Risk of Myocarditis or Pericarditis With High-Dose vs Standard-Dose Influenza Vaccine: A Prespecified Secondary Analysis of the Randomized DANFLU-2 Trial

Manan Pareek; Niklas Dyrby Johansen; Daniel Modin; Matthew M Loiacono; Rebecca C Harris; Marine Dufournet; Carsten Schade Larsen; Lykke Larsen; Lothar Wiese; Michael Dalager-Pedersen; Brian L Claggett; Kira Hyldekær Janstrup; Katja Vu Bartholdy; Katrine Feldballe Bernholm; Julie Inge-Marie Helene Borchsenius; Filip Søskov Davidovski; Lise Witten Davodian; Maria Dons; Lisa Steen Duus; Caroline Espersen; Frederik Holme Fussing; Anne Marie Reimer Jensen; Nino Emanuel Landler; Adam Cadovius Femerling Langhoff; Mats Christian Højbjerg Lassen; Anne Bjerg Nielsen; Camilla Ikast Ottosen; Morten Sengeløv; Kristoffer Grundtvig Skaarup; Scott D Solomon; Martin J Landray; Gunnar H Gislason; Lars Køber; Pradeesh Sivapalan; Cyril Jean-Marie Martel; Jens Ulrik Stæhr Jensen; Tor Biering-Sørensen

doi:10.1001/jamanetworkopen.2025.36889

. 2025 Aug 30;8(8):e2536889. doi: 10.1001/jamanetworkopen.2025.36889

Risk of Myocarditis or Pericarditis With High-Dose vs Standard-Dose Influenza Vaccine

A Prespecified Secondary Analysis of the Randomized DANFLU-2 Trial

Manan Pareek ^1,², Niklas Dyrby Johansen ^1,², Daniel Modin ^1,², Matthew M Loiacono ³, Rebecca C Harris ⁴, Marine Dufournet ⁵, Carsten Schade Larsen ^6,^7,²¹, Lykke Larsen ⁸, Lothar Wiese ⁹, Michael Dalager-Pedersen ^10,¹¹, Brian L Claggett ¹², Kira Hyldekær Janstrup ^1,², Katja Vu Bartholdy ^1,², Katrine Feldballe Bernholm ^1,², Julie Inge-Marie Helene Borchsenius ^1,², Filip Søskov Davidovski ^1,², Lise Witten Davodian ^1,², Maria Dons ^1,², Lisa Steen Duus ^1,², Caroline Espersen ^1,², Frederik Holme Fussing ^1,², Anne Marie Reimer Jensen ^1,², Nino Emanuel Landler ^1,², Adam Cadovius Femerling Langhoff ^1,², Mats Christian Højbjerg Lassen ^1,², Anne Bjerg Nielsen ^1,², Camilla Ikast Ottosen ^1,², Morten Sengeløv ^1,², Kristoffer Grundtvig Skaarup ^1,², Scott D Solomon ¹², Martin J Landray ¹³, Gunnar H Gislason ^1,^14,^15,¹⁶, Lars Køber ^14,¹⁷, Pradeesh Sivapalan ^14,¹⁸, Cyril Jean-Marie Martel ¹⁹, Jens Ulrik Stæhr Jensen ^14,¹⁸, Tor Biering-Sørensen ^1,^2,^17,^20,^✉

¹Department of Cardiology, Copenhagen University Hospital–Herlev and Gentofte, Copenhagen, Denmark

²Center for Translational Cardiology and Pragmatic Randomized Trials, Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark

³Sanofi, Swiftwater, Pennsylvania

⁴Sanofi, Singapore

⁵Sanofi, Lyon, France

⁶Department of Clinical Medicine, Aarhus University Hospital, Aarhus, Denmark

⁷Danske Lægers Vaccinations Service, European LifeCare Group, Søborg, Denmark

⁸Department of Infectious Diseases, Odense University Hospital, Odense, Denmark

⁹Department of Infectious Diseases, Zealand University Hospital, Roskilde, Denmark

¹⁰Department of Infectious Diseases, Aalborg University Hospital, Aalborg, Denmark

