Herpes zoster (HZ) incidence among only children with past varicella infection is higher than previously reported, according to findings of a national population-based study published online February 23 in Pediatrics. Children contracting varicella at 2 years of age or younger have an even greater incidence of HZ.

“Studies regarding HZ among children are limited, and no previous population-based epidemiologic studies of pediatric HZ among only those infected with varicella have been published,” write Su-Ying Wen, MD, and Wen-Liang Liu, PhD, from Taipei City Hospital in Taiwan. “The aim of this study was to establish population-based pediatric HZ data from only those who had varicella infection and assess the early effect of routine varicella vaccinations on the incidence of pediatric zoster.”
The retrospective cohort included children younger than 12 years with varicella infections between 2000 and 2006. The investigators also included vaccinated children without medically attended varicella between 2004 (when a routine varicella vaccination program began) and 2006. Follow-up for a diagnosis of HZ occurred through December 2008.

Overall, 428 of 27,517 children with medically attended varicella developed HZ, for an incidence of HZ of 262.1 per 100,000 person-years. HZ also occurred in 106 of 25,132 vaccinated children without medically attended varicella, for an incidence of 93.3 per 100,000 person-years.

The mean time from varicella infection to HZ was 4.12 years. Compared with children diagnosed with varicella at age 2 years or older, those diagnosed with varicella at age younger than 2 years had a higher incidence of HZ (P < .001) and shorter time between varicella infection and HZ (P = .04).

Children diagnosed with varicella between 2 and 8 years of age had an 85% greater risk for HZ after the vaccination program began compared with before initiation of the vaccination program (relative risk, 1.85 at 3 years of follow-up; P = .03).

Risk for HZ was more than twice as high in children with varicella infections than in vaccinated children without a history of varicella (relative risk, 2.31 at 4 years of follow-up; P < .001).
“The HZ incidence increased for children contracting varicella aged <2 years,” the study authors write. “After a vaccination program, the HZ risk increased for those contracting varicella aged ≥2 years.”

The researchers also found that children given systemic antiviral treatment for varicella had a greater risk of developing HZ than those who did not receive antiviral treatment.

The authors note several study limitations, including unreported cases of HZ resulting in an underestimated incidence, reliance on physician diagnosis coding, and limited duration of follow-up.

“The results of the current study may serve as baseline data for the early effects of varicella vaccinations on pediatric HZ,” the study authors conclude. “Long-term studies are required to monitor the impact of a varicella vaccination program on pediatric HZ.”