A decreased incidence in herpes zoster infection among vaccinated and unvaccinated children corresponds to the implementation of a universal varicella vaccination program, according to results of a study published in the European Journal of Clinical Microbiology & Infectious Diseases.
Researchers calculated age-specific cumulative herpes zoster incidence rates according to vaccination status for children born before (1993-1999) and after (2000-2018) the vaccine program was initiated. Population-based administrative health data were used and included 924,124 children aged younger than 20 years, contributing a total of 9,257,601 person-years of follow-up time.
Before universal vaccination, cases of herpes zoster were distributed relatively evenly across all age groups older than 1 year. After introduction of the vaccine program, approximately 50% of cases occurred in children aged 5 years and younger. Overall, the rate of herpes zoster among unvaccinated children was 259.4 (95% CI, 253.0-266.0) per 100,000 person-years before universal vaccination and 104.4 (95% CI, 99.2-109.8) per 100,000 person-years after vaccination. For vaccinated children, the rates of herpes zoster before and after universal vaccination were 92.8 (95% CI, 85.3-100.0) and 71.6 (95% CI, 69.2-74.0) per 100,000 person-years, respectively.
The incidence of herpes zoster was 65% higher in the unvaccinated group before universal vaccination (incidence rate ratio [IRR]: 0.36; 95% CI, 0.33-0.39), and 32% higher following universal vaccination (IRR: 0.68; 95% CI, 0.64-0.73). The annual incidence of herpes zoster cases declined approximately 64% after implementation of universal vaccination, and decreases were seen across all age groups. The rates of herpes zoster among unvaccinated groups were lower for those born during the universal vaccination period; however, no significant difference was reported in children older than 9 years. Compared with children receiving 1 dose, those receiving 2 doses of vaccine had a 41% lower risk of herpes zoster (IRR: 0.59; 95% CI, 0.53-0.65).
Study limitations include an inability to determine the history of exposure to wild-type varicella zoster virus. Privately purchased vaccines were not included, nor were First Nations children. Immunocompromised children were not differentiated in the analysis, the diagnostic codes used were not verified, and unvaccinated children may be more likely to be diagnosed with herpes zoster.
According to researchers, “[T]he indirect benefits of the [vaccine program] are crucially important for maintaining the safety of immunocompromised children and children [younger than] 1 year of age, as they may be ineligible for vaccination and at higher risk for [herpes zoster] incidence and associated complications.”
Rafferty E, Reifferscheid L, Russell ML, Booth S, Svenson LW, MacDonald SE. The impact of varicella vaccination on paediatric herpes zoster epidemiology: a Canadian population-based retrospective cohort study. Eur J Clin Microbiol Infect Dis. Published online June 26, 2021. doi:10.1007/s10096-021-04298-z