Chickenpox Outbreak in a Highly Vaccinated School Population

Abstract

A chickenpox outbreak occurred in a school in which 97% of students without a prior history of chickenpox were vaccinated. Students vaccinated >5 years before the outbreak were at risk for breakthrough disease. Booster vaccination may deserve additional consideration.

A varicella outbreak in a school with high one-dose vaccination coverage, Beijing, China

“…This study confirms that children vaccinated with a single-dose of varicella vaccine may both contract and transmit varicella, and high singledose varicella vaccination coverage may not provide sufficiently high population immunity to prevent outbreaks [10][11][12][13]. This study also demonstrates, as have others, that single-dose varicella vaccination is highly effective in preventing cases and mitigating the severity of breakthrough cases that occurs [9].…”

“…During the single-dose varicella vaccination era in the United States, attack rates in school settings with high and low varicella vaccination coverage differed dramatically, from 9% to 15% in schools with coverage levels higher than 95% to 55% in a school with vaccination coverage of just 45% [6,[11][12][13][14]. In China, comparisons of areas with high varicella vaccination coverage to those with low coverage or no varicella vaccination also clearly demonstrate the impact of varicella vaccination on reducing attack rates during outbreaks.…”

Methodological issues in design and analysis of a matched case–control study of a vaccine's effectiveness

“…Risk set sampling and use of conditional logistic regression models similar to those described above will allow investigators to assess the effects of a wide range of clinically important variables, such as time since vaccination and age at the time of vaccination, on the effectiveness of the vaccine. Table 1 Coding structure and covariate pattern, including interaction term, for Equation (9). …”

“…Therefore, this time point is inherently of interest because if important differences were found, clinically relevant recommendations could be made. Furthermore, recent reports of outbreaks of varicella in which young age at the time of vaccination was assessed as a potential risk factor for vaccine failure have not used a consistent cutpoint for younger age at vaccination (range <14 to <18 months [9][10][11][12]); our choice is consistent with and in the middle of the range of ages used by others.…”

“…log odds (case) = α i + ß 1 (vaccine) + ß 2 (Y2toY7) + ß 3 (age) + ß 4 ( Y2toY7 * age ) (9) In this model, if the ß 4 parameter is statistically significant, it suggests that the waning effect after one year is different between the two different categories of age at vaccination, and the analyses should be stratified by age at vaccination (and/or by time since vaccination). In our study, the coefficient ß 4 was significantly different from zero (P=.02), so it was concluded that there was a significant interaction between age at vaccination and time since vaccination.…”