National Center for Emerging and Zoonotic Infectious Diseases

Update on COVID-19 and influenza vaccine safety

Advisory Committee on Immunization Practices (ACIP) meeting

October 25, 2023

Tom T. Shimabukuro, MD, MPH, MBA

Director, Immunization Safety Office

Division of Healthcare Quality Promotion 

Centers for Disease Control and Prevention (CDC)
Key points up front

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▪ A statistical signal for ischemic stroke after Pfizer-BioNTech bivalent mRNA COVID-19 vaccine was 

detected in CDC’s Vaccine Safety Datalink in persons aged ≥65 years during fall 2022; information 

was presented at prior ACIP meetings and efforts have been underway to evaluate the signal*

▪ Available data do not provide clear and consistent evidence of a safety problem for ischemic stroke 

with bivalent mRNA COVID-19 vaccines when given alone or given simultaneously with influenza 

vaccines, or when influenza vaccine is given alone

• Variable and inconsistent results were obtained in some analyses of the risk of ischemic stroke following 

bivalent mRNA COVID-19 vaccination, simultaneous bivalent mRNA COVID-19 and influenza vaccination, 

and influenza vaccination alone†

• Most study results have not shown an association between vaccination and ischemic stroke, and no clear 

pattern demonstrating increased risk has emerged

▪ Any real or theoretical risks of vaccine adverse events need to be placed in the context of the 

known benefits of COVID-19 and influenza vaccination in preventing COVID-19 and influenza 

disease and their potentially serious complications, including stroke

▪ Our vaccine safety monitoring systems are designed to be sensitive, and the detection and 

assessment of potential safety signals and communication of safety information to the public is an 

example of the vaccine safety monitoring process working 

* https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2023-02/slides-02-24/COVID-02-Shimabukuro-508.pdf; 

https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2023-04-19/03-COVID-Shimabukuro-508.pdf

† For the purpose of this presentation, “influenza vaccination alone” is defined as not with simultaneous COVID-19 vaccination but may include other types of non-COVID-19 vaccines 
Topics

▪ Summary of analyses of ischemic stroke and bivalent mRNA COVID-19 

and influenza vaccination

▪ Additional data on ischemic stroke

▪ Interpretation of data on ischemic stroke and bivalent mRNA COVID19 and influenza vaccination and next steps

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Selected analyses of ischemic stroke and bivalent COVID-19 and 

influenza vaccination

1. Vaccine Safety Datalink (VSD) Rapid Cycle Analysis (RCA) of ischemic stroke after bivalent 

mRNA COVID-19 vaccination (Centers for Disease Control and Prevention)

2. Analysis of ischemic stroke after bivalent mRNA COVID-19 vaccination (Kaiser Permanente 

Southern California)

3. Analysis of stroke risk following bivalent mRNA COVID-19 vaccination among U.S. adults aged 

≥65 years in CMS data (U.S. Food and Drug Administration)

4. Analysis of ischemic stroke after bivalent mRNA COVID-19 vaccination in patients aged ≥65 

years using nation-wide patient electronic health records (Case Western Reserve University 

School of Medicine)

5. Analysis of stroke, myocardial infarction, and pulmonary embolism after bivalent PfizerBioNTech COVID-19 vaccination (EPI-PHARE Scientific Interest Group, France)

6. Analysis of bivalent mRNA COVID-19 vaccination and stroke in England (UK Health Security 

Agency and the London School of Hygiene and Tropical Medicine)

7. Analysis of the safety of monovalent and bivalent Pfizer-BioNTech COVID-19 vaccination in atrisk populations (Israel)

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1.) VSD COVID-19 Rapid Cycle Analysis: Analyses of Ischemic Stroke 

after Pfizer-BioNTech Bivalent Booster Dose*

(April 2023 ACIP https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2023-04-19/03-COVID-Shimabukuro-508.pdf)

▪ Data source: VSD data contributing sites

▪ Methodology

• Primary: Vaccinated concurrent comparator (i.e., bivalent vaccinated vs. bivalent vaccinated) using a 

1–21-day risk interval and a 22–42-day comparison interval 

• Supplemental: Bivalent vaccinated vs. bivalent unvaccinated but eligible for bivalent vaccine in a 1–

21-day risk interval

• Age groups included 18–64 and ≥65 years

▪ Main findings

• Statistical signal for ischemic stroke detected in primary analysis after Pfizer-BioNTech in the age 

group ≥65 years using a 1–21-day risk interval; signal attenuated over time

• Post-signal assessment detected an elevated risk in the age group ≥65 years receiving same-day 

