Varicella susceptibility in a Canadian population

To determine susceptibility to varicella zoster virus (VZV) infection among children, pregnant women and health care workers in Newfoundland.

Cohort and cross-sectional, province-wide, population-based seroprevalence study.

A cohort of 586 children aged one year, a cross-sectional group of 1135 school children aged five to 15 years, 3643 pregnant women aged 15 to 45 years, and 5386 persons in health care settings aged 15 to 55 years. Susceptibility to varicella was determined by enzyme immunoassay based on serum antibody to VZV.

Among the cohort of 586 children, 565 (96.4%) did not have detectable VZV antibody at one year of age. The proportion with VZV antibody increased thereafter to 12.8% and 33.9%, respectively, at age two and four years, indicating the extent of exposure to VZV at these ages. Among the 1135 school-age children, the proportion testing positive for VZV antibody increased from 44% at five years of age to 88.9% at 15 years of age, indicating the cumulative incidence of varicella in this age group. Among pregnant women, 92.1% tested positive for VZV antibody, and the corresponding figure for the health care group was 93.1%. In both groups, the proportion testing positive for VZV antibody increased with advancing age, from 89.6% for the 15- to 19-year age group to 96.5% for those over the age of 40 years.

The risk of VZV infection increases steadily from one year of age, reaching a peak during school years. The study data support the recent Canadian recommendation to vaccinate any person older than 12 months of age who is susceptible to VZV. Among the adult population, the proportion susceptible will be under 10% for the foreseeable future, and for those at risk, selective vaccination based on their immune status would be a cost effective approach.