On December 20, 2011, an outbreak of influenza occurred among residents in a nursing home in Burlington, Vermont. Overall, 86 cases occurred among 325 residents at the home. Fifteen of the cases occurred among the 195 residents who were immunized with the influenza vaccine earlier in the year. What is the vaccine effectiveness (efficacy) of the influenza vaccine among residents in this nursing home?

a. 86%

b. 55%

c. 92%

d. 14%