The Insightful Corner Hub: FDA Cautions: A Deeper Look into the Association of Certain Flu Vaccines, Strokes in the Elderly, and Seizures in Children Post-Coronavirus Vaccination

Executive Summary

In public health practice, vaccine safety monitoring is a cornerstone of immunization policy. While vaccines represent one of the most effective interventions for preventing infectious disease morbidity and mortality globally, rare adverse events occur and must be rigorously investigated, contextualized, and communicated transparently. Recently, FDA and CDC scientists have identified preliminary safety signals in specific populations that warrant closer examination specifically stroke risk in older adults following co-administration of influenza and COVID-19 vaccines, and seizure events in children following COVID-19 vaccination.

This article examines these concerns comprehensively, drawing on regulatory surveillance data, peer-reviewed research, and clinical understanding of immunization safety. The aim is to interpret findings in context for clinicians, public health professionals, and informed readers.

1. Vaccine Safety Surveillance in the United States: An Overview

1.1 The Framework

In the United States, vaccine safety is monitored through multiple, complementary systems:

• Vaccine Adverse Event Reporting System (VAERS): A national passive reporting system co-managed by CDC and FDA. VAERS collects reports of potential adverse events post-vaccination from healthcare providers, patients, and caregivers. Importantly, VAERS data do not establish causation but serve as a signal detection and hypothesis-generation mechanism.
• Vaccine Safety Datalink (VSD): A collaboration between CDC and integrated healthcare systems that conducts near real-time surveillance using electronic health records to identify trends and potential associations.
• Post-licensure Rapid Immunization Safety Monitoring (PRISM): Another large database used for epidemiologic assessment of vaccine safety.
• Clinical Trials and Post-Marketing Studies: Provide structured controlled data before and after licensure.

This multi-layered approach increases the likelihood of identifying rare adverse events while corroborating or disputing signals across independent data sources.

2. Stroke Risk and Vaccination in Older Adults

2.1 What is an Ischemic Stroke?

Ischemic stroke the most common type occurs when blood flow to part of the brain is blocked, usually by a clot, leading to tissue damage. Risk increases with age and comorbidities such as hypertension, diabetes, and atrial fibrillation.

2.2 Preliminary Safety Signal Identified by FDA/CDC

In late 2022 and early 2023, CDC and FDA scientists reported a preliminary safety signal from the Vaccine Safety Datalink (VSD) indicating a possible association between the Pfizer-BioNTech bivalent COVID-19 vaccine and ischemic stroke in adults aged 65 years or older within 21 days following vaccination, compared with a later post-vaccination period. This signal did not emerge for Moderna’s bivalent vaccine, and it remained unconfirmed by multiple other safety systems.

Crucially:

• VSD flagged a potential pattern that merited investigation.
• Other safety systems, including CV-19 VAERS, CMS analyses, and VA databases, did not replicate the signal.
• No change in vaccine recommendations occurred as a result of this specific signal.

2.3 Risk of Stroke With Concomitant Influenza and COVID-19 Vaccines

Several observational analyses including large Medicare cohort studies have examined stroke risk related to co-administration of high-dose or adjuvanted influenza vaccines with bivalent COVID-19 boosters:

• A study including millions of Medicare beneficiaries aged 65+ found no significant increase in stroke following COVID-19 bivalent vaccination alone, but noted slightly elevated risks when influenza vaccines were given at the same visit.
• In absolute terms, when influenza and COVID-19 vaccines were given together, there were approximately 3 extra strokes per 100,000 doses administered compared with later risk windows a very small absolute increase.

Further detailed analysis revealed that the elevated risk signal in these co-administration studies may have been driven primarily by the influenza vaccine, particularly high-dose formulations used for older adults, rather than the COVID-19 vaccine itself.

2.4 Interpretation of Stroke Signals

While any potential risk signal deserves scientific scrutiny, several points are critical:

• Association is not causation: Observed associations in surveillance systems can reflect confounding by underlying health status, seasonal risk factors, or healthcare-seeking behavior.
• Absolute risk remains low: Even when a signal is reported, the excess stroke events per 100,000 vaccinations are small relative to the baseline incidence in elderly populations.
• Infections themselves (influenza, SARS-CoV-2) pose much higher stroke risk: Acute respiratory infections significantly increase thromboembolic events, including strokes, particularly in older adults.
• No change in clinical recommendations: Regulatory agencies continue to endorse influenza and COVID-19 vaccination for eligible adults, including co-administration, because the benefits greatly outweigh the rare risks.

3. Seizures Following COVID-19 Vaccination in Children

3.1 What Types of Seizures Are Observed Post-Vaccination?

A seizure is an electrical disturbance in the brain that can manifest in many ways. Febrile seizures a subset are seizures associated with fever and are common in young children, especially during viral infections. Most febrile seizures are brief and benign.