¹¹Department of Clinical Medicine, Aalborg University, Aalborg, Denmark

¹²Cardiovascular Division, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts

¹³Nuffield Department of Population Health, University of Oxford, Oxford, United Kingdom

¹⁴Department of Clinical Medicine, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark

¹⁵Danish Heart Foundation, Copenhagen, Denmark

¹⁶National Institute of Public Health, University of Southern Denmark, Copenhagen, Denmark

¹⁷Department of Cardiology, Copenhagen University Hospital–Rigshospitalet, Copenhagen, Denmark

¹⁸Respiratory Medicine Section, Department of Medicine, Copenhagen University Hospital–Herlev and Gentofte, Copenhagen, Denmark

¹⁹Epidemiological Infectious Disease Preparedness, Statens Serum Institut, Copenhagen, Denmark

²⁰Steno Diabetes Center Copenhagen, Herlev, Denmark

²¹Department of Infectious Diseases, Aarhus University Hospital, Aarhus, Denmark

Accepted for Publication: August 15, 2025.

Published: August 30, 2025. doi:10.1001/jamanetworkopen.2025.36889

Open Access: This is an open access article distributed under the terms of the CC-BY-NC-ND License, which does not permit alteration or commercial use, including those for text and data mining, AI training, and similar technologies. © 2025 Pareek M et al. JAMA Network Open.

^✉

Corresponding Author: Tor Biering-Sørensen, MD, MSc, MPH, PhD, Center for Translational Cardiology and Pragmatic Randomized Trials, Department of Biomedical Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, Gentofte Hospitalsvej 8, 3.th., 2900 Hellerup, Denmark (tor.biering@gmail.com).

Author Contributions: Drs Johansen and Biering-Sørensen had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: Pareek, Johansen, Loiacono, Harris, Dufournet, L. Larsen, Dalager-Pedersen, Davidovski, Skaarup, Solomon, Køber, Sivapalan, Biering-Sørensen.

Acquisition, analysis, or interpretation of data: Pareek, Modin, Harris, C. Larsen, Wiese, Dalager-Pedersen, Claggett, Janstrup, Bartholdy, Bernholm, Borchsenius, Davidovski, Davodian, Dons, Duus, Espersen, Fussing, Reimer Jensen, Landler, Langhoff, Lassen, Nielsen, Ottosen, Sengeløv, Skaarup, Landray, Gislason, Martel, Jensen, Biering-Sørensen.

Drafting of the manuscript: Pareek.

Critical review of the manuscript for important intellectual content: All authors.

Statistical analysis: Pareek, Johansen, Modin, Dufournet, Janstrup, Biering-Sørensen.

Obtained funding: Biering-Sørensen.

Administrative, technical, or material support: C. Larsen, Dalager-Pedersen, Bernholm, Borchsenius, Davidovski, Dons, Duus, Fussing, Nielsen, Gislason, Martel, Jensen, Biering-Sørensen.

Supervision: Dalager-Pedersen, Claggett, Sengeløv, Solomon, Køber, Sivapalan, Jensen, Biering-Sørensen.