Pfizer-BioNTech and high-dose inactivated influenza (HD-IIV4) or adjuvanted inactivated influenza 

vaccine (aIIV4); finding attenuated over time

• No elevated risk for ischemic stroke detected in supplemental analysis using secondary comparators

• Additional supplemental analyses suggested that comparison interval (22–42 days) rates of 

ischemic stroke were lower than expected 

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* Conducted through the CDC Vaccine Safety Datalink Indefinite Delivery, Indefinite Quantity (IDIQ) contract
2.) Ischemic Stroke after Bivalent COVID-19 Vaccination: A SelfControlled Case Series Study*

(Xu et al., 2023 https://www.medrxiv.org/content/10.1101/2023.10.12.23296968v1)

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▪ Data source: Kaiser Permanente Southern California electronic health record data

▪ Methodology

• Modified self-controlled case series using age groups ≥12, 12–64 and ≥65 years 

▪ Main findings

• No elevated risks detected for either Pfizer-BioNTech or Moderna in any age groups in automated data 

with a 21-day risk interval

• No elevated risks detected for either Pfizer-BioNTech or Moderna in the age group ≥65 years in automated 

data with a 42-day risk interval

• There were several analyses with statistically significant elevated risks in the age group 12–64 years in 

automated data with a 42-day risk interval

‒ Pfizer-BioNTech and simultaneous influenza vaccination, overall and in those with a history of SARSCoV-2 infection 

‒ Moderna in those with a history of SARS-CoV-2 infection

• After limiting signaling analyses to chart-verified cases, the findings were no longer statistically significant

* Funding provided through NIAID/NIH Award Number R01 AI168209
3.) Evaluation of Stroke Risk Following COVID-19 mRNA Bivalent 

Vaccines Among U.S. Adults Aged ≥65 Years*

(Lu et al., 2023 https://www.medrxiv.org/content/10.1101/2023.10.10.23296624v1)

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▪ Data source: Medicare claims data 

▪ Populations: Community-dwelling Medicare beneficiaries aged ≥65 years

• Primary population: Recipients of bivalent mRNA COVID-19 vaccine

• Secondary population: Recipients of high-dose (HD-IIV4) or adjuvanted (aIIV4) influenza vaccine

▪ Methodology: Modified self-controlled case series with Farrington adjustment 

▪ Main findings

• Primary population analyses showed no consistent stroke risk after mRNA COVID-19 vaccination

• An increased risk was observed with concomitant (same day administration) influenza vaccination 

‒ Non-hemorrhagic stroke after Pfizer-BioNTech + HD-IIV4/aIIV4 with a 22–42-day risk interval 

‒ Transient ischemic attack after Moderna + HD-IIV4/aIIV4 with a 1–21-day risk interval

• Secondary population analyses showed a small increased risk of non-hemorrhagic stroke after 

HD-IIV4 or aIIV4 with a 22–42-day risk interval, and the risk remained for people without 

concomitant bivalent mRNA COVID-19 vaccination 

* Funding provided by the FDA as part of the SafeRx Project, a joint initiative of the CMS and FDA
4.) Ischemic stroke after COVID-19 bivalent vaccine administration 

in patients aged 65 years and older: analysis of nation-wide 

patient electronic health records in the United States*

(Gorenflo et al., 2023 https://www.medrxiv.org/content/10.1101/2023.02.11.23285801v1)

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▪ Data source: TriNetX, a cloud-based analytics platform that includes electronic 

health record data from >90 million unique patients in the United States

▪ Methodology

• Retrospective cohort study among people aged ≥65 years 

▪ Main findings

• Patients who received bivalent Pfizer-BioNTech COVID-19 vaccination had a similar 

hazard for ischemic stroke encounters compared to those who received bivalent 

Moderna COVID-19 vaccination, but had a lower hazard than those who received the 

monovalent Pfizer-BioNTech or Moderna COVID-19 booster vaccines in the 1–21 or 

22–42 days post-vaccination

* Support provided through NIA/NIH (grants nos. RF1AG076649), NIAAA/NIH (grant no. R01AA029831), CTSC of Cleveland (grant no. 1UL1TR002548)
5.) Stroke, Myocardial Infarction, and Pulmonary Embolism 

after Bivalent Booster 

(Jabagi et al., 2023 https://www.nejm.org/doi/full/10.1056/NEJMc2302134)

