FIELD EFFECTIVENESS OF ERYTHROMYCIN PROPHYLAXIS TO PREVENT PERTUSSIS WITHIN FAMILIES

To evaluate the field effectiveness of erythromycin prophylaxis for pe rtussis within families, a retrospective cohort study was conducted am ong 246 families. Overall 41% of the subjects (387 of 940) had been si ck. The secondary attack rate was 65% for infants younger than 2 years , 54% for those 2 to 4 years old and 39% for children 5 to 9 years old , and it declined thereafter. The secondary attack rate decreased from 25% in families without prophylaxis to 17% in families with prophylax is. The protection induced by prophylaxis did not vary with age or vac cination status. When prophylaxis was used before the onset of a secon dary case, the secondary attack rate was 4% compared with 35% when giv en after a secondary case (P < 0.001). Erythromycin prophylaxis seems to be efficient in preventing secondary cases but is most useful when administered before the occurrence of the first secondary case.