Within Anecdotes
What safety reports can and cannot prove
Safety reports can reveal warning signals, but raw reports cannot prove that a vaccine or drug caused an event.
On this page
- Why raw reports are not confirmed causes
- How safety systems look for signals
- The mistake of counting reports as risk rates
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Introduction
Adverse-event reporting systems are often misunderstood because they collect stories of things that happened after a drug or vaccine was used. To many people, a report seems like evidence that the product caused the outcome. In reality, these systems are designed to cast a very wide net. They record possible safety concerns so that experts can investigate them later. A report can show that two events occurred in sequence; it does not, by itself, show that one caused the other. Public-health agencies, regulators and researchers repeatedly warn that raw reports are a starting point for investigation, not a verdict on causation. [CDC]cdc.govEstablishing a causal relationship requires rigorous…Read more… [VAERS]vaers.hhs.govVAERSGuide to Interpreting VAERS DataHHS.gov8 May 2025 — When reviewing data from VAERS, please keep in mind the following limitations: VAERS is a passive reporting system, m…
This distinction matters because misconceptions often arise when individual reports are treated as proof. A dramatic case can feel more persuasive than statistical evidence, especially when the event is serious or emotionally charged. The result is a common error: turning a surveillance signal into a causal claim before the evidence has been examined.
Why raw reports are not confirmed causes
The key misunderstanding is the difference between a temporal association and a causal relationship.
An adverse-event report typically records that a health event happened after a medicine or vaccine was given. However, many illnesses, injuries and deaths occur every day in large populations regardless of medical treatment. Some of those events will inevitably happen shortly after vaccination or medication use simply by chance. Reporting systems are designed to capture those events rather than filtering them out at the reporting stage. [Canada]canada.caCanadaAdverse events following immunization (AEFI) reporting form22 Apr 2025 — A causal relationship does not need to be proven, and subm… [ScienceDirect]sciencedirect.comAn approach to death as an adverse event following…by MS Gold · 2016 · Cited by 32 — An adverse event following immunization (AEFI) is…
For this reason, agencies that run reporting databases explicitly state that a submitted report does not prove causation. The US Vaccine Adverse Event Reporting System (VAERS), for example, notes that its data alone cannot determine whether a vaccine caused a reported event. Establishing causation requires additional scientific investigation. [CDC]cdc.govchapter 21 vaersReporting rates can provide context for AEs observed after vaccination. · Quality of information.Read more… [VAERS]vaers.hhs.govFAQs - HHS.govOne of the main limitations of VAERS data is that it cannot determine if the vaccine caused the reported adverse event. Thi…
Several features of these databases make simple causal conclusions unreliable:
- Reports can be incomplete. Early reports may lack medical records, laboratory findings or alternative explanations. [VAERS]vaers.hhs.govAdverse Event Reporting System (VAERS)VAERS is co-sponsored by the Centers for Disease Control and Prevention (CDC), and the Food and Dru…
- Reporting is intentionally broad. Healthcare professionals and members of the public are often encouraged to report significant events even when they are unsure whether the product was responsible. Canada [World Health Organization]who.intWorld Health OrganizationSerious AEFIAn adverse event following immunization (AEFI) is defined as any untoward medical occurrence followi…
- Some events would have happened anyway. A heart attack, stroke or death occurring after treatment does not automatically mean the treatment caused it. Investigators must compare observed events with expected background rates. [vaccinesafety.edu]vaccinesafety.eduMonitoring Vaccine SafetyCausal relationships between vaccines and adverse events can be established by demonstrating an increased risk o…
- Reports may contain errors or duplicates. Large safety databases require extensive review because duplicate records and reporting inconsistencies can distort apparent totals. [arXiv]arxiv.orgarXivA Scalable Predictive Modelling Approach to Identifying Duplicate Adverse Event Reports for Drugs and VaccinesMarch 31, 2025…
The crucial point is that a report is evidence that a concern exists, not evidence that the concern has been confirmed.
How safety systems look for signals
Safety reporting systems are best understood as early-warning networks.
