THE INCIDENCE OF INFECTION AMONG CONTACTS OF POLIOMYELITIS CASES 1

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Abstract
Incidence of infection among 91 contacts of poliomyelitis cases was studied during an epidemic due to Type 1. Virus isolations as a measure of infection were correlated with immune status, i.e., presence or absence of antibodies. 2. Incidence of infection among household and daily contacts of diagnosed cases was high. If only antibody negative (and therefore presumably susceptible) contacts are considered, infection rates for those under 15 years of age were 100% for household contacts, and 87% for daily contacts. 3. Type 1 poliomyelitis virus was isolated with greater frequency from those with symptoms than from those who remained asmptomatic. However, many of the asymptomatic group were demonstrated already to possess antibodies to the family type of virus; when these were eliminated, there was no significant difference in ease of virus isolation from individuals with symptoms as compared to those who remained asymptomatic. 4. In analyzing clinical response to poliomyelitis virus infection among family associates of cases, it was found that the ratio of inapparent to apparent infection was approximately 3:1 (and not higher than 7:1). It is suggested that the high incidence of apparent infections in the group studied (compared to the ratio for the general population) might be related, in part at least, to factors of exposure and dosage: in a family in which a frank case occurs, exposure is of a high degree, dosage of virus is presumably large, and incidence of paralytic and nonparalytic infections is correspondingly high. 5. On the basis of virus isolations and serological evidence, poliomyelitis infection may be said to be as contagious as measles among susceptible individuals in a family setting.