A trial of formalin-killed trachoma vaccines was conducted between 1959 and 1962 in Portugal, using two immunologically distinct virus strains isolated from Saudi Arabia, SA-1 and SA-2.
In an area known to be free of trachoma 13 percent of individuals were shown to have circulating antibodies to group complement-fixing (CF) antigen, while in an area where trachoma was present approximately 25 percent of individuals possessed such antibodies, regardless of whether or not they were trachomatous. Slightly over 50 percent of persons without antibody prior to vaccination showed a detectable serologic response following one, two or three inoculations. The group immunized with strain SA-1 antigen showed a two-fold rise in the geometric mean titer of antibodies to the common group CF antigen, while those immunized with strain SA-2 showed a three-fold rise. On the basis of slit-lamp examination of individuals receiving three inoculations of vaccine, more cases of trachoma developed over a 3-year period in a group receiving normal yolk sac vaccine than in groups receiving SA-1 or SA-2 vaccines, but the difference was not statistically significant.