HIV vaccine trials: some design issues including sample size calculation

The World Health Organization estimates that 40 million people will be infected with HIV by the year 2000. Since there is no cure for AIDS, preventing the infection is the most urgent public health goal. More than 20 different AIDS vaccines are in development. Most are based on proteins from the viral envelope, which is the outer layer of the virus. Tests on human volunteers show that many of the vaccines have few side effects. Future studies will need to show that the vaccines can prevent infection. The number of volunteers enrolled in these studies will vary depending on several factors. The rate of infection in the US population varies from .026% in Army personnel to 2% to 5% in intravenous drug users. The lower the infection rate, the larger the study sample will need to be. Sample size can be reduced by lengthening the follow-up period.