Conflict of Interest Disclosures: Dr Pareek reported receiving personal fees from AstraZeneca, Bayer, Boehringer Ingelheim, Janssen-Cilag, and Novo Nordisk and grants from the Danish Cardiovascular Academy and Danish Heart Foundation outside the submitted work. Dr. Harris reported that as an employee of Sanofi, she may hold Sanofi shares outside the submitted work. Dr. Dufournet reported owning stock options from Sanofi outside the submitted work. Dr. Claggett reported receiving personal fees from Alnylam, Cardior, Cardurion, Cytokinetics, CVRx, Intellia, Rocket, Lilly, and Alexion outside the submitted work. Dr. Bartholdy reported receiving grants from the Danish Cardiovascular Academy outside the submitted work. Dr Langhoff reported receiving grants from the Novo Nordisk Foundation and personal fees from Novo Nordisk outside the submitted work. Dr. Skaarup reported receiving personal fees from Sanofi and AstraZeneca outside the submitted work. Dr. Solomon reported receiving grants from Alexion, Alnylam, Applied Therapeutics, AstraZeneca, Bellerophon, Bayer, Bristol Myers Squibb (BMS), Boston Scientific, Cytokinetics, Edgewise, Eidos/BridgeBio, Gossamer, GSK, Ionis, Lilly, the National Institutes of Health/National Heart, Lung, and Blood Institute, Novartis, Novo Nordisk, Respicardia, Sanofi Pasteur, Tenaya, Theracos, and US2.AI and personal fees from Abbott, Action, Akros, Alexion, Alnylam, Amgen, Arena, Askbio, AstraZeneca, Bayer, BMS, Bridgebio, Cardior, Cardurion, Corvia, Cytokinetics, GSK, Intellia, Lilly, Novartis, Roche, Theracos, Quantum Genomics, Tenaya, Sanofi Pasteur, Dinaqor, Tremeau, CellProThera, Moderna, American Regent, Sarepta, Lexicon, Anacardio, Akros, Valo, Synhale, and Recordati outside the submitted work. Dr Landray reported receiving grants from Moderna, Regeneron, GSK, Sanofi, Boehringer Ingelheim, and Novartis to the University of Oxford and Protas outside the submitted work. Dr Køber reported receiving personal fees from Novo Nordisk, Novartis, AstraZeneca, and Boehringer outside the submitted work. Dr Biering-Sørensen reported receiving grants from Sanofi for the DANFLU-2 trial during the conduct of the study and grants from Sanofi Pasteur, Novo Nordisk, Novartis, Pfizer, GSK, AstraZeneca, Boston Scientific, GE HealthCare, and Bayer and personal fees from Sanofi Pasteur, Novo Nordisk, Novartis, GSK, IQVIA, Parexel, Amgen, CSL Seqirus, AstraZeneca, Bayer, and GE HealthCare outside the submitted work.

Funding/Support: The DANFLU-2 trial was funded by Sanofi.

Role of the Funder/Sponsor: Sanofi participated in the design of the study; protocol development; preparation, review, and approval of the manuscript; and decision to submit the manuscript for publication but had no role in trial conduct and collection, management, analysis, and interpretation of the data.

Data Sharing Statement: See Supplement 2.

Additional Contributions: The authors wish to thank all personnel at European LifeCare Group for collaborating and assisting with study procedures. This group was compensated by a grant from Sanofi.

^✉

Corresponding author.

PMCID: PMC12398704 PMID: 40884443

Abstract

This prespecified secondary analysis of a randomized clinical trial investigates the risk of myocarditis or pericarditis with high-dose vs standard-dose inactivated influenza vaccine among older adults.

Introduction

Influenza infection is associated with an increased risk of myocarditis and pericarditis.¹ Although such cardiac complications are rare and usually mild, they may lead to cardiogenic shock or cardiac tamponade. The high-dose inactivated influenza vaccine (HD-IIV) significantly reduces the incidence of laboratory-confirmed influenza infection compared with standard-dose IIVs (SD-IIVs) among older adults.² However, it is unknown whether HD-IIV reduces the risk of myocarditis or pericarditis, and it has not previously been possible to explore this hypothesis given the large sample size needed. This prespecified analysis of the trial A Pragmatic Randomized Trial to Evaluate the Effectiveness of High-Dose Influenza Vaccine vs Standard-Dose Influenza Vaccine in Older Adults (DANFLU-2)³ examined the risk of myocarditis or pericarditis with HD-IIV vs SD-IIV.

Methods

This prespecified secondary analysis of a randomized clinical trial used data from the DANFLU-2 trial,³ as did another secondary analysis.⁴ The trial is registered (NCT05517174), and the protocol indicating prespecification of this analysis is included in Supplement 1. In brief, DANFLU-2 was a pragmatic (ie, with broadened generalizability via more representative populations studied in routine settings), registry-based, open-label, randomized clinical trial of adults aged 65 years or older conducted in Denmark during the 2022-2023, 2023-2024, and 2024-2025 influenza seasons. Participants were randomized 1:1 to HD-IIV or SD-IIV. The trial was approved by the Danish Medical Research Ethics Committees and Danish Medicines Agency. All participants provided informed consent, granting access to registry data and medical records. Approval and consent extend to this study, which is reported according to the CONSORT reporting guideline.

