a formaldehyde-inactivated suspension of Rickettsia prowazekii grown in embryonated eggs; effective against louse-borne (epidemic) typhus; primary immunization consists of two subcutaneous injections 4 or more weeks apart; booster doses are required every 6-12 months, as long as the possibility of exposure exists. A vaccine containing living rickettsiae of an attenuated strain of R. prowazekii has also been used.
