Plague vaccines have been used since the late nineteenth century, but their effectiveness is uncertain. Vaccination reduces the incidence and severity of disease resulting from the bite of infected fleas, but it isn't 100 percent effective. The plague vaccine is licensed for use in the United States and is available for adults at high risk—people who live in the western United States, people who will be in parts of the world where plague is still endemic, and people who are around rodents. Severe inflammatory reactions are common, and plague vaccine should not be given to anyone with a known hypersensitivity to beef protein, soya, casein, or phenol. Finally, the vaccination routine is complex and requires frequent boosters to maintain its effectiveness.