Varicella struck 11.8% of vaccinated children and 42.9% of unvaccinated children. What is the vaccine effectiveness?

1. Vaccine effectiveness is the risk among the unvaccinated minus the risk among the vaccinated, divided by the risk among the unvaccinated. 2. The numerator is 42.9 minus 11.8, which equals 31.1. 3. 31.1 divided by 42.9 equals 0.72, so effectiveness is 72%. 4. The same answer comes from 1 minus the risk ratio, since the risk ratio here is 0.28 and 1 minus 0.28 equals 0.72. 5. 31% stops at the risk difference and 28% is the risk ratio expressed as a percentage. 6. 57% divides the risk difference by the sum of the two risks instead of by the risk among the unvaccinated. _Source: CDC Principles of Epidemiology in Public Health Practice (3rd ed., US federal, public domain), Lesson 3 "Measures of Risk", section Vaccine efficacy or vaccine effectiveness_