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▪ Data source: French National Health Data System linked to the national coronavirus 

disease 2019 (Covid-19) vaccination database

▪ Methodology

• Matched cohort study (1:5) in people aged ≥50 years

• Recipient of monovalent vaccine matched to recipients of bivalent mRNA COVID-19 

vaccine*; followed for 21 days after vaccination 

▪ Main findings

• Compared to monovalent Pfizer-BioNTech COVID-19 vaccination, bivalent Pfizer-BioNTech 

COVID-19 vaccination was not associated with an increased risk of ischemic stroke, 

hemorrhagic stroke, myocardial infarction, or pulmonary embolism in people aged ≥50 years 

• The authors previously found no increase in the incidence of stroke, acute myocardial 

infarction, or pulmonary embolism after administration of the monovalent Pfizer-BioNTech 

COVID-19 vaccine

*contains omicron BA.4–BA.5 strains; used in the United States 
6.) BA.1 Bivalent COVID-19 Vaccine Use and Stroke in England*

(Andrews et al., 2023 https://jamanetwork.com/journals/jama/fullarticle/2806456)

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▪ Data source: National Health Service hospital admissions in England linked to the 

National Immunisation Management System

▪ Methodology

• Self-controlled case-series design in people aged ≥50 and ≥65 years (1–21-day risk 

window) with further analysis in people aged ≥65 years given simultaneous bivalent mRNA 

COVID-19† and influenza vaccination‡

▪ Main findings

• No increased risk of stroke in the 21 days after vaccination with either the Pfizer-BioNTech 

or the Moderna bivalent mRNA COVID-19 vaccines

• Similar results obtained for ischemic and hemorrhagic stroke for the subset of people aged 

≥65 years given influenza vaccine on the same day as the bivalent mRNA COVID-19 vaccine 

* Funded by the UK Health Security Agency; † Contains omicron BA.1 strain; not used in United States; ‡ persons aged ≥ 65 years 95% received quadrivalent or 

trivalent adjuvanted influenza vaccine (no high-dose influenza vaccine used)
7.) Safety of monovalent and bivalent BNT162b2 mRNA COVID-19 

vaccine boosters in at-risk populations in Israel: a large-scale, 

retrospective, self-controlled case series study*

(Yamin et al., 2023 https://www.sciencedirect.com/science/article/pii/S1473309923002074?via%3Dihub)

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▪ Data source: Clalit Health Services medical records (largest healthcare 

organization in Israel with over 3.5 million enrollees, 1.2 million aged ≥60 years)

▪ Methodology

• Self-controlled case-series

▪ Main findings

• No safety signals detected for ischemic stroke after either monovalent or bivalent 

Pfizer-BioNTech COVID-19 vaccines used in Israel in the overall analysis or in people 

aged ≥65 years

* Supported by Israel Science Foundation (grant 3409/19), within the Israel Precision Medicine Partnership
Additional data on ischemic stroke

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▪ No unusual or unexpected reporting patterns observed, and no evidence of a safety concern 

detected for ischemic stroke with either of the bivalent mRNA COVID-19 vaccines in Vaccine 

Adverse Event Reporting System (VAERS) monitoring

▪ FDA monitoring in the CMS data and Department of Veterans Affairs monitoring in the VA 

system did not detect any safety signals for ischemic stroke following bivalent mRNA COVID-19 

vaccination using historical comparator designs

▪ A separate ad hoc CDC analysis during the bivalent Pfizer-BioNTech ischemic stroke signal 

assessment did not detect an elevated risk for ischemic stroke after influenza vaccination alone 

▪ Surveillance conducted by international regulatory and public health partners did not detect a 

safety concern for ischemic stroke following bivalent mRNA COVID-19 vaccination 

▪ No evidence of a safety signal for ischemic stroke detected in the manufacturers’ global 

monitoring of bivalent mRNA COVID-19 vaccination

▪ No safety signals were detected for ischemic stroke for primary series or monovalent boosters 

for Pfizer-BioNTech or Moderna COVID-19 vaccines in U.S. and global monitoring)

▪ Data suggest COVID-19 and influenza disease are associated with an increased risk of stroke 

