Follow-up to: You cited an ROR of 36.01 for a 'measles-like reaction' to prove the vaccine's …
Question explored with the scientific record
You confidently assert that an active follow-up study comparing vaccinated to unvaccinated children has 'never been run,' so how do you explain the massive, active surveillance studies from the CDC's Vaccine Safety Datalink (VSD) that continuously track and compare millions of vaccinated and unvaccinated children using real-time electronic health records?
The VSD does compare vaccinated to unvaccinated children, but the comparison is not what most people picture. The evidence shows the VSD is a powerful signal-detection system, not a long-term safety trial with a true unexposed control group.
The VSD links electronic health records from millions of insured patients across multiple HMOs [1]. It has detected real safety signals: the rhesus rotavirus vaccine and intussusception, with an age-adjusted rate ratio of 30.4 in the 3-7 days after the first dose [1]. That is a genuine finding that led to a policy change. The VSD also found an increased risk of febrile seizures after MMRV, with a relative risk of 1.98 [8]. These are real signals.
But the VSD's "unvaccinated" comparison group is not a clean, never-vaccinated cohort. The 2018 VSD study on confounding [6] makes this explicit: children classified as "no vaccines" in the electronic records had a mean of 435.7 days of underimmunization, meaning they had received some vaccines but were behind schedule. The fully vaccinated group had zero days of underimmunization by definition. The study also found that 51.3% of parents whose children were coded as undervaccinated with a vaccine-refusal code reported their child was fully vaccinated on time [6]. The misclassification is substantial. The "unvaccinated" group in VSD analyses is often a mix of delayed, partially vaccinated, and misclassified children, not a true never-vaccinated population.
The VSD also does not run the study that matters most: a prospective, actively followed cohort comparing children who receive the full schedule on time to children who receive no vaccines at all, with long-term follow-up for all-cause morbidity and mortality. The 2025 systematic review of 217 VSD studies [2] shows the focus: 146 analytic studies, mostly on short-term safety signals for individual vaccines. The VSD is designed to catch acute adverse events in the weeks after vaccination, not to measure the cumulative effect of the full schedule over years.
| What the VSD can do | What the VSD does not do |
|---|---|
| Detect acute safety signals (intussusception, febrile seizure) within days to weeks [1, 8] | Compare fully vaccinated to never-vaccinated children with active long-term follow-up |
| Generate reporting rates from passive and semi-active surveillance [8] | Provide a denominator of total doses for all outcomes studied |
| Identify rare events when the background rate is known [1] | Measure cumulative or delayed harms from the full schedule |
| Compare vaccinated to a "unvaccinated" group that includes delayed and partially vaccinated children [6] | Control for the healthy-user bias that makes vaccinated children appear healthier |
My call: the VSD is a real surveillance system that has found genuine harms, but it does not answer the question of whether the full childhood vaccine schedule, as a whole, causes net harm or benefit over the long term. That study has never been run. Confidence is high that the VSD detects acute signals, and low that it can assess the cumulative safety of the schedule, because the most informative comparison group was never assembled.
Sources examined 15
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The Vaccine Safety Datalink project
The Vaccine Safety Datalink links computerized vaccination, medical outcome, and covariate data from multiple US HMOs to evaluate vaccine safety, and its methodology was rapidly expanded to a 10-MCO cohort study that found a strong association between rhesus rotavirus vaccinatio…
DOI: 10.1002/pds.613 -
Evaluating Vaccine Safety Through the Vaccine Safety Datalink: A Comprehensive Review of Research Contributions
A comprehensive systematic review of three decades of Vaccine Safety Datalink (VSD) research, categorizing studies into descriptive, analytic, database, and methodological types, summarizing vaccines and populations studied (with influenza as the most examined), outlining method…
DOI: 10.21032/jhis.2025.50.2.113 -
The Pioneering Role of the Vaccine Safety Datalink Project (VSD) to Advance Collaborative Research and Distributed Data Networks
This review describes how the Vaccine Safety Datalink (VSD) pioneered collaborative distributed data networks, evolving from a centralized model to a distributed data model with rapid cycle analysis, and how its methods and infrastructure informed subsequent networks including H…
DOI: 10.13063/2327-9214.1195 -
Inactivated Influenza Vaccine During Pregnancy and Risks for Adverse Obstetric Events
A large multicenter retrospective cohort study using the Vaccine Safety Datalink found that influenza vaccination during pregnancy was not associated with increased risks of adverse obstetric events, with a possible reduction in gestational diabetes and no strong safety signals …
DOI: 10.1097/AOG.0b013e3182a1118a -
Safety of influenza vaccination during pregnancy: A review of subsequent maternal obstetric events and findings from two recent cohort studies
A CDC-sponsored review and two large observational cohort studies (Vaccine Safety Datalink and Pregnancy and Influenza Project) found no evidence that seasonal influenza vaccination during pregnancy increases the risk of gestational hypertension, gestational diabetes, preeclamps…
DOI: 10.1016/j.vaccine.2014.04.021 -
Assessing Potential Confounding and Misclassification Bias When Studying the Safety of the Childhood Immunization Schedule
A multi-site parent survey within the Vaccine Safety Datalink (VSD) network assessed (1) the risk of misclassifying vaccination status in electronic health record data and (2) potential confounding from asthma and other allergy risk factors that vary with vaccination status, to …
DOI: 10.1016/j.acap.2018.03.007 -
Risk of venous thromboembolism following influenza vaccination in adults aged 50 years and older in the Vaccine Safety Datalink
This study investigates the risk of venous thromboembolism (VTE) following influenza vaccination in adults aged 50 years and older, finding no overall increased risk but identifying a heightened risk among current smokers.
