In 1954, the virus causing the disease was isolated from a 13-year-old boy from the United States, David Edmonston, and adapted and propagated on chick embryo tissue culture.[78] To date, 21 strains of the measles virus have been identified.[79] While at Merck, Maurice Hilleman developed the first successful vaccine.[80] Licensed vaccines to prevent the disease became available in 1963.[81] An improved measles vaccine became available in 1968.[82]