Of the 29 strains isolated, the investigators traced all to the oral vaccine, and 16 of these--55 percent--included more than 2 percent revertant viruses.

Boosters are recommended every five years for the oral vaccine and every two years for the injectable form.

Oral vaccines left in food are preferable, but have yet to be developed.

The contamination actually occurred in both vaccines at very low levels, but because the oral vaccine was ingested rather than injected, it did not result in infections or any harm.

These injected whole cell vaccine became increasingly popular until they were replaced by oral vaccines starting in the 1980s.