Intrafamilial and Interfamilial Spread of Living Vaccine Strains of Polioviruses

Fifty-six household units participated in a study of the spread of poliovirus within families and between families. Within this group all persons above the age of 6 months and not naturally immune had previously received the Salk vaccine. Each of the families used included at least three naturally susceptible members. Sabin vaccine was administered by squirting 1 ml. of the single-strain virus suspension into the back of the recipient's mouth. All but one adult was successfully infected after receiving the recommended dose. Virologic and serologic methods demonstrated transmission to many contacts, fecal excretion of the virus being the most sensitive indicator of that fact. The virus commonly appeared in the pharyngeal secretion within the first two days. The most important factor favoring intrafamilial transmission was low socioeconomic status. No evidence was found to suggest that any illness, however minor, followed the administration or spread of any of the three vaccine virus strains used.