ACIP Ischemic Stroke, COVID-19 and Influenza in Adults Ages greater than or Equal to 65 years-Feb. 24, 2023 (cdc.gov)
Monthly incidence rates of non-hemorrhagic stroke (NHS) 

in Medicare claims data suggesting seasonality*

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* From eFigure 1. Monthly Incidence Rates of NHS, TIA, NHS/TIA, and HS per 100,000 Person-Years at Risk, in Lu et al., 2023 https://www.medrxiv.org/content/10.1101/2023.10.10.23296624v1
Outpatient visits for respiratory illness in the United States from ILINet

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Interpretation of analyses of ischemic stroke and bivalent 

COVID-19 and influenza vaccination 

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▪ Variable and inconsistent results were obtained in some analyses of risk of ischemic 

stroke following bivalent COVID-19 vaccination, simultaneous bivalent COVID-19 and 

influenza vaccination, and influenza vaccination alone

• There is a lack of consistency in findings from different data systems, when using different methods, 

across age groups, and across sub-group analyses

• The most common findings across studies are findings of no association

• Multiple comparisons were conducted in studies without adjusting for multiplicity; few reached 

statistical significance

• The studies were not designed to account for a potential protective effect of vaccination on stroke in 

later post-vaccination periods

• Adjusting for seasonality and restricting analyses to chart-verified cases frequently resulted in 

attenuated findings or findings that were no longer statistically significant

• Remaining statistically significant findings tended to be relatively small in magnitude (i.e., RRs<2)

• Ischemic stroke cases in the analyses are predominantly occurring in older people and in people in 

the upper ranges of the age groups studied (e.g., upper range of a 12–64-year-old age group); 

relatively few cases are in younger people
Interpretation of analyses of ischemic stroke and bivalent 

COVID-19 and influenza vaccination (cont.) 

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▪ Available data do not provide clear and consistent evidence of a safety problem for ischemic 

stroke with bivalent mRNA COVID-19 vaccines when given alone or given simultaneously with 

influenza vaccines, or when influenza vaccine is given alone

▪ Most study results have not shown an association between vaccination and ischemic stroke, and 

no clear pattern demonstrating increased risk has emerged

▪ Seasonality of stroke risk and an unusual respiratory illness pattern in 2022-2023 could be 

impacting the results of some of these analyses; unrecognized SARS-CoV-2 infection could also 

play a role in occurrence of stroke after vaccination

▪ Any real or theoretical risk needs to be placed in the context of the known benefits of COVID-19 

and influenza vaccination in preventing COVID-19 and influenza disease and their potentially 

serious complications, including stroke

▪ Simultaneous vaccination provides substantial benefits in keeping patients up-to-date with 

recommended vaccines and protected from vaccine preventable diseases

▪ Our vaccine safety monitoring systems are designed to be sensitive, and the detection and 

assessment of the ischemic stroke signal and communication to the public is an example of the 

vaccine safety monitoring process working
Next steps

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▪ Conduct additional analyses on the possible relationship between 

ischemic stroke and bivalent mRNA COVID-19 vaccination, 

simultaneous administration of bivalent mRNA COVID-19 and 

influenza vaccines, and influenza vaccine alone

▪ Continue vigilant safety monitoring of 2023–2024 COVID-19 and 

influenza vaccines, including for ischemic stroke
Acknowledgements

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▪ CDC Immunization Safety Office

• VAERS Team

• Vaccine Safety Datalink (VSD) Team

• Clinical Immunization Safety Assessment 

(CISA) Project

▪ FDA Center for Biologics Evaluation and 

Research, Office of Biostatistics and 

Pharmacovigilance

▪ Department of Veterans Affairs

▪ VSD sites

• Kaiser Permanente Northern California, Oakland, CA

• Kaiser Permanente Southern California, Los Angeles, CA

• Marshfield Clinic Research Institute, Marshfield, WI

• HealthPartners Institute, Minneapolis, MN

• Kaiser Permanente Colorado, Denver, CO

• Kaiser Permanente Northwest, Portland, OR

• Kaiser Permanente Washington, Seattle, WA

• Denver Health, Denver, CO

• Harvard Pilgrim Health Care Institute, Boston
Disclaimer

▪ The findings and conclusions in this presentation are those of the presenter and 

do not necessarily represent the official position of CDC or FDA

▪ Mention of a product or company name is for identification purposes only and 

does not constitute endorsement by CDC or FDA

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For more information, contact CDC

1-800-CDC-INFO (232-4636)

TTY: 1-888-232-6348 www.cdc.gov

The findings and conclusions in this report are those of the authors and do not necessarily represent the 

official position of the Centers for Disease Control and Prevention.

Photo credit: James Gathany

(https://phil.cdc.gov/Details.

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