Serological responses among teenagers after natural exposure to Norwalk virus.

Twenty-one teenagers exposed to a contaminated water supply during an outbreak of gastroenteritis were tested for seroconversion to Norwalk virus. Serum specimens were collected within 72 hr of exposure and four weeks later. Each of the 11 individuals who developed symptoms and five of the 10 who remained well had a whole-antibody response in serum. None of the remaining five teenagers became ill or seroconverted. Neither seroconversion nor susceptibility to illness was associated with an absence of detectable antibody from acute-phase serum specimens. These findings support the view that immunity to Norwalk virus is not determined by serum antibody. Furthermore, the results are consistent with the possibility, suggested by previous studies in volunteers, that susceptibility is determined by Norwalk virus-specific intestinal receptor sites. IgM responses to the Norwalk virus were detected in only seven persons who became ill (64%) and nine who seroconverted (56%). The seroassay for the Norwalk IgM component might have proved a more sensitive diagnostic tool in this outbreak if convalescent-phase specimens had been collected sooner than four weeks after the onset of illness.