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HOST IMMUNE RESPONSE

Experimental studies in animals have demonstrated that feeding colostrum containing antibody to rotavirus during challenge is protective. The colostrum is not protective if given prior to challenge, however. Epidemiological studies in humans suggest that breast-fed infants are similarly protected, supporting the role of intestinal antibody in the response to rotavirus.

Disease due to rotavirus occurs primarily in the 6 to 24 months age group, and by the third year of life essentially all members of populations in developing countries have serologic evidence of prior infection (Black et al., 1982b). Limited data from experimental infections in human adults indicate that homologous protection from clinical manifestations persists for at least 19 months (Kapikian et al., 1983). Both heterotypic and heterosubgroup serologic responses also have been found (Kapikian et al., 1983). Prechallenge serum neutralizing antibody titer is associated with a lower frequency of symptomatic infection and virus shedding following virus challenge. A titer of 1:320 or greater in children less than 2 years of age is indicative of protective immunity and results in a relative risk of 0.3 for rotavirus diarrhea compared to individuals with low titers. The antibody measured may not be directed to the actual protective antigen, however, because titers of 1:320 in the child under 2 are still associated with a relative risk of 6.1 for rotavirus diarrhea compared to older children with similarly high titers (Black et al., 1982b). Although less well documented, an inverse relationship also appears to exist between intestinal antibody level and susceptibility to rotavirus diarrhea.

Asymptomatic, naturally acquired, neonatal rotavirus infection has been shown to reduce the severity of subsequent infections, but not to confer immunity against reinfection (Bishop et al., 1983). Recent studies employing a live oral bovine rotavirus vaccine (RIT 4237) indicate that a heterologous antibody response occurs in humans as well (Vesikari et al., 1983). Significant protection in immunized compared to nonimmunized infants was observed during a natural outbreak of rotavirus infection following the immunogenicity and safety trials of this vaccine in Finland (Vesikari et al., 1984).

At present no longitudinal data are available to address the question of the duration of protection. However, the period of vulnerability to symptomatic rotavirus infection is largely restricted to the first 2 to 3 years of life, indicating that immunity is acquired and may last for decades, if not for a lifetime.

DISTRIBUTION OF DISEASE

Geographic Distribution

Rotavirus infection has worldwide distribution. The 6 to 24 months age group is the principal target of infection in all regions. In temperate climates, the disease has a distinct seasonality, occurring



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