Their purpose is to detect unusual patterns that might otherwise go unnoticed. If many reports describe a similar event after exposure to the same product, investigators may identify a potential signal worthy of further study. The signal is a hypothesis, not a conclusion. [ScienceDirect]sciencedirect.comAn approach to death as an adverse event following…by MS Gold · 2016 · Cited by 32 — An adverse event following immunization (AEFI) is… [PubMed]pubmed.ncbi.nlm.nih.govVAERS data interpreted alone or out of context canPubMedSafety monitoring in the Vaccine Adverse Event Reporting…by TT Shimabukuro · 2015 · Cited by 713 — Generally, VAERS data cannot…
Once a signal appears, researchers use other methods to test whether a real causal relationship exists. These methods can include:
- Comparing rates of the event among exposed and unexposed groups.
- Analysing large healthcare databases.
- Reviewing medical records and clinical details.
- Examining whether the timing and biological mechanism make sense.
- Assessing whether the pattern is consistent across multiple datasets. [vaccinesafety.edu]vaccinesafety.eduMonitoring Vaccine SafetyCausal relationships between vaccines and adverse events can be established by demonstrating an increased risk o… [World Health Organization]who.intWorld Health OrganizationSerious AEFIAn adverse event following immunization (AEFI) is defined as any untoward medical occurrence followi…
The World Health Organization has developed formal causality-assessment frameworks because determining whether an adverse event was caused by a vaccine or medicine requires structured evaluation rather than simple counting. Investigators examine competing explanations, existing scientific evidence, timing, medical plausibility and population-level data before reaching conclusions. [World Health Organization]who.intWorld Health OrganizationSerious AEFIAn adverse event following immunization (AEFI) is defined as any untoward medical occurrence followi… [MDPI]mdpi.comThe WHO Algorithm for Causality Assessment of Adverse…by P Bellavite · 2024 · Cited by 7 — The WHO algorithm assesses whether there is…
A useful way to think about reporting systems is that they are smoke detectors. A smoke alarm may indicate a fire, but the alarm itself is not proof that a fire exists. Someone still has to investigate.
The mistake of counting reports as risk rates
One of the most common misconceptions is to treat the number of reports as the number of injuries caused by a product.
This approach sounds intuitive but ignores how surveillance databases work. A raw count of reports lacks the information needed to calculate risk accurately. For example, a large number of reports may simply reflect the fact that millions of people received a treatment. Without knowing how many people were exposed and how often the same event occurs normally, the report count alone is difficult to interpret. [CDC]wonder.cdc.govAdverse Events Reporting System (VAERS) HelpKey considerations and limitations of VAERS data: The number of reports alone cannot be inter…
Reporting behaviour also changes over time. Media coverage, lawsuits, public concern and heightened awareness can increase reporting even when the underlying risk has not changed. Researchers refer to this as stimulated reporting. Passive reporting systems are also affected by under-reporting, meaning some events never enter the database at all. These factors make raw totals unsuitable as direct measures of risk. [PMC]pmc.ncbi.nlm.nih.govPMCSafety monitoring in the Vaccine Adverse Event Reporting…by TT Shimabukuro · 2015 · Cited by 697 — Like all spontaneous public heal… [ScienceDirect]sciencedirect.comVAERS data should be interpreted with caution. Limitations include reporting bias (e.g., underreporting and stimulated reporting)…Read… [PMC]pmc.ncbi.nlm.nih.govPMCThe reporting sensitivity of the Vaccine Adverse EventPMCby ER Miller · 2020 · Cited by 55 — Underreporting is a limitation common to passive surveillance systems, including the Vaccine Adver…
A simple example illustrates the problem. If ten million people receive a vaccine and one thousand adverse-event reports are submitted, that figure alone says little. Some reports may be unrelated coincidences, some may describe the same case, some may be incomplete, and many genuine events may never be reported. The number of reports therefore cannot be treated as a count of confirmed injuries or used as a reliable estimate of risk. [VAERS]vaers.hhs.govHHS.gov8 May 2025 — The number of reports alone cannot be interpreted as evidence of a causal association between a vaccine and an advers… [PMC]pmc.ncbi.nlm.nih.govPMCImprovement in the Analysis of Vaccine Adverse EventPMCby L Zhao · 2020 · Cited by 13 — However, not all AEs are reported to VAERS. Therefore, there is a potential under-reporting bias in V…
How reports become false proof in public debate
The path from report to misconception is often straightforward.
