# Population immunity to varicella in Canada: A Canadian Immunization Research Network (CIRN) study

## TL;DR

Varicella immunity in Canada is high overall, but young children and immigrants are at higher risk of infection due to lower vaccination rates.

## Contribution

This study provides the first national estimate of varicella immunity in Canada using representative serosurveys.

## Key findings

- Population immunity was 93.6%, but lowest in children aged 3–5 years at 54.3%.
- Individuals born outside Canada had more than twice the odds of being susceptible to varicella.
- Immunity estimates from CHMS were significantly higher than residual specimens for 12–19 year-olds in Ontario.

## Abstract

The incidence of varicella in Canada has decreased by almost 99% since vaccination was introduced. However, variation in the timing and eligibility of vaccination programs across the country has resulted in some cohorts being under-vaccinated and therefore potentially susceptible to infection.

We used nationally representative specimens from the Biobank of Statistics Canada’s Canadian Health Measures Survey (CHMS) as well as residual specimens from Ontario collected between 2009–2014 to estimate population immunity across age-groups and geography, and identify any groups at increased risk of varicella infection.

The weighted proportion of specimens with antibody levels above the threshold of protection was 93.6% (95% CI: 92.4, 95.0). Protection was lowest among those aged 3–5 years (54.3%; 95% CI: 47.3, 61.4), but increased with age. Individuals born outside Canada had more than twice the odds of varicella susceptibility than those born in Canada (aOR: 2.7; 95% CI: 1.4, 5.0; p = 0.004). There were no differences by sex or geography within Canada, and there were no statistically significant differences when Ontario CHMS sera were compared to Ontario residual sera, apart from in participants aged 12–19 year age-group, for whom the CHMS estimate (91.2%; 95% CI: 86.7, 95.7) was significantly higher (p = 0.03) than that from residual specimens (85.9%, 95% CI: 81.1, 90.8).

Varicella immunity in Canada is changing. Children appear to have low population immunity, placing them at greater risk of infection and at increased risk of severe disease as they age. Our results underscore the importance of performing periodic serosurveys to monitor further population immunity changes as the proportion of vaccine-eligible birth-cohorts increases, and to continually assess the risk of outbreaks.