than those in unvaccinated children.
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The benefit became largely apparent after children received the second vaccination in the recommended series, and persisted throughout childhood, Sheila Weinmann, PhD, of Kaiser Permanente Northern California, Oakland, and colleagues said.*
The analysis included 6.37 million children in the Kaiser Permanente database, 50% of whom were vaccinated for all or some of the study period stretching from 2003 to 2014. Overall, the crude lab-confirmed herpes zoster (HZ) incidence rate was 74/100,000 person-years. When stratified by vaccine status, the crude rate of HZ among vaccinated children was 78% lower than among unvaccinated children (38 vs. 170 cases per 100,000 person years).
Herpes zoster was more common among girls than boys and up to six times more common in immunosuppressed children than in nonimmunosuppressed children.
The authors also found that unvaccinated children benefited from the high rate of vaccination around them. Although the HZ rate was always lower among vaccinated children, the rate among unvaccinated children fell sharply after 2007.
“The trend of decreasing HZ incidence among children who were unvaccinated is likely due to a lack of primary VZV [varicella-zoster virus] infection resulting from herd immunity in a highly vaccinated population,” Dr. Weinmann and her associates said.
There was some variability among age groups, especially among the youngest who were not fully vaccinated.
“In the group aged 1-2 years, the confirmation-adjusted HZ rate among children who were vaccinated was 70% higher than among those who were unvaccinated,” the authors said. In the “older groups, HZ rates were significantly higher in children who were unvaccinated than in those who were vaccinated,” the researchers noted.
The highest incidence was among vaccinated 1-year-olds, who had a 140% higher risk of HZ than did unvaccinated 1-year-olds. But this risk elevation disappeared by age 2 years. For everyone else, aged 2-17 years, the rate of HZ remained significantly lower in vaccinated children.
“Among the small number of children vaccinated at 11 months of age (for whom the vaccine is not recommended), the HZ incidence rate was significantly higher than in children vaccinated at 1 year of age and older. Similarly, children who contract wild-type varicella infection at younger than 1 year of age also have a higher risk of HZ (relative risk, 13.5). The immature adaptive T-cell response in children less than 1 year of age appears less able to contain VZV as a latent infection, compared with older children.
“Our findings for 11-month-olds who were vaccinated should be interpreted with caution because this population included only three cases of HZ and could have included children participating in a prelicensure study with a vaccine formulation different from Varivax,” Dr. Weinmann and her associates said.
Dr. Weinmann and her associates reported no relevant financial disclosures. The study was supported by the Centers for Disease Control and Prevention.
SOURCE: Weinmann S et al. Pediatrics. 2019 Jun 10. doi: 10.1542/peds.2018-2917.
* This article was updated 6/14/2019