A person searches a public database, finds a large number of reports linked to a particular product, and concludes that the reports demonstrate harm. The apparent simplicity is appealing: if thousands of events were reported, surely the product caused thousands of events. Yet this skips the entire investigative process that the reporting system was built to support. [Bloomberg School of Public Health]publichealth.jhu.eduwhat vaers is and isntBloomberg School of Public HealthWhat VAERS Is (And Isn't) | Johns Hopkins3 May 2022 — While VAERS cannot determine whether an adverse ev… [ScienceDirect]sciencedirect.comScienceDirectSafety monitoring in the Vaccine Adverse Event Reporting…by TT Shimabukuro · 2015 · Cited by 713 — Generally, VAERS data…
This misunderstanding is especially powerful because reports resemble personal testimonies. They are concrete, emotional and memorable. A database containing thousands of such accounts can feel like overwhelming evidence even when the entries have not been verified as caused by the product. The emotional weight of the stories can overshadow the methodological limitations of the dataset.
Public-health experts have repeatedly noted that misuse of raw adverse-event databases can generate misleading claims. The problem is not that the reports are worthless; the problem is treating preliminary surveillance data as if it were the final result of scientific investigation. [Bloomberg School of Public Health]publichealth.jhu.eduwhat vaers is and isntBloomberg School of Public HealthWhat VAERS Is (And Isn't) | Johns Hopkins3 May 2022 — While VAERS cannot determine whether an adverse ev…
What safety reports can and cannot prove
Safety-report databases serve an essential role in modern pharmacovigilance, the monitoring of medicine and vaccine safety after approval. They can reveal unexpected patterns, identify rare problems and trigger investigations that lead to genuine discoveries. Several important safety issues have first emerged as reporting signals before being confirmed through further study. [Bloomberg School of Public Health]publichealth.jhu.eduwhat vaers is and isntBloomberg School of Public HealthWhat VAERS Is (And Isn't) | Johns Hopkins3 May 2022 — While VAERS cannot determine whether an adverse ev…
What they cannot do is transform every reported sequence into a demonstrated cause-and-effect relationship. A report can suggest a question. It cannot, by itself, answer that question.
Understanding this distinction helps prevent a common misconception: confusing evidence that something deserves investigation with evidence that the case has already been proven. In safety surveillance, the report is the beginning of the inquiry, not the end. CDC [World Health Organization]who.intWorld Health OrganizationSerious AEFIAn adverse event following immunization (AEFI) is defined as any untoward medical occurrence followi…
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Endnotes
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Source: cdc.gov
Link: https://www.cdc.gov/vaccine-safety-systems/vaers/index.htmlSource snippet
Establishing a causal relationship requires rigorous...Read more...
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Source: canada.ca
Link: https://www.canada.ca/en/public-health/services/immunization/reporting-adverse-events-following-immunization.htmlSource snippet
CanadaAdverse events following immunization (AEFI) reporting form22 Apr 2025 — A causal relationship does not need to be proven, and subm...
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Source: sciencedirect.com
Link: https://www.sciencedirect.com/science/article/abs/pii/S0264410X15016382Source snippet
An approach to death as an adverse event following...by MS Gold · 2016 · Cited by 32 — An adverse event following immunization (AEFI) is...
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Source: vaers.hhs.gov
Title: VAERSGuide to Interpreting VAERS Data
Link: https://vaers.hhs.gov/data/dataguide.htmlSource snippet
HHS.gov8 May 2025 — When reviewing data from VAERS, please keep in mind the following limitations: VAERS is a passive reporting system, m...
