Ty21a

Ty21a is a live attenuated oral vaccine used to prevent typhoid fever caused by Salmonella enterica serovar Typhi. It is derived from the Ty2 strain and attenuated by multiple mutations that reduce virulence while preserving immunogenic properties. Because it is a live vaccine, it stimulates mucosal and systemic immune responses after oral administration.

Administration of the vaccine involves a series of four enteric-coated capsules taken on alternate days over about one week. It is approved for individuals aged 6 years and older and should be completed before potential exposure. The capsules should be swallowed whole with water, and the product must be stored in the refrigerator to maintain viability.

Effectiveness and duration vary by population and study, but Ty21a generally provides roughly 50–70% protection against culture-confirmed typhoid fever for several years. Immunity is not lifelong, and booster doses may be recommended for ongoing risk, depending on local guidelines and exposure.

Safety and contraindications: Ty21a is usually well tolerated. Common adverse events are mild gastrointestinal symptoms or fever. It is contraindicated in children under 6 years, in pregnant or severely immunocompromised individuals, in those with a history of severe allergic reaction to a vaccine component, and in those with moderate to severe acute illness. Because it is a live vaccine, antibiotics administered around vaccination may reduce efficacy; avoid unnecessary antibiotic use during the vaccination course.

Notes: Vaccination does not replace other preventive measures in endemic areas, such as safe water and food practices. The schedule may vary by country; always follow local guidelines.