Baseline and outcome data were collected through linkage to nationwide health registries. During each study season, participants were followed up from day 14 after vaccination through May 31 the following year. The primary outcome was hospitalization for influenza or pneumonia. As an exploratory outcome, this study examined the risk of hospitalization for myocarditis (defined as International Statistical Classification of Diseases and Related Health Problems, Tenth Revision [ICD-10] codes I40-I41) or pericarditis (defined as ICD-10 codes I30-I31) in the 2 randomization groups. Relative vaccine effectiveness ( [1 − relative risk] × 100%) based on first events was reported, with 95% CIs constructed using the exact Clopper-Pearson method for binomial proportions. Events occurring before 14 days after vaccination were included in a sensitivity analysis. Consistency of the treatment effect was assessed across baseline characteristics and conditions, with no adjustment for multiplicity. Interactions were assessed using the Cochran-Mantel-Haenszel test for homogeneity. We considered 2-sided P values < .05 statistically significant. Statistical analysis was performed using SAS version 9.4 (SAS Institute), Stata MP version 19.5 (StataCorp), and R version 4.3.3 (R Project for Statistical Computing).

Results

Of 332 438 participants randomized, 1295 (mean [SD] age, 74.2 [5.7] years; 884 men [68.3%]) had a prior history of myocarditis or pericarditis, including 97 with prior myocarditis and 1218 with prior pericarditis, and 331 143 did not (mean [SD] age, 73.7 [5.8] years; 170 016 men [51.3%]) (Table). Baseline characteristics were balanced across HD-IIV and SD-IIV groups. Individuals with prior myocarditis or pericarditis were older, more often men, and had a higher prevalence of comorbidities. The incidence of myocarditis or pericarditis was lower among participants randomized to HD-IIV vs SD-IIV (19 vs 35 events; relative vaccine effectiveness, 45.71%; 95% CI, 2.46%-70.67%; P = .04) (Figure). Results were consistent when events occurring before 14 days were included. No interactions were detected.

Table. Participant Baseline Characteristics.

Characteristic	Participants, No. (%) (N = 332 438)		P value
Characteristic	No prior myocarditis or pericarditis (n = 331 143)	Prior myocarditis or pericarditis (n = 1295)	P value
Age, mean (SD), y	73.7 (5.8)	74.2 (5.7)	.001
Gender
Men	170 016 (51.3)	884 (68.3)	<.001
Women	161 127 (48.7)	411 (31.7)	<.001
Comorbidities
Hypertension	63 416 (19.2)	491 (37.9)	<.001
Diabetes	43 639 (13.2)	242 (18.7)	<.001
Chronic cardiovascular disease	90 086 (27.2)	940 (72.6)	<.001
Ischemic heart disease	30 718 (9.3)	394 (30.4)	<.001
Cerebrovascular disease	16 300 (4.9)	81 (6.3)	.03
Peripheral artery disease	2956 (0.9)	23 (1.8)	<.001
Heart failure	10 212 (3.1)	198 (15.3)	<.001
Atrial fibrillation	33 550 (10.1)	535 (41.3)	<.001
Chronic lung disease	26 948 (8.1)	204 (15.8)	<.001
Chronic kidney disease	46 452 (14.0)	336 (25.9)	<.001
Chronic liver disease	4950 (1.5)	38 (2.9)	<.001
Cancer	45 693 (13.8)	225 (17.4)	<.001
Immunodeficiency	14 193 (4.3)	122 (9.4)	<.001
Coadministration with COVID-19 vaccine	203 916 (61.6)	807 (62.3)	.59

Open in a new tab

Figure. — HD-IIV indicates high-dose inactivated influenza vaccine; NA, not applicable; rVE, relative vaccine effectiveness; SD-IIV, standard-dose inactivated influenza vaccine.