Published: May 2025
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Source: cdc.gov
Title: chapter 21 vaers
Link: https://www.cdc.gov/surv-manual/php/table-of-contents/chapter-21-vaers.htmlSource snippet
Reporting rates can provide context for AEs observed after vaccination. · Quality of information.Read more...
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Source: vaccinesafety.edu
Link: https://www.vaccinesafety.edu/monitoring-vaccine-safety/Source snippet
Monitoring Vaccine SafetyCausal relationships between [vaccines]({{ 'vaccines/' | relative_url }}) and adverse events can be established by demonstrating an increased risk o...
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Source: arxiv.org
Link: https://arxiv.org/abs/2504.03729Source snippet
arXivA Scalable Predictive Modelling Approach to Identifying Duplicate Adverse Event Reports for Drugs and VaccinesMarch 31, 2025...
Published: March 31, 2025
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Source: sciencedirect.com
Link: https://www.sciencedirect.com/science/article/abs/pii/S0264410X15006209Source snippet
VAERS data should be interpreted with caution. Limitations include reporting bias (e.g., underreporting and stimulated reporting)...Read...
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Source: sciencedirect.com
Link: https://www.sciencedirect.com/science/article/abs/pii/S0264410X15009822Source snippet
ScienceDirectSafety monitoring in the Vaccine Adverse Event Reporting...by TT Shimabukuro · 2015 · Cited by 713 — Generally, VAERS data...
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Source: mdpi.com
Link: https://www.mdpi.com/2077-0383/13/23/7291Source snippet
The WHO Algorithm for Causality Assessment of Adverse...by P Bellavite · 2024 · Cited by 7 — The WHO algorithm assesses whether there is...
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Source: sciencedirect.com
Link: https://www.sciencedirect.com/science/article/abs/pii/S0264410X99001322Source snippet
The study of vaccine safety is complicated by underreporting, erroneous reporting...Read more...
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Source: pmc.ncbi.nlm.nih.gov
Link: https://pmc.ncbi.nlm.nih.gov/articles/PMC4632204/Source snippet
PMCSafety monitoring in the Vaccine Adverse Event Reporting...by TT Shimabukuro · 2015 · Cited by 697 — Like all spontaneous public heal...
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Source: pmc.ncbi.nlm.nih.gov
Title: PMCThe reporting sensitivity of the Vaccine Adverse Event
Link: https://pmc.ncbi.nlm.nih.gov/articles/PMC11252891/Source snippet
PMCby ER Miller · 2020 · Cited by 55 — Underreporting is a limitation common to passive surveillance systems, including the Vaccine Adver...
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Source: pmc.ncbi.nlm.nih.gov
Title: PMCImprovement in the Analysis of Vaccine Adverse Event
Link: https://pmc.ncbi.nlm.nih.gov/articles/PMC8054210/Source snippet
PMCby L Zhao · 2020 · Cited by 13 — However, not all AEs are reported to VAERS. Therefore, there is a potential under-reporting bias in V...
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Source: sciencedirect.com
Title: VAERS data interpreted alone or out of context can lead to erroneous
Link: https://www.sciencedirect.com/science/article/abs/pii/S0264410X12014181Source snippet
Causality assessment of adverse events reported to the...by AM Loughlin · 2012 · Cited by 46 — Generally, VAERS data cannot be used to d...
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Source: sciencedirect.com
Link: https://www.sciencedirect.com/science/article/abs/pii/S0264410X13011997Source snippet
Assessment of causality of individual adverse events...by AE Tozzi · 2013 · Cited by 131 — This proposed tool should support the classif...
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Source: canada.ca
Link: https://www.canada.ca/en/public-health/services/publications/healthy-living/canadian-immunization-guide-part-2-vaccine-safety/adverse-events-following.htmlSource snippet
well as when and how to report...
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Source: wonder.cdc.gov
Link: https://wonder.cdc.gov/wonder/help/vaers.htmlSource snippet
Adverse Events Reporting System (VAERS) HelpKey considerations and limitations of VAERS data: The number of reports alone cannot be inter...
