FDA investigations found higher rates of stroke in older adults who received the COVID-19 vaccine and a flu shot, and a slightly increased risk of seizures among young children after COVID vaccination.
Earlier this year, the CDC's Vaccine Safety Datalink (VSD)—a real-time vaccine surveillance system — identified a potential link between Pfizer's COVID-19 bivalent vaccine and an increased risk of ischemic stroke in adults 65 or older.
Further investigations of the safety signal found that most confirmed stroke cases occurred when the COVID-19 booster and high dose or adjuvant flu vaccine were given at the same time.
Now, a new FDA analysis evaluating Medicare records of adults 65 years and older has found a slightly increased risk of non-hemorrhagic stroke (NHS) among older adults who received the COVID-19 and high-dose/adjuvanted influenza vaccines simultaneously. According to the study data, combining the flu shot with a COVID vaccination was linked to one to two strokes per every 100,000 doses.
However, the analysis also showed that older adults receiving the COVID vaccine alone did not have an increased stroke risk, unless they were over 85.
In addition, the researchers found a slightly elevated risk of stroke among those who only received an influenza vaccine, which suggests that flu shots likely drove the stroke risks found in the analysis.
In light of these findings, which appear as a preprint awaiting peer review, the authors say the benefits of COVID-19 vaccination among older adults outweigh the possible stroke risks. However, they also suggest the need for additional investigations into the safety of high dose/adjuvanted influenza vaccines.
According to a CNN report, Dr. Peter Marks, head of FDA's Center for Biologics Evaluation and Research, said, "If you want to minimize the chance of interactions and minimize confusing the side effects from one with another, you wait about two weeks between the vaccines."
Vaccination and seizures in children
In a second FDA analysis, which included 4,102,016 participants aged six months to 17 years, researchers found that children aged 12 to 17 years old who received Pfizer's BNT162b2 vaccine had a higher risk of myocarditis or pericarditis, two types of heart inflammation.
These results aligned with previous studies that found the same associations. For example, 2022 research involving 23.1 million Nordic residents found that people who received Pfizer's BNT162b2 and Moderna's mRNA-1273 COVID shots had a higher risk of myocarditis within 28 days after vaccination than those who were not vaccinated.
This risk was highest among 16- to 24-year-old males who received a second vaccine dose.
In addition, the FDA analysis found 2- to 5-year-old children had a slightly higher risk of experiencing a seizure after COVID vaccination when compared to 2020 base seizure rates. However, the researchers did not observe this association when they compared the data to seizure rates in 2022.
Fevers from common childhood infections can cause seizures, and vaccinations can also lead to fevers. The study's authors suggest differences in infection rates between 2020 and 2022 could explain why they didn't see links between COVID-19 vaccination and seizures when comparing the study data to 2022. They note that because of less socialization during COVID lockdowns, children weren't exposed to as many colds and flu viruses and didn't get sick as often.
However, the authors wrote, "The myocarditis or pericarditis signals are consistent with previously published reports. The new signal detected for seizures/convulsions among younger children should be further investigated in a robust epidemiological study with better confounding adjustment."
The FDA authorized emergency use of Moderna and Pfizer COVID-19 vaccines for children six months and older in June 2022.
Other rare side effects possibly associated with COVID-19 vaccines
Aside from a slightly higher risk of myocarditis in teens and young adults, there's also emerging interest among the scientific community about whether COVID vaccines can lead to persistent post-vaccination symptoms, including headaches, severe fatigue, and abnormal heart rate and blood pressure. This collection of symptoms, coined "Long-Vax," appears to be rare. However, some scientists say it's a real phenomenon that needs more research.
Scientists have also found associations between COVID-19 vaccines and Bell's palsy and altered menstrual cycles. In addition, while some research, including a 2022 study published in Open Life Sciences, suggests a possible connection between COVID vaccines and blood clots, a later FDA study did not find evidence of a direct association.
Unrelated to adverse effects, a study published on October 23 in JAMA Pediatrics found that vaccinated children were infectious for the same length of time as children who didn't receive the COVID-19 vaccine. Because of the findings, the authors suggest that school vaccine mandates are largely unnecessary.
According to the CDC, COVID-19 vaccines are effective and proven safe through evaluations of tens of thousands of participants in clinical trials, and the benefits of vaccination continue to outweigh any potential risks.
The FDA and CDC continue to monitor COVID-19 vaccine safety signals to detect and investigate any potential adverse effects. For example, the CDC is investigating vaccine-related myocarditis in an ongoing study. In addition, the CDC and FDA monitor COVID-19 safety through active and passive surveillance systems, including the Vaccine Adverse Event Reporting System (VAERS) and V-safe system.
- MedRxiv. Evaluation of Stroke Risk Following COVID-19 mRNA Bivalent Vaccines Among U.S. Adults Aged ≥65 Years.
- CNN. Covid shots may slightly increase risk of stroke in older adults, particularly when administered with certain flu vaccines.
- MedRxiv. Safety of Monovalent BNT162b2 (Pfizer-BioNTech), mRNA-1273 (Moderna), and NVX-CoV2373 (Novavax) COVID-19 Vaccines in US Children Aged 6 months to 17 years.
- FDA. Coronavirus (COVID-19) Update: FDA Authorizes Moderna and Pfizer-BioNTech COVID-19 Vaccines for Children Down to 6 Months of Age.
- JAMA Pediatrics. Duration of SARS-CoV-2 Culturable Virus Shedding in Children.
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- JAMA Cardiology. SARS-CoV-2 Vaccination and Myocarditis in a Nordic Cohort Study of 23 Million Residents.
- Open Life Sciences. Relationship between blood clots and COVID-19 vaccines: A literature review.
- Science. Rare link between coronavirus vaccines and Long Covid–like illness starts to gain acceptance.
- FDA. COVID-19 Vaccine Safety Surveillance.