PEDIATRICS Vol. 114 No. 4 October 2004, pp. 1131 (doi:10.1542/peds.2004-1341)
Chickenpox Outbreak in a Highly Vaccinated School Population: In Reply
Paul R. Cieslak, MDKatrina Hedberg, MD, MPH
Lore Elizabeth Lee, MPH
Oregon Department of Human Services
Portland, OR 97232
We appreciate the opportunity to address the points made by Drs Jumaan and Harpaz and by Mr Heath and Dr Watson.
Certainly varicella is highly contagious and may be spread not only in the classroom but in areas throughout a school. However, we did not "presume that there was zero exposure for children in the unaffected classrooms," as Drs Jumaan and Harpaz suggest; rather, we presumed neither that they were exposed nor that they were unexposed and excluded them from analysis a priori. Recalculating the vaccine efficacy including children in unaffected classrooms yields a value of 80%, which is higher than our calculated efficacy of 72%, although not significantly so, given the small number of chickenpox cases. As we indicated, however, the 80% figure involves making the presumption that children in all classrooms were exposed and, for that reason, is probably an overestimate.
Although we agree that it would be handy to know the degree of exposure in each classroom, in reality this is probably unknowable; Drs Jumaan and Harpaz note themselves that "it is difficult to ascertain the likelihood of exposure." Other Centers for Disease Control and Prevention experts have observed that "it is not possible to identify in advance persons who could be highly infectious."1(p1914) We reanalyzed time since vaccination among children in affected classrooms after excluding 6 students (including 3 cases) with household exposure (to control for the presumably more extensive exposure that occurs in the home) and 22 students with classroom-only exposure to unvaccinated case students (to control for the possibility that unvaccinated cases are more contagious than vaccinated cases), as suggested by Drs Jumaan and Harpaz and by Mr Heath and Dr Watson. Our results were the same as first reported: the attack rates were 23.2% (vs 23.1%) among students vaccinated 
5 years previously and 3.1% (vs 3.4%) among those vaccinated <5 years previously. Likewise, after excluding these students, the relative risk of disease among those vaccinated 5 years earlier was 7.3 (95% confidence interval: 1.7–31.0) and was not significantly different from the first-reported relative risk of 6.7 (95% confidence interval: 2.2–22.9). In short, time since vaccination remained associated with breakthrough chickenpox, which is consistent with that of a recent Centers for Disease Control and Prevention investigation of a chickenpox outbreak at a Michigan elementary school.2
Citing Kuter et al,3 Drs Jumaan and Harpaz suggest that time since vaccination might be confounded by increased exposure as a function of the age of the students. We do not believe that this was the case in this outbreak. Even after we stratified the analysis by classroom and again excluded those students with potentially more-intense exposures as noted above, time since vaccination remained a risk factor for breakthrough chickenpox. Kuter et al noted that the relatively high attack rates that they observed 2 to 5 years after vaccination occurred before the vaccine was licensed, at time as which rates of disease transmission were higher. This transmission could have resulted in immunologic boosting, causing attack rates to decline with longer time since vaccination. On the other hand, our outbreak occurred in the setting of widespread vaccination of the student body, lower disease-transmission rates, and little or no immunologic boosting. This may explain why, unlike Kuter et al, we did find breakthrough rates increasing with time since vaccination. Our outbreak, although small, may better reflect the performance of the vaccine in current disease-transmission circumstances.
Finally, Drs Jumaan and Harpaz point out that the results of a small, observational study are unlikely to be definitive. We agree, and for that reason we commented that "our finding is suggestive of waning immunity," and "if the interval between vaccination and exposure is significantly associated with breakthrough disease in future outbreak investigations, routine booster vaccination for children might be warranted."4(p458–459) Others have expressed similar sentiments.5,6
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- Galil K, Lee B, Strine T, et al. Outbreak of varicella at a day-care center despite vaccination.
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[Abstract/Free Full Text] - Centers for Disease Control and Prevention. Outbreak of varicella among vaccinated children—Michigan, 2003. MMWR Morb Mortal Wkly Rep. 2004;53 :389 –392[Medline]
- Kuter B, Matthews H, Shinefield H, et al. Ten year follow-up of healthy children who received one or two injections of varicella vaccine. Pediatr Infect Dis J. 2004;23 :132 –137[Web of Science][Medline]
- Tugwell BD, Lee LE, Gillette H, Lorber EM, Hedberg K, Cieslak PR. Chickenpox outbreak in a highly vaccinated school population.
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[Abstract/Free Full Text] - Vázquez M, LaRussa PS, Gershon AA, et al. Effectiveness over time of varicella vaccine.
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[Abstract/Free Full Text] - Gershon AA. Varicella vaccine—are two doses better than one?
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[Free Full Text]
PEDIATRICS (ISSN 1098-4275). ©2004 by the American Academy of Pediatrics
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Related articles in Pediatrics:
- Chickenpox Outbreak in a Highly Vaccinated School Population
- Aisha O. Jumaan and Rafael Harpaz
Pediatrics 2004 114: 1130.[Extract] [Full Text]
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