Risk factors for susceptibility to varicella in newly arrived adult migrants in Canada.

Varicella occurs at an older age in tropical compared to cold climates. Migrants from tropical countries provide the opportunity to gain insights into observed global differences in varicella epidemiology. Severity of varicella increases with age thus, description of risk factors for varicella susceptibility will identify those who would benefit most from vaccination. A total of 1480 migrants, with a mean age of 32 years, were recruited in the pre-vaccination period (2002-2004) in Montreal, Canada. A questionnaire was administered and serum varicella antibodies were measured. Overall 6% were susceptible and ranged from 0·8% to 14·1% in subgroups. Risk factors for susceptibility were younger age, recent arrival, and originating from a tropical country. This could be modified by conditions that increased the probability of person-to-person spread of varicella through direct contact in source countries such as larger community size or household crowding. Many new young adult migrants would benefit from targeted varicella vaccination programmes.