Worked: Vaccine Effectiveness in the Field
Calculate vaccine effectiveness from vaccinated and unvaccinated attack rates.
A school outbreak reports 18 varicella cases among 152 vaccinated students and 3 cases among 7 unvaccinated students. Vaccine effectiveness = (risk among unvaccinated - risk among vaccinated) / risk among unvaccinated. It can also be calculated as 1 - risk ratio. Do not judge effectiveness from the count of vaccinated cases alone. When vaccination coverage is high, vaccinated people may make up many cases while still having much lower individual risk. Before There were 18 vaccinated cases, so the vaccine failed. After Vaccinated students had 72% lower disease occurrence than expected from the unvaccinated risk in this outbreak. Step…
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