Risk factors for secondary transmission in households after a common-source outbreak of Norwalk gastroenteritis.

In November 1984, a foodborne outbreak of Norwalk gastroenteritis occurred in a K-12 public school in northern Vermont. The outbreak offered an opportunity to systematically study in detail secondary transmission rates in households. Eating salad at Tuesday's school-sponsored Thanksgiving Banquet was associated with illness among students and staff members (p less than 0.025). Seven of 11 serum pairs from ill persons showed a fourfold or greater rise in antibody titer to Norwalk virus compared with one of nine controls (p = 0.028). The study of secondary household transmission revealed that households with persons with primary illness were 5.5 times more likely to experience secondary illness than households with well school children or adults. As the number of individuals with primary illness in the household increased, the secondary illness rates increased. Pre-school children were twice as likely as adults to develop secondary illness.