Safety and immunogenicity of live attenuated varicella vaccine in 9-month-old children

Background: The present study was conducted to evaluate the safety and immu nogenicity of live attenuated varicella vaccine (Oka-strain) in 9-month-old infants. Methods: One hundred and fourteen infants were vaccinated once with live at tenuated varicella vaccine (Valrix(R); SmithKline Beecham Biologicals, Rixe nsart, Belgium) containing a mean virus titer of 10(4.0) plaque-forming uni ts (p.f.u.) per dose. Signs and/or symptoms after vaccination were followed for 42 days. Home visits were made to detect solicited local reactions (0- 3 days) and solicited general reactions (0-21 days), as well as unsolicited reactions. Specific varicella antibodies were determined by an indirect im munofluorescence method. The geometric mean titer and seroconversion rate w ere calculated. Results: Signs and/or symptoms were reported in 47.4% (54/114) of cases fol lowing vaccination. The only local symptom reported was pain on digital pre ssure at the injection site and this was reported in 28.1% (32/114) of infa nts. General symptoms were reported in 38.6% (44/114) of cases. The most fr equently reported findings were fever (27.2%), which was mostly mild, restl essness (20.2%) and cough (11.4%). Only four unsolicited symptoms were repo rted and they were all unrelated to vaccination. No serious adverse event w as reported. Of the 109 infants included in the immunogenicity analysis, 10 5 were seronegative and four were seropositive for antibodies against varic ella before vaccination. The vaccine elicited seroconversion in 97.1% of in itially seronegative cases. The post-vaccination geometric mean titer for t hese infants was 30.9 geometric mean titer (GMT). Conclusions: The vaccine was found to be safe and immunogenic when given to infants as young as 9 months of age. This may be of clinical significance during outbreaks of varicella and especially for developing countries.