Definition of vaccine effectiveness (VE)
Cohort study (cumulative-risk ratio)
We define absolute VE of a vaccine as
1 - Relative risk of being a case in vaccinated subjects over unvaccinated subjects
. Similarly, we define relative VE as
1 - Relative risk of being a case in subjects vaccinated with one vaccine over another vaccine
.
References
The calculations for minimum detectable VE are done using the
epi.sscohortc
function of the R package
epiR
which internally uses the approach described in
Woodward M (2005). Epidemiology Study Design and Data Analysis. Chapman & Hall/CRC, New York, pp. 381 - 426.
The calculations for expected lower and upper limits of confidence intervals for VE are based on simulations. The confidence intervals are obtained using the approach described in
Morris, J. A., & Gardner, M. J. (1988). Statistics in medicine: Calculating confidence intervals for relative risks (odds ratios) and standardised ratios and rates. British medical journal (Clinical research ed.), 296(6632), 1313.
Please note that this method does not use an exact confidence interval.
Cohort study (incidence rate ratio)
We define absolute VE of a vaccine as
1 - Ratio of incidence rate of being a case in vaccinated subjects over unvaccinated subjects
. Similarly, we define relative VE as
1 - Ratio of incidence rate of being a case in subjects vaccinated with one vaccine over another vaccine
.
References
The calculations for minimum detectable VE are done using the
epi.sscohortt
function of the R package
epiR
which internally uses the approach described in
Lwanga S, Lemeshow S (1991). Sample Size Determination in Health Studies. World Health Organization, Geneva.
The calculations for expected lower and upper limits of confidence intervals for VE are based on simulations. The confidence intervals are obtained using the approach described in
Rothman KJ (2012) Epidemiology: An Introduction. 2nd Ed., Oxford University Press, Oxford.
Please note that this method does not use an exact confidence interval.
Test negative design studies (odds ratio)
We define absolute VE of a vaccine as
1 - odds of being vaccinated among cases divided by the odds of being vaccinated among control subjects
. Similarly, we define relative VE as
1 - odds of being vaccinated with a particular vaccine versus the reference vaccine among cases divided by odds being vaccinated with a particular vaccine versus the reference vaccine among controls
.
References
The calculations for minimum detectable VE are done using the
epi.sscc
function of the R package
epiR
which internally uses the approach described in
Dupont WD (1988) Power calculations for matched case-control studies. Biometrics 44: 1157 - 1168.
The calculations for expected lower and upper limits of confidence intervals for VE are based on simulations. The confidence intervals are obtained using the approach described in
Morris, J. A., & Gardner, M. J. (1988). Statistics in medicine: Calculating confidence intervals for relative risks (odds ratios) and standardised ratios and rates. British medical journal (Clinical research ed.), 296(6632), 1313.
Please note that this method does not use an exact confidence interval.