A researcher investigating the occurrence of influenza enrolled 200 subjects into his study on January 1, 2011 and followed them up for 12 months. A total of 50 subjects were found to be vaccinated for the disease at study entry. By September 30th, 2011, 30 subjects developed influenza. Between October and December, 60 more subjects developed the disease.
Calculate the incidence rate of influenza between October and December 2011.
