Effectiveness of the typhoid Vi vaccine in overseas travelers from England.

BACKGROUND: Approximately 500 cases of enteric fever, caused by Salmonella enterica serovar Typhi and Paratyphi, are reported in the UK each year. The majority are associated with travel to the Indian subcontinent. The typhoid Vi vaccine protects against S. Typhi and is available to travelers from their general practice or private clinics. The effectiveness of this vaccine has been assessed previously in endemic regions of the world but not in travelers. METHODS: Data from the enhanced surveillance scheme concerning persons in England aged ≥2 years who traveled from the UK and contracted culture-confirmed enteric fever were used to calculate the effectiveness of the vaccine in travelers. A "case-case" case-control design was used, in which patients with typhoid comprised the "cases" and those with paratyphoid acted as "controls." RESULTS: The overall effectiveness of the vaccine, adjusted for age group, sex, ethnicity, birth in a typhoid-endemic country, and year (of receipt of specimen), was 65% (95% confidence interval 53%-73%). Effectiveness did not vary across subgroups of any of the factors in the model, but there was some evidence of waning effectiveness of the vaccine with increasing time since receipt (trend p = 0.05). CONCLUSIONS: The vaccine has been demonstrated to have a similar effectiveness in travelers as that found in endemic populations. It appears to be protective in all ages, including in young children (aged 2-5 years), a finding not consistently replicated in other studies. However, good hygiene practices are necessary in addition to vaccination to prevent infection. The "case-case" case-control design provides a valuable method of calculating the effectiveness of this vaccine in travelers, given the availability of paratyphoid controls, a population with similar demographics and risk exposures.