Between 22 January and 4 April 1997, 467 hepatitis A cases were reported to the New South
Wales Health Department, Australia. To identify the cause of the outbreak, we conducted a
matched case-control study, and an environmental investigation. Among 66 cases and 66
postcode-matched controls, there was a strong association between illness and consumption of
oysters (adjusted odds ratio 42; 95% confidence interval 5–379). More than two-thirds of cases
reported eating oysters, including one third of cases and no controls who reported eating
oysters in the Wallis Lake area. A public warning was issued on 14 February, and Wallis Lake
oysters were withdrawn from sale. Hepatitis A virus was subsequently identified in oyster
samples taken from the lake. Hepatitis A virus poses a special risk to consumers who eat raw
oysters because it can survive for long periods in estuaries and cause severe disease.