The Vaccine Safety Datalink (VSD) project was established in 1990 to monitor safety of vaccines and investigate the occurrence of rare and serious side effects after vaccination. Even though all vaccines go through extensive studies for safety prior to licensure (learn more about this process), uncommon and rare side effects may only be appreciated after the vaccine has been administered to millions of persons. To ensure such side effects are detected, vaccines undergo postlicensure evaluation through the use of passive (for example VAERS) and non-passive surveillance systems such as VSD. VSD was created by the Centers for Disease Control and Prevention (CDC) and is managed by the Immunization Safety Office (ISO) of the CDC in collaboration with eight large managed care organizations (HMOs) and America’s Health Insurance Plans (AHIP). The eight HMO sites in the project are:
The VSD project consists of a large linked database that gathers information (while maintaining patient confidentiality) for a population of more than 9 million persons who are affiliated with the eight HMOs (approximately 5.5 million persons each year). The medical records of individuals receiving vaccines at the eight sites are reviewed for potential vaccine-related adverse events. Some examples of the type of data available to researchers from this database include:
Because the database has access to the comprehensive history of HMO participants, it can be assumed that virtually all medical encounters will be available to investigators. Thus it is not subject to the bias that adverse events are reported only when the reporting person recognizes the possibility that an event is potentially vaccine-related. Furthermore, VSD has access to the information required to investigate the hypotheses raised by VAERS or other reporting systems, or by reports in the medical literature that certain events are vaccine related.
Despite the many advantages of VSD the project is limited by a number of factors such as being unable to detect extremely rare or very mild adverse effects. It also is not completely representative of the entire US population and does not allow comparison with an unvaccinated control population. However, VSD has proven its value in addressing immunization safety issues.
VSD also has identified some priority studies for investigation. These include:
More information on the hypotheses behind and status of these and other VSD studies is available here.
In 2005, the Vaccine Safety Datalink Project team launched an active surveillance team called Rapid Cycle Analysis (RCA) that is used to monitor vaccines that have recently been licensed and new vaccine recommendations. RCA uses data from HMO sites that participate in VSD and these data are updated every week. The aim is to monitor possible side effects of vaccination in real time so that the public can be informed quickly of potential risks.
RCA works in the following way:
It decides which possible adverse events to monitor using information from studies done on vaccines before they are licensed, reports to the Vaccine Adverse Event Reporting system (VAERS) and published scientific articles.
Every week the rate of adverse events in people who have received a particular vaccine are compared with the rate of adverse events in a similar group of people who did not receive the vaccine.
If the rate of adverse events is significantly higher among the vaccinated group, VSD Project scientists perform a formal epidemiological study to determine if a vaccine truly increases the risk of that adverse event.RCA is currently monitoring the safety of all new vaccines including meningococcal conjugate, rotavirus, MMRV, Tdap and HPV vaccines. In the future it will also be used to monitor seasonal influenza vaccines.
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