Active surveillance of vaccine safety using linked electronic health records across the United States
The Vaccine Safety Datalink (VSD) is a collaborative project between the CDC's Immunization Safety Office and nine integrated healthcare organisations across the United States. Established in 1990, the VSD conducts active surveillance of vaccine safety using linked electronic health records covering approximately 12 million people annually.
Unlike VAERS, which relies on passive voluntary reporting, the VSD actively monitors vaccinated populations using real medical records — allowing it to calculate actual rates of adverse events and compare them to unvaccinated populations.
VAERS accepts reports from anyone; VSD actively monitors a defined population using electronic health records. VSD can calculate rates; VAERS cannot.
VSD knows exactly how many doses were administered within its population, allowing true incidence rates to be calculated. VAERS has no denominator.
VSD compares vaccinated vs. unvaccinated groups and uses epidemiological methods to assess whether a vaccine increases risk of a specific outcome.
VSD data covers approximately 3% of the U.S. population. Findings are generalisable but may not capture rare events in smaller subpopulations.
The VSD currently includes nine healthcare organisations:
Kaiser Permanente Northern California
Kaiser Permanente Southern California
Kaiser Permanente Colorado
Kaiser Permanente Northwest
HealthPartners (Minnesota)
Marshfield Clinic Research Institute (Wisconsin)
Harvard Pilgrim Health Care (New England)
Denver Health
Jackson Health System (Florida)
Selected findings from published VSD studies:
VSD studies confirmed a small increased risk of febrile seizures following MMRV (combined measles-mumps-rubella-varicella) vaccine compared to separate MMR and varicella vaccines, leading to updated ACIP guidance.
VSD studies in the 1990s contributed to the withdrawal of the first rotavirus vaccine (RotaShield) after detecting increased intussusception risk.
VSD studies confirmed myocarditis/pericarditis signal following mRNA vaccines, particularly in males aged 12–29 after the second dose.
VSD has repeatedly confirmed the safety profile of annual influenza vaccines across age groups.
VSD study results are published in peer-reviewed journals. The VSD does not release individual-level data publicly due to patient privacy protections.
VSD research is published in journals including Pediatrics, Vaccine, JAMA, and NEJM. Search PubMed for "Vaccine Safety Datalink" to access the full publication record.
Search PubMed →The CDC maintains a public summary of VSD methods, member organisations, and recent findings.
cdc.gov/vaccinesafety →Qualified researchers can apply for access to VSD data through the CDC's research collaboration process.
cdc.gov/vaccinesafety →