Objective　This paper presents formula for determining the sample size required in case-control study to estimate vaccine efficacy VE with adjusting the precision of confidence interval.Methods　Formula to calculate sample size is derived from the principle of relative width of the confidence interval.Results　Prespecified relative width may regulate and control the precision of confidence interval and may reflects magnitude of the sample size.Conclusion　Sample size calculated by formula will assure that the investigator understand accurately the degree of point estimates of VE close to the true value of VE in the population.