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Source: vaers.hhs.gov
Link: https://vaers.hhs.gov/faq.htmlSource snippet
FAQs - HHS.govOne of the main limitations of VAERS data is that it cannot determine if the vaccine caused the reported adverse event. Thi...
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Source: who.int
Link: https://www.who.int/groups/global-advisory-committee-on-vaccine-safety/topics/aefi/serious-aefiSource snippet
World Health OrganizationSerious AEFIAn adverse event following immunization (AEFI) is defined as any untoward medical occurrence followi...
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Source: pubmed.ncbi.nlm.nih.gov
Title: VAERS data interpreted alone or out of context can
Link: https://pubmed.ncbi.nlm.nih.gov/26209838/Source snippet
PubMedSafety monitoring in the Vaccine Adverse Event Reporting...by TT Shimabukuro · 2015 · Cited by 713 — Generally, VAERS data cannot...
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Source: publichealth.jhu.edu
Title: what vaers is and isnt
Link: https://publichealth.jhu.edu/2022/what-vaers-is-and-isntSource snippet
Bloomberg School of Public HealthWhat VAERS Is (And Isn't) | Johns Hopkins3 May 2022 — While VAERS cannot determine whether an adverse ev...
Published: May 2022
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Source: who.int
Link: https://www.who.int/publications/i/item/9789241516990Source snippet
World Health OrganizationCausality assessment of an adverse event following...16 Apr 2021 — Causality assessment of an adverse event fol...
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Source: pmc.ncbi.nlm.nih.gov
Link: https://pmc.ncbi.nlm.nih.gov/articles/PMC9805973/Source snippet
adverse event reporting system (VAERS) - PMC - NIHby OA Almadani · 2022 · Cited by 10 — The aim of this study was to investigate VAERS pa...
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Source: pubmed.ncbi.nlm.nih.gov
Link: https://pubmed.ncbi.nlm.nih.gov/30026925/Source snippet
World Health Organization (WHO)'s causality...by J Puliyel · 2018 · Cited by 54 — The World Health Organisation (WHO) has recently revis...
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Source: vaers.hhs.gov
Link: https://vaers.hhs.gov/Source snippet
Adverse Event Reporting System (VAERS)VAERS is co-sponsored by the Centers for Disease Control and Prevention (CDC), and the Food and Dru...
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Source: vaers.hhs.gov
Link: https://vaers.hhs.gov/data.htmlSource snippet
HHS.gov8 May 2025 — The number of reports alone cannot be interpreted as evidence of a causal association between a vaccine and an advers...
Published: May 2025
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Source: Wikipedia
Title: Vaccine Adverse Event Reporting System
Link: https://en.wikipedia.org/wiki/Vaccine_Adverse_Event_Reporting_SystemSource snippet
Vaccine Adverse Event Reporting SystemVAERS is a postmarketing surveillance program, collecting information about adverse events (poss...
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Source: Wikipedia
Title: Vaccine adverse event
Link: https://en.wikipedia.org/wiki/Vaccine_adverse_eventSource snippet
Vaccine adverse eventA vaccine adverse event (VAE), sometimes referred to as a vaccine injury, is an adverse event believed to have be...
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Source: science.org
Title: antivaccine activists use government database side effects scare public
Link: https://www.science.org/content/article/antivaccine-activists-use-government-database-side-effects-scare-publicSource snippet
Antivaccine activists use a government database on side...26 May 2021 — To counter misinterpretation of its data, the VAERS website prom...
Published: May 2021
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Source: fda.gov
Title: vaccine adverse event reporting system vaers questions and answers
Link: https://www.fda.gov/vaccines-blood-biologics/vaccine-adverse-events/vaccine-adverse-event-reporting-system-vaers-questions-and-answersSource snippet
VAERS reports generally cannot be used to determine if a vaccine caused or contributed to an adverse event or illness. Some events may oc...
