Effect of Age at Vaccination on Rotavirus Vaccine Effectiveness in Bolivian Infants.

BACKGROUND: Rotavirus vaccines are less effective in developing countries versus developed countries. One hypothesis for this difference in performance is that higher levels of maternal antibodies in developing countries may interfere with vaccine response, suggesting that delayed dosing could be beneficial. The present analysis aims to assess whether rotavirus vaccine effectiveness (VE) varies by age at vaccination during routine use in Bolivia.

METHODS: Data were merged from 2 postlicensure evaluations of monovalent rotavirus vaccine (RV1) in Bolivia, where 2 doses of RV1 are recommended at 2 and 4 months of age. For each dose, children were classified as receiving each dose "early," "on-time" or "late." Stratified unconditional logistic regression models were used to estimate VE, using unvaccinated children as the referent. VE was calculated as (1 - odds ratio) × 100%. Models were adjusted for hospital, age and time since RV1 introduction (via including terms for month and year of birth).

RESULTS: VE for 2 doses of RV1 tended to be higher in infants receiving the first dose early (VE, 92%; 95% confidence interval: 70%-98%), when compared with infants receiving their first dose on-time [72% (62%-81%)] or late [68% (51%-79%)]. Estimates of VE were not substantially different when comparing children by age at second dose [early: VE, 76% (50%-89%); on-time: VE, 70% (50%-89%); late: VE, 75% (60%, 84%)], including all children.

CONCLUSIONS: Our results indicate that early administration may improve VE and support the current World Health Organization recommendations for the RV1 schedule.