of choice. in southeast asia, where doxycycline and chloramphenicol resistance have been experienced, azithromycin is recommended for all patients. a randomized controlled trial and systematic review showed that azithromycin is the safest medication. vaccine no licensed o. tsutsugamushi vaccines are currently available. the first vaccines were developed in the late 1940s, but failed in clinical trials. considered an ideal target, the unique tsa56 itself is highly variable in its chemical composition in different strains. an effective vaccine for one strain is not useful for another. an ideal vaccine should give protection to all the strains present locally. this complexity makes it difficult to produce a usable vaccine. a vaccine targeting the 47-kda outer membrane protein (omp) is a promising candidate with experimental success in mice against the boryong strain. combined targeting of tsa56 and scaa is also a good candidate for mixed-strain infection. immunity there is no lasting immunity to o. tsutsugamushi infection. antigenic variation prevents the development of cross immunity to the various strains of o. tsutsugamushi. an infected individual may develop a short-term immunity but that disappears after a few months, and immunity to one strain does not confer immunity to another. an immunisation experiment was done in 1950 in which 16 volunteers still developed the infection after 11–25 months of primary infection. it is now known that the longevity of immunity depends on the strains of the bacterium. when reinfection occurs with the same strain as the previous infection, there can be immunity for 5–6 years in monkeys. but in humans, immunity declines after one year, and disappears within two years.