While only approximately 400 cases of typhoid fever are diagnosed and reported each year in the United States, nine of 10 occur in returned travelers, three-fourths of whom had been in India, Pakistan, or Bangladesh, and with the majority having visited family and friends. While the number of cases is small, the infection can be life-threatening. Furthermore, the frequency of antibiotic resistance, including to fluoroquinolones, is increasing among isolates of Salmonella serotype Typhi.
There are currently two modestly protective vaccines available in the United States — each with an overall estimated protective efficacy of approximately 50%. Furthermore, the vaccines provide no or very limited protection against Paratyphoid infection at a time when the incidence of disease due to Paratyphoid A is increasing. Despite this relatively limited protection, vaccination is recommended for travelers to a number of countries in Asia, Africa, and Latin America — the individual countries can be seen at the CDC web site.1 The vaccines are a Vi capsular polysaccharide vaccine for intramuscular administration (Typhim Vi) and an orally administered live-attenuated vaccine (Vivotif) derived from the Ty21a strain of Salmonella serotype Typhi. The latter should not be administered to immunocompromised individuals and should not be given together with antibacterial agents.
The Vi vaccine is administered as a single injection, while the attenuated Ty21a vaccine is contained in an enteric-coated capsule (which must be kept refrigerated, but not frozen). A single capsule of Ty21a is taken with water that is less than 37.0°C approximately one hour before a meal on alternate days for a total of four doses. Vi should be administered at least two weeks prior to potential exposure, while Ty21a administration should be completed at least one week before potential exposure.
In addition to vaccination of travelers to at-risk countries, CDC also recommends immunization of individuals, such as household contacts, with close exposure to chronic typhoid carriers. In addition, microbiologists and other laboratory workers routinely exposed to cultures of Salmonella serotype Typhi or specimens containing this organism, or who work in laboratory environments where these cultures or specimens are routinely handled, should be vaccinated.
The manufacturer of the Vi polysaccharide vaccine recommends a repeat dose every two years after the primary dose if continued or repeat exposure is expected. In contrast, the manufacturer of Ty21a recommends revaccination with the entire four-dose series every five years if continued or repeated exposure to Salmonella serotype Typhi is expected.
The relatively poor protection provided by these vaccines highlights the importance of the use of discretion by travelers in attempting to avoid potentially contaminated food and water.