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A new study led by Tanya R. Myers, Ph.D., of the U.S. Centers for Disease Control and Prevention (CDC), published in Vaccine, presents the most comprehensive summary yet of the CDC’s V-safe active surveillance program, analyzing reports from 9.61 million Americans who completed at least one health survey after receiving a COVID-19 vaccine between December 2020 and June 2023.
The descriptive surveillance study found that short-term health impacts — including inability to perform normal activities or attend work or school — were most common during the first week after vaccination, particularly after second mRNA doses and more frequently following Moderna than Pfizer-BioNTech vaccination.
Emergency department visits and hospitalizations were reported infrequently. However, because V-safe is a voluntary, self-reported surveillance system involving only 3.6% of vaccinated Americans, the study cannot determine whether reported medical events were caused by vaccination, nor can it estimate the true incidence of adverse events in the broader U.S. population.
A massive active surveillance effort
Unlike the passive Vaccine Adverse Event Reporting System (VAERS), V-safe actively invited vaccinated individuals to complete smartphone-based health surveys following COVID-19 vaccination.
Between December 2020 and June 2023, 9,609,282 participants completed at least one survey, with 95% receiving an mRNA vaccine.
Most participants were women (62%), adults ages 25-64 years, and predominantly non-Hispanic White. The study was conducted as a public health surveillance activity rather than human subjects research and summarizes participant-reported symptoms, healthcare utilization and health perceptions across multiple vaccine doses.
Unlike randomized clinical trials, V-safe was designed primarily to characterize short-term reactogenicity — expected post-vaccination symptoms such as fatigue, fever, soreness and temporary disruption of daily activities.
It was not designed to diagnose or adjudicate rare serious adverse events, which would instead be evaluated through complementary surveillance systems such as VAERS and the Vaccine Safety Datalink.
What the study found
The findings largely reinforce previous CDC reports. Health impacts were concentrated during the first week after vaccination, especially following second mRNA doses. Adults receiving Moderna generally reported higher rates of temporary disruption to daily activities than Pfizer recipients.
Seeking medical care during the first week remained uncommon, occurring in approximately 0.8%-1.2% of adults after mRNA vaccination, while emergency department visits and hospitalizations were reported infrequently.
Children experienced similar or slightly fewer short-term health impacts than adults, and pregnant participants demonstrated reactogenicity patterns comparable to the general adult population.
Importantly, the authors emphasize that V-safe did not collect diagnoses explaining medical visits or hospitalizations. Consequently, healthcare encounters reported in surveys cannot be assumed to have been caused by vaccination, and the platform cannot determine whether a reported hospitalization was vaccine-related.
Hidden signals in the supplement
The supplementary appendix reveals findings that deserve careful interpretation. At six weeks following dose 1, among the relatively small proportion of participants who continued completing surveys (1.4% of Pfizer recipients and 2.5% of Moderna recipients), 21.8% and 13.6%, respectively, reported feeling worse than before vaccination.
Among those respondents, 17.9% of Pfizer and 10.5% of Moderna participants believed their health problems might be related to vaccination. However, these observations arise from exceptionally low follow-up participation and are therefore highly susceptible to response bias.
The investigators themselves caution against overinterpreting these findings because participants who experienced ongoing symptoms may have been more likely to continue responding than those who recovered or disengaged from the platform.
Another important observation emerges from the supplemental data: participation steadily shifted toward older, highly vaccinated individuals as successive booster doses were administered, further limiting the generalizability of later-dose findings to the broader U.S. population.
TrialSite reflection
One question this study cannot answer is perhaps the one patients care about most: how many people developed persistent, life-altering health problems after COVID-19 vaccination?
Based on TrialSite News’ ongoing review of hundreds of peer-reviewed studies, pharmacovigilance reports, registries and mechanistic investigations worldwide, we estimate that approximately 0.2%-0.8% of mRNA vaccine recipients may have developed a chronic post-COVID-19 vaccination syndrome (PVS) or related persistent condition — a figure that remains controversial and requires further prospective validation.
Even at the low end of that estimate, applied to roughly 270 million Americans who completed a primary mRNA vaccination series and at least one booster, the number of potentially affected individuals would reach hundreds of thousands, with the upper estimate exceeding two million vaccine-injured Americans.
