safety studies. The major advantage is the large sample size that can be achieved. The major disadvantage is that some health conditions are not captured. Depending on the potential adverse event under study and the confounders that need to be adjusted for, this may or may not be a problem.
Because of their similarities, EMRs and health insurance claims data will be treated as the same type of data in this appendix under the name “health plan data.”
Study-Specific Data Collection
Sometimes, new data are collected specifically for vaccine safety studies, such as a self-controlled case series, a case-control study, a cohort study, or a postmarketing randomized trial. An intermediate option is to obtain some of the data from health plans, disease registries, and/or vaccine registries, while the remaining data are collected from study-specific patient surveys or measurements. The available options are too many to provide a detailed description of each.
TIMING OF SPECIFIC VACCINES
In a randomized childhood vaccine trial, the age at which the vaccine is given is tightly controlled by the study design, to correspond to the future planned vaccine schedule. This is appropriate, but once a vaccine is on the market, it is also given at a wide variety of other ages, for a variety of reasons. There are two scenarios in which it is of great interest to evaluate the risk of a vaccine as a function of the age at which the vaccine was given. (i) If a vaccine safety study has shown that there is a statistically significant excess risk of an adverse event, we want to know if the excess risk varies by the age at which the vaccine was given. (ii) Even if a general safety study covering all age groups has not shown a statistically significant excess risk of the adverse event, there could still be an excess risk if the vaccine is given at certain ages outside the recommended schedule. Such a safety problem could be masked by the noneffect among the most populous age group, and a special study looking at age-specific risks would be warranted.
Known Adverse Events with Early Onset
Some vaccines have been shown to cause an acute adverse event within a few weeks after vaccination. Examples include intussusception 3 to 7 days after vaccination with rotavirus vaccine (RotaShield) (Kramarz et al., 2001; Murphy et al., 2001) and febrile seizure 7 to 10 days after vaccination