An Elementary School Outbreak of Varicella Attributed to Vaccine Failure: Policy Implications. Lee BR, Feaver SL, Miller CA, Hedberg CW, and Ehresmann KR. The Journal of Infectious Diseases 2004;190:477-483
How effective was the chickenpox (varicella) vaccine during an outbreak of chickenpox among students attending a Minnesota elementary school?
This study investigated a chickenpox outbreak that occurred in a Minnesota school with 319 students in November 2002.
Researchers conducted phone surveys with students’ parents to collect demographic information, vaccination history, medical risk factors, history of chickenpox, extracurricular and social activities, the amount of time spent at the school, and use of school transportation.
Other information collected included the number of students who contracted chickenpox during the outbreak, the date of rash onset, illness severity, rash duration and type (macule, papule, and vesicle), and outcomes of the illness.
The researchers defined chickenpox severity according to how many lesions the child had. A mild case was defined as a child with less than 50 lesions and a moderate case as a child with 51 500 lesions. A “breakthrough case” was defined as chickenpox in a previously vaccinated student whose illness began more than 42 days after vaccination.
The child who probably started the outbreak (the primary case) had a mild breakthrough case of chickenpox. During the outbreak there were a total of 54 cases. Twenty-nine (53%) of 118 (25%) previously vaccinated children developed chickenpox lesions while 16 of 20 (56%) of unvaccinated students developed lesions.
Chickenpox vaccine was 56% effective in preventing any chickenpox illness and was 90% effective in protecting against moderate illness.
Immunized students had fewer lesions, were less likely to have a fever, were sick for fewer days, and missed less school than unvaccinated students. One unvaccinated child who was receiving immunosuppressive therapy required hospitalization.
Age and length of time since vaccination were risk factors for vaccinated students developing breakthrough chickenpox. Students who received the varicella vaccine at less than 15 months of age had a higher risk of breakthrough varicella illness.
In a chickenpox outbreak, vaccinated students are less likely to develop chickenpox but if they do, they are likely to have milder symptoms than unvaccinated students. An isolated case of breakthrough varicella, even though mild, should be considered infectious.
This and other recent studies pose questions such as: What is the ideal immunization strategy for chickenpox? Should there be a second dose of varicella vaccine given for all children and should varicella vaccination be initiated at or after 15 months of age? Breakthrough varicella can be infectious for both unvaccinated and vaccinated children.