Three-year follow-up of protection rates in children given varicella vaccine.

OBJECTIVE

To determine the rate and severity of subsequent varicella and zoster among children given a varicella vaccine.

DESIGN

Retrospective survey conducted three years after vaccination, using standardized telephone interviews.

SETTING

Three urban Canadian centres (Halifax, Ottawa and Vancouver).

PARTICIPANTS

Of 475 eligible children, 431 aged three to 15 years participated.

INTERVENTION

Participants had received one dose of a live, attenuated varicella vaccine (Oka/GSK) an average of 36 months earlier.

MAIN OUTCOME MEASURES

Interviewers asked about known varicella exposures and obtained descriptions of any illnesses reported as varicella or zoster. Reported varicella was classified as definite (vesicles present, physician confirmation), probable (vesicles reported), suspected (nonvesicular rash, recent contact with varicella) or unlikely (nonvesicular rash, no known contact).

RESULTS

Parents reported 257 within-home exposures to varicella and alleged that 80 subjects had had varicella rash after exposure. Of these, 40 cases were classified as definite or probable (vesicles present; rate 9.3% or 3.1% per year on average), 31 as suspected (no vesicles; rate 2.4% per year) and nine as unlikely. All rash illnesses were mild: among children with vesicles, 75% were said to have up to 10 and none had more than 100. The outcome of 76 exposures to household members with varicella was evaluable: 25 (32.9%) resulted in a rash, and 13 (17.1%) of those involved vesicles (maximum 40 lesions). Parents alleged that three subjects had zoster, but only one illness was definite (six vesicles, physician confirmed).

CONCLUSIONS

Varicella vaccination completely prevented the development of typical chickenpox during the three years of observation. Breakthrough infections were infrequent (about 5% per year) and mild. Zoster was rare.