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Source: health.qld.gov.au
Title: adverse event
Link: https://www.health.qld.gov.au/clinical-practice/guidelines-procedures/diseases-infection/immunisation/adverse-eventSource snippet
s following immunisation (AEFI) are a notifiable condition.Read more...
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Source: downloads.regulations.gov
Title: attachment 3
Link: https://downloads.regulations.gov/CDC-2022-0111-84601/attachment_3.pdfSource snippet
VAERS) are subject to multiple limitations, including underreporting, reporting of temporal...Read more...
Additional References
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Source: adultvaccinesnow.org
Link: https://adultvaccinesnow.org/resources/cdc-releases-data-showing-safety-of-covid-vaccines/Source snippet
CDC Releases Data Showing Safety of COVID VaccinesFirst, VAERS analyses are based on passive surveillance, and reporting biases are possi...
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Source: pch.health.wa.gov.au
Link: https://pch.health.wa.gov.au/For-health-professionals/Referrals-to-PCH/Prereferral-guidelines/Adverse-event-following-immunisationSource snippet
event following immunisationAn adverse event following immunisation (AEFI) is defined by the World Health Organization as any 'untoward m...
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Source: factcheck.org
Link: https://www.factcheck.org/2023/06/scicheck-what-vaers-can-and-cant-do-and-how-anti-vaccination-groups-habitually-misuse-its-data/Source snippet
What VAERS Can and Can't Do, and How Anti-Vaccination...6 Jun 2023 — “VAERS data interpreted alone or out of context can lead to erroneo...
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Source: digital.ahrq.gov
Title: electronic support public health vaccine adverse event reporting system
Link: https://digital.ahrq.gov/ahrq-funded-projects/electronic-support-public-health-vaccine-adverse-event-reporting-systemSource snippet
Adverse Event Reporting System (ESP:VAERS)The Electronic Support for Public Health - Vaccine Adverse Event Reporting System (ESP: VAERS)...
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Source: kcra.com
Link: https://www.kcra.com/article/fact-check-viral-posts-misuse-vaers-data-to-make-false-claims-about-covid-19-vaccines/36068010Source snippet
The fact that VAERS cannot assess causality is not its only weakness. “It's the worst possible data that you could...
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Source: facebook.com
Link: https://www.facebook.com/DurhamHealthNC/posts/if-vaccines-are-safe-why-are-there-harmful-effects-reported-in-vaers-its-importa/1162242512582508/Source snippet
The biggest limitation is it usually cannot help us assess causation, it provides signals.Read more...
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Source: ajmc.com
Title: challenges in the us vaccine adverse event reporting system
Link: https://www.ajmc.com/view/challenges-in-the-us-vaccine-adverse-event-reporting-systemSource snippet
Challenges in the US Vaccine Adverse Event Reporting...14 Nov 2023 — A study published in The BMJ found that VAERS may not be meeting it...
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Source: health.nsw.gov.au
Title: nsw.gov.au Adverse events following immunisation (AEFI)
Link: https://www.health.nsw.gov.au/immunisation/Pages/aefi.aspxSource snippet
events following immunisation (AEFI) - NSW Health10 Nov 2023 — It does not necessarily have a causal relationship with the vaccine". AEFI...
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Source: fda.gov
Title: covid 19 vaccine safety surveillance
Link: https://www.fda.gov/vaccines-blood-biologics/safety-availability-biologics/covid-19-vaccine-safety-surveillanceSource snippet
COVID-19 Vaccine Safety Surveillance10 Jan 2025 — The limitations of data mining include but are not limited to the following: Results ma...
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Source: sahealth.sa.gov.au
Link: https://www.sahealth.sa.gov.au/wps/wcm/connect/public%2Bcontent/sa%2Bhealth%2Binternet/clinical%2Bresources/clinical%2Bprograms%2Band%2Bpractice%2Bguidelines/immunisation%2Bfor%2Bhealth%2Bprofessionals/immunisation%2Bsection%2Breporting/adverse%2Bevent%2Bfollowing%2BimmunisationSource snippet
Most reactions, such as low-grade fever and pain at...Read more...
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