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Varicella vaccine eligibility according to chickenpox history examined in England
Adolescent chickenpox history can discriminate between varicella immunity and susceptibility, but vaccine wastage may occur if only history is used to determine vaccine eligibility, according to recent study findings published in Vaccine.
“Cost-effectiveness of an adolescent varicella vaccination program in the United Kingdom is likely to depend on the proportion of vaccine doses given unnecessarily to individuals with prior natural immunity,” researchers wrote. “We therefore assessed the validity of reported chickenpox history to determine vaccine eligibility by asking parents about their child’s history of chickenpox, explicitly setting the context in terms of the implications for vaccination.”
The study included 247 adolescents recruited through secondary schools in England from February to September 2012. Parents completed a short questionnaire about their child’s chickenpox history, explicitly setting the context in terms of implication for chickenpox vaccination. Oral fluid for varicella zoster virus-specific immunoglobulin-G (VZV-IgG) tests were performed on all participants.
The researchers found that 109 (90.8%; 95% CI, 85.6-96) participants with a positive chickenpox history; 52 (67.5%; 95% CI, 57-78.1) with a negative history; and 42 (84%; 95% CI, 73.7-94.3) with an uncertain history had VZV-IgG results suggesting prior infection. Combining negative and uncertain histories, 74% had VZV-IgG (best case). Eighty-four percent had VZV-IgG when discounting low total-IgG samples and counting equivocals as positive (worst case). Further modeling was undertaken to investigate the effect of varying the negative predictive value of the assay; 74% to 87% of participants under the best case and 84% to 92% under the worst-case scenario would receive an unnecessary vaccine.
“We show that asking parents to report their child’s chickenpox history can significantly discriminate between adolescents who are immune and susceptible to varicella infection,” researchers wrote. “However, significant wastage would occur if this approach alone were used to determine vaccine eligibility. These data will be used to determine by modeling whether reported history, with or without oral fluid testing in those with negative or uncertain history, is sufficiently discriminatory to underpin a cost-effective varicella program that will protect susceptibles against chickenpox in the United Kingdom.”
Nigel Field, MPH, PhD, can be reached at nigel.field@ucl.ac.uk.
Disclosure: The study was funded in part by the DH Research and Development Directorate. Field reports support from a NIHR Academic Clinical Lectureship.