Many report debilitating fatigue, dysautonomia, cognitive impairment, neuropathy, exercise intolerance or other multisystem symptoms, yet few have access to formal diagnostic criteria, evidence-based treatments or recognition within mainstream medicine.
This V-safe analysis neither proves nor disproves that estimate.
However, its striking loss of long-term follow-up — with participation falling to approximately 1-3% by six weeks after the first dose — means the surveillance system was not positioned to meaningfully evaluate persistent syndromes.
Consequently, while the study provides valuable evidence regarding short-term reactogenicity, it leaves unresolved the central scientific and humanitarian question: how many people experienced chronic illness after vaccination, and what biological mechanisms underlie those cases?
Emerging work from groups such as Yale and others suggests that a small but real subset of patients may have identifiable immunologic abnormalities deserving rigorous investigation rather than dismissal.
Red flags & limitations
Despite its unprecedented size, V-safe has important methodological limitations.
First, V-safe represents a voluntary convenience sample, not a representative U.S. population. Only 3.6% of vaccinated Americans enrolled, introducing substantial selection bias.
Participants were disproportionately female, White, middle-aged, smartphone users and individuals motivated to participate in repeated surveys. Survey completion declined dramatically over time.
Most notably, by six weeks after dose 1, only 1%-3% of eligible participants remained engaged, creating substantial survivorship and attrition bias for longer-term observations. This makes interpretation of health perceptions beyond the immediate post-vaccination period particularly challenging.
Second, all outcomes were self-reported and generally not independently verified through medical records.
Third, because V-safe primarily measured symptoms and healthcare utilization — not medically confirmed diagnoses — it was not designed to estimate the incidence of rare serious adverse events or establish causality.
Finally, the absence of an unvaccinated comparison group precludes meaningful assessment of background event rates.
From the TrialSite News Human Consequence Index perspective, V-safe represents an important innovation in engaging the public in active vaccine safety monitoring.
However, participation depended on smartphone access, digital literacy and sustained engagement, potentially excluding important demographic groups.
Regarding the Pluralism Index, the investigators openly acknowledge major methodological limitations, although reported outcomes were not independently adjudicated and alternative explanations for reported health events were not systematically investigated within the platform.
Conclusion & implications
This landmark publication demonstrates both the strengths and limitations of the CDC’s largest active COVID-19 vaccine surveillance program. The data support previous observations that transient post-vaccination reactogenicity was common while emergency medical care was reported infrequently, yet with the numbers involved, noticeable regardless within the V-safe population.
At the same time, the study’s voluntary enrollment, demographic imbalance, dramatic participant attrition and inability to establish causality underscore why V-safe should be viewed as one component of vaccine pharmacovigilance rather than definitive evidence regarding long-term vaccine safety.
For future pandemic responses, the study offers an important lesson: digital active surveillance can rapidly collect real-world safety information from millions of participants, but maintaining representative participation over time remains essential if such systems are to provide robust evidence beyond the immediate post-vaccination period.
In summary, this study provides moderate-strength descriptive evidence regarding short-term post-vaccination experiences among millions of V-safe participants. It should not be interpreted as establishing either the absence or presence of rare or long-term vaccine-related adverse effects, questions that require different epidemiologic designs and complementary surveillance systems.
TrialSite Evidence Strength Indicator (ESI 2.0)
- Methodological Rigor & Risk of Bias: 6.0/10
- Exceptionally large surveillance dataset but fundamentally limited by voluntary enrollment, self-reporting, attrition, and lack of controls.
- Consistency & Effect Size: 8.5/10
- Findings are internally consistent and generally align with previous CDC reports and international active surveillance systems.
- External Validity & Applicability: 5.5/10
- Limited by demographic imbalance, convenience sampling, and declining participation over time.
- Human Consequence Index: 8.0/10
- Innovative public engagement in active vaccine safety monitoring, though digital access requirements may have excluded some populations.
- Pluralism Index: 7.0/10
- Authors transparently discuss limitations, but reported outcomes were not independently adjudicated within the surveillance system.
- Transparency & Disclosure: 8.5/10
- Methods, supplementary analyses, and principal limitations are clearly disclosed.
- Summary (Weighted View): 7.1/10 (71%)
- Strong descriptive surveillance evidence for short-term reactogenicity, but substantial methodological constraints limit causal inference and estimation of rare adverse event incidence.
Originally published by TrialSite News.
