Question

Question 2

Among 84 children greater than 12 months of age who were exposed to the varicella virus at a day-care center, 25 contracted varicella (N Engl J Med 2002; 347:1909–15). There were no instances of illness in the 16 children who had previously had varicella. Among those who had not previously had the disease, 8 of 18 unvaccinated children developed varicella, as did 17 of 49 vaccinated children. (There was one child whose vaccination status was unknown.)

Vaccine efficacy is calculated as:

Vaccine efficacy = (I _unvaccinated – I _vaccinated) / I _{unvaccinated x 100%}

What was the efficacy of varicella vaccination in this population?

Answer 1

	Total Population	Cases
Unvaccinated	18	8
Vaccinated	49	17

Vaccine efficacy = (I _unvaccinated – I _vaccinated) / I _{unvaccinated x 100%}

I _{unvaccinated = Incidence of Unvaccinated = Number of cases unvaccinated/Number of population unvaccinated = 8/18}

I _{vaccinated = Incidence of vaccinated = Number of cases vaccinated/Number of population vaccinated = 17/49}

Vaccine efficacy = (8/18) - (17/49) /( (8/18) * 100

= (1- 0.780612245)* 100 = 0.219387755* 100 = 21.94%