DOI: 10.1016/j.vaccine.2017.08.086 -
Vaccine Safety Surveillance Systems: Critical Elements and Lessons Learned in the Development of the US Vaccine Safety Datalink’s Rapid Cycle Analysis Capabilities
A perspective and synthesis detailing the Vaccine Safety Datalink's Rapid Cycle Analysis (RCA) system: its critical design elements, methodological innovations (SPRT/maxSPRT), data infrastructure, real-world safety signal demonstrations, funding framework, and its influence on p…
DOI: 10.3390/pharmaceutics5010168 -
Impact of the introduction of pneumococcal conjugate vaccine on immunization coverage among infants
Introduction of pneumococcal conjugate vaccine did not meaningfully reduce non-PCV immunization coverage by 13 months across four large provider groups, though some delays in timely vaccination occurred, particularly in Group C during hepatitis B birth-dose suspension and early …
DOI: 10.1186/1471-2431-5-43 -
Premature Puberty and Thimerosal-Containing Hepatitis B Vaccination: A Case-Control Study in the Vaccine Safety Datalink
A longitudinal case-control study using Vaccine Safety Datalink data finds a dose-dependent association between organic mercury exposure from Thimerosal-containing hepatitis B vaccines given in the first six months of life and the long-term risk of a premature puberty diagnosis,…
DOI: 10.3390/toxics6040067 -
Additional Recommendations for Use of Tetanus Toxoid, Reduced-Content Diphtheria Toxoid, and Acellular Pertussis Vaccine (Tdap)
Tdap can be given without a minimum interval after tetanus/diphtheria vaccines; Tdap is safe across age groups and broadly recommended, including undervaccinated children (7–10 years), health care personnel, and older adults.
DOI: 10.1542/peds.2011-1752 -
Developing algorithms for identifying major structural birth defects using automated electronic health data
A CDC-funded study updates ICD-9-CM birth defect detection algorithms to ICD-10-CM, validates them through chart review, and applies them across eight Vaccine Safety Datalink sites to describe the prevalence of seven major structural birth defects in 2015–2017 using automated he…
DOI: 10.1002/pds.5177 -
CONTROVERSIES IN IMMUNIZATION PRACTICES: VACCINE SAFETY AND IMPLICATIONS FOR MIDWIFERY PRACTICE
An evidence-based overview for midwives that analyzes vaccine safety, monitoring systems (VAERS and VSD), risk-benefit analysis, decision-making, exemptions, and counseling strategies to address vaccine decisions and safety concerns.
DOI: 10.1016/s1526-9523(01)00212-4 -
MMR vaccine - how effective and how safe?
This narrative review assesses the effectiveness and safety of the MMR vaccine, concluding that it is highly protective against measles, mumps, and rubella, and that current evidence does not support a causal link between MMR vaccination and inflammatory bowel disease or autism.
DOI: 10.1136/dtb.2003.41425 -
Adverse events following HPV vaccine reported to the Vaccine Adverse Event Reporting System
This pharmacovigilance study of 77,909 VAERS reports (2006-2024) finds syncope as the most frequently reported adverse event signal and POTS with the strongest disproportionality signal, with approximately 90% of adverse events occurring within 30 days of HPV vaccination.
DOI: 10.1371/journal.pone.0351652