3.2 CDC’s Findings on Seizures After COVID-19 Vaccination

In CDC’s surveillance and a recent Morbidity and Mortality Weekly Report (MMWR) summary on safety monitoring for COVID-19 vaccines in children aged 6 months to 5 years:

• Eight seizures were reported to VAERS among children in this age group post-COVID-19 vaccination; only one was associated with a high fever.
• Febrile seizures are a known clinical phenomenon in this age group and occur with many illnesses that cause fever — including influenza, respiratory syncytial virus (RSV), and common colds.
• The observed seizure reports were small in number and did not alter CDC recommendations.

CDC and FDA continue to monitor febrile seizures after COVID-19 vaccination, especially among very young children, but current evidence does not suggest a causative relationship beyond what might be expected by coincidence or fever-related events in this age group.

3.3 Influenza Vaccines and Febrile Seizure Risk

Seasonal influenza vaccines have been studied extensively:

• Most influenza vaccine seasons show no significant association with febrile seizures in children.
• Some studies, particularly involving co-administration of influenza vaccine with other routine vaccines (e.g., pneumococcal or DTaP), have observed a small increase in febrile seizures between 12–23 months old, but no permanent neurological harm.

Importantly, children with natural influenza infection are at greater risk of fever and febrile seizures than those receiving vaccination, and flu vaccine protects against severe influenza and its complications.

3.4 Analysis of VAERS Data on New-Onset Seizures

A peer-reviewed analysis of VAERS data found a higher rate of new-onset seizures reported after COVID-19 vaccination compared with influenza vaccines (3.19 vs. 0.09 per 100,000 persons per year). However, researchers emphasize the limitations of VAERS data, including reporting biases, incomplete information, and inability to establish causality.

4. Scientific Context: Interpreting Signals and Risks

4.1 Principle of Pharmacovigilance

In pharmacovigilance (drug and vaccine safety monitoring), a signal is defined as information on a possible causal relationship between an adverse event and a vaccine that is previously unknown or incompletely documented and that warrants further investigation. Signals are preliminary they do not imply confirmed causality. They trigger rigorous epidemiologic and clinical evaluation.

4.2 Rare Events Are Expected

With billions of vaccine doses administered annually, rare health events such as strokes or seizures will occur by coincidence after vaccination because these conditions also occur commonly in the general population. The critical analytic task is discerning whether observed patterns exceed expected background rates:

• Statistical signal detection (in systems like VSD) uses near-real-time data to compare risk windows before and after vaccination.
• Controlled epidemiologic studies (cohort studies, case-control studies) adjust for confounding variables to assess risk.

Both types of data are necessary for comprehensive evaluation.

5. Regulatory and Clinical Implications

5.1 Reporting and Monitoring Continue

Regulators continue to monitor vaccine safety data proactively:

• CDC and FDA scientists publish periodic safety updates.
• Clinicians are reminded to report any suspected adverse events to VAERS.
• Global systems, such as WHO’s VigiBase, provide international safety signal detection.

5.2 Clinical Recommendations Remain Unchanged

To date, neither CDC nor FDA has changed vaccination recommendations for COVID-19 or influenza vaccines because of stroke or seizure signals in specific subgroups. These vaccines have demonstrable benefits:

• Substantial reduction in hospitalizations and deaths from influenza and COVID-19.
• Protection of vulnerable populations, including older adults, infants, and children with underlying conditions.

High-risk individuals (e.g., elderly, immunocompromised) receive preferential guidance, but vaccination remains strongly advised.

6. Communicating With Patients and Communities

Healthcare professionals should communicate about vaccine safety with accuracy and empathy:

• Acknowledge monitoring systems. Explain that rigorous safety surveillance is ongoing and includes multiple checks.
• Clarify risk context. Rare events are possible but must be weighed against the far higher risk of serious disease outcomes from influenza and SARS-CoV-2 infection itself.
• Differentiate coincidence vs. causation. Not every event following vaccination is caused by vaccination. Baseline incidence and timing are critical.

7. Conclusion

The FDA and CDC have identified some preliminary safety signals regarding stroke in elderly adults receiving combined influenza and COVID-19 vaccines, and seizure reports in children post-COVID-19 vaccination. However:

• Evidence does not establish causality. Signals are hypotheses requiring further study.
• Absolute risks are low. Even observed increases are small in the context of millions of administered doses.
• Benefits of vaccination remain substantial. Vaccines prevent severe disease, hospitalization, and death.

Ongoing safety monitoring, transparent communication, and nuanced interpretation of pharmacovigilance data remain essential to sustaining public trust and achieving optimal health outcomes.