A two-part study of echovirus-12 was done in volunteers. In the first part the human infectious dose of the virus was determined in 149 healthy adults with undetectable serum antibody, each of whom drank 0-330,000 plaque-forming units (pfu) of virus in 100 ml of nonchlorinated water. Infection was defined as fecal shedding of virus or significant (fourfold or greater) increases in serum antibody titer. The HID50 (i.e., the dose required for infection of 50% of the volunteers) was 919 pfu. Through statistical analysis of the data by probit transformation, a 1% human-infectious dose of 17 pfu was predicted. These results were used in the second portion of the study to determine the effect of previous infection on the infectious dose. Previously infected volunteers (those with neutralizing serum antibody) were given a dose of echovirus-12 (1,500 pfu) that had been found to infect 60% of persons with undetectable serum antibody. The presence of serum antibody caused no significant change in the percentage of volunteers infected by this dose. Furthermore, the concentration of serum antibody did not affect the rate of infection or the duration of viral shedding. These results indicate that previous infection with echovirus-12 does not provide lasting protection against reinfection.

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