Research in context
Evidence before this study
We searched PubMed Central for reports of varicella among immigrants using the terms “Chickenpox” or “Varicella” and “Emigrants and Immigrants” or “Refugees”. The search was done on June 3, 2019, with no search date or language restrictions. A large proportion of young adult immigrants (about 15%) born in tropical climates could be susceptible to varicella upon arrival to Canada because of different transmission dynamics in their countries of origin and the absence of vaccinations. Varicella childhood vaccination has resulted in a decrease in varicella incidence rates in all age groups because of herd immunity and is cost-effective from a societal perspective. The severity of varicella increases with age and adults are more likely to be admitted to hospital and to die from varicella, and pregnant women are the individuals at the highest risk. The key potential disadvantage of childhood vaccination is shifting the age of varicella acquisition to older age groups and increasing the risk of severe varicella in adults. This risk can be further potentiated by the presence of large numbers of susceptible unvaccinated adult immigrants. Immigrants born in tropical countries constitute a substantial and increasing proportion of the population in Canada and in other cold high-income countries, and could benefit from varicella vaccination. There are no population-based data on the epidemiology of varicella in the immigrant compared with the non-immigrant population nor are there data on the impact of the childhood vaccination programme. Data from our study could provide insights on which subgroups of immigrants would benefit most from targeted vaccination programmes.
Added value of this study
We present results from a large population-based study of all medically attended varicella cases over an 18-year period (1996–2014) and three vaccination periods that ascertained immigrant status accurately through linkage with the provincial immigrant database. Our findings showed that immigrants from tropical countries have an increased health disparity because of varicella compared with non-immigrants, which increased after introduction of a childhood vaccination programme. Immigrants were older at varicella diagnosis and had higher age-standardised rates of varicella over the study period than did non-immigrants. More than half of all immigrant cases occurred within 2 years of arrival to Canada. Although varicella incidence decreased in both non-immigrants and immigrants after introduction of public vaccination the varicella burden shifted disproportionately to older immigrants. Certain subgroups of immigrants including women of childbearing age, young adults from Latin America and the Caribbean or south Asia, and refugees were at the highest risk.
Implications of all the available evidence
Our findings emphasise the need to provide targeted varicella vaccination programmes for immigrants younger than 50 years of age from tropical regions as soon as possible after arrival. These study findings are applicable to cold or temperate high-income countries with and without a childhood vaccination programme who are receiving large number of immigrants from temperate or tropical climates.