of the outbreak, and gender were not risks.
Although differences in disease severity between vaccinated and “susceptible” (had neither varicella vaccination nor a history of chickenpox) students could not be examined statistically due to small numbers in this outbreak, other studies have shown that varicella vaccine is effective in preventing fever and large numbers of lesions. One immunized child with varicella developed skin infection, a common complication of chickenpox. No students were hospitalized.
Vaccine protection against chickenpox was 72%. Since the interval between vaccination and exposure was associated with breakthrough disease, routine booster varicella vaccination needs to be considered.
In this study, varicella vaccine was effective in preventing disease in a large population of students. Breakthrough chickenpox cases occurred, however, in some vaccinated students. Most children who were immunized but who later developed chickenpox had very mild disease but some had more typical illness with fever, many lesions and a (mild) complication.
The questions that arise from this and other recent studies are what is the ideal immunization strategy for chickenpox; whether there should be a second dose of varicella vaccine for all children, whether varicella vaccination should be given at or after 15 months of age, and how contagious so-called breakthrough disease is for unvaccinated children.