Which statement about the efficacy of E. coli O157 vaccines in cattle is true?

• A. They are 100% effective.
• B. They reduce incidence but are not 100% effective.
• C. They have no effect on incidence.
• D. They increase incidence.

Answer: (B)

Vaccines against E. coli O157:H7 in cattle are used to lower colonization and fecal shedding, which helps reduce contamination risk in the beef production chain. They do not provide perfect protection, though; data show a reduction in the incidence of shedding and in the level and duration of shedding, but they do not completely prevent infection or eliminate all shedding. Efficacy can vary depending on the vaccine type, timing and number of doses, the age and health of the cattle, and the level of exposure in the environment. Because protection is not complete, some animals may still become colonized and shed organisms even after vaccination. Therefore, the statement that best describes the reality is that vaccines reduce incidence but are not 100% effective. In practice, vaccination is one component of an overall preharvest strategy that includes good management, hygiene, and other control measures to further reduce human exposure.