Vaccine Effectiveness

Vaccine Effectiveness is the approximate measure of the percentage of cases that a vaccine prevents in an outbreak. It is calculated using the following formula:

VE = (ARU - ARV)/ARU [x100%]

Where VE is the vaccine effectiveness, ARU is the attack rate in the unvaccinated and ARV is the attack rate in the vaccinated. The attack rate is the proportion of the given population (unvaccinated or vaccinated) that are infected with the disease.

Figure 1: ARU 0.3, ARV 0.06

In Figure 1 it's easy to visualize how the formula works. The shaded area is the unvaccinated attack rate applied to the vaccinated population. The people within the shaded area would have been infected had there been no vaccinations. All of the healthy people in the grey area represents an infection that the vaccine prevented, this can be calculated using (ARU - ARV). Then to get the proportion of cases prevented as a proportion of the cases that would have occurred the full formula is used: (ARU - ARV)/ARU.

In the above example the ARU is 0.3 (3 out of 10) and the ARV is 0.06 (6 out of 100) so the proportion of cases prevented (ARU - ARV) is 0.3 - 0.06 = 0.24 which means the vaccine effectiveness is 0.24/0.3 = 0.8 or 80% (the vaccine prevented 24 out of 30 cases that would have occurred in an unvaccinated population).

Another thing to notice is that even with an effective vaccine, when there is a high vaccination rate it is expected that there will be more cases in the vaccinated population than the unvaccinated population. Only if the ARV was equal to the ARU would the vaccine be ineffective.