Discussion

In this prespecified analysis of the DANFLU-2 trial, the risk of incident myocarditis or pericarditis was lower among individuals randomized to HD-IIV vs SD-IIV. Despite sporadic reports of myocarditis and pericarditis associated with influenza vaccination,⁵ the consistency of our results with vs without inclusion of events occurring immediately after vaccination negates a dose-response association and a causal link. Study limitations include the small number of events, lack of knowledge regarding the etiology and severity of each event, and inability to compare the effects of vaccines vs no vaccine. Implications for myocarditis are unclear given that only 2 such events occurred. The generalizability to younger individuals, who typically have a higher risk of inflammatory cardiac conditions, is also unknown.⁶ HD-IIV was associated with protection against myocarditis or pericarditis compared with SD-IIV in the largest individually randomized trial of an enhanced influenza vaccine.

Supplement 1.

Trial Protocol and Statistical Analysis Plan

jamanetwopen-e2536889-s001.pdf^{(1.5MB, pdf)}

Supplement 2.

Data Sharing Statement

jamanetwopen-e2536889-s002.pdf^{(54.1KB, pdf)}

References

1.Mamas MA, Fraser D, Neyses L. Cardiovascular manifestations associated with influenza virus infection. Int J Cardiol. 2008;130(3):304-309. doi: 10.1016/j.ijcard.2008.04.044 [DOI] [PubMed] [Google Scholar]
2.DiazGranados CA, Dunning AJ, Kimmel M, et al. Efficacy of high-dose versus standard-dose influenza vaccine in older adults. N Engl J Med. 2014;371(7):635-645. doi: 10.1056/NEJMoa1315727 [DOI] [PubMed] [Google Scholar]
3.Johansen ND, Modin D, Loiacono MM, et al. High-dose influenza vaccine effectiveness against hospitalization in older adults. N Engl J Med. Published online August 30, 2025. doi: 10.1056/NEJMoa2509907 [DOI] [Google Scholar]
4.Johansen ND, Modin D, Loiacono MM, et al. High-dose vs standard-dose influenza vaccine and cardiovascular outcomes in older adults: a prespecified secondary analysis of the DANFLU-2 randomized clinical trial. JAMA Cardiol. Published online August 30, 2025. doi: 10.1001/jamacardio.2025.3460 [DOI] [PMC free article] [PubMed] [Google Scholar]
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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplement 1.

Trial Protocol and Statistical Analysis Plan

jamanetwopen-e2536889-s001.pdf^{(1.5MB, pdf)}

Supplement 2.

Data Sharing Statement

jamanetwopen-e2536889-s002.pdf^{(54.1KB, pdf)}

[zld250224r1] 1.Mamas MA, Fraser D, Neyses L. Cardiovascular manifestations associated with influenza virus infection. Int J Cardiol. 2008;130(3):304-309. doi: 10.1016/j.ijcard.2008.04.044 [DOI] [PubMed] [Google Scholar]

[zld250224r2] 2.DiazGranados CA, Dunning AJ, Kimmel M, et al. Efficacy of high-dose versus standard-dose influenza vaccine in older adults. N Engl J Med. 2014;371(7):635-645. doi: 10.1056/NEJMoa1315727 [DOI] [PubMed] [Google Scholar]

[zld250224r3] 3.Johansen ND, Modin D, Loiacono MM, et al. High-dose influenza vaccine effectiveness against hospitalization in older adults. N Engl J Med. Published online August 30, 2025. doi: 10.1056/NEJMoa2509907 [DOI] [Google Scholar]

[zld250224r4] 4.Johansen ND, Modin D, Loiacono MM, et al. High-dose vs standard-dose influenza vaccine and cardiovascular outcomes in older adults: a prespecified secondary analysis of the DANFLU-2 randomized clinical trial. JAMA Cardiol. Published online August 30, 2025. doi: 10.1001/jamacardio.2025.3460 [DOI] [PMC free article] [PubMed] [Google Scholar]

[zld250224r5] 5.Engler RJ, Nelson MR, Collins LC Jr, et al. A prospective study of the incidence of myocarditis/pericarditis and new onset cardiac symptoms following smallpox and influenza vaccination. PLoS One. 2015;10(3):e0118283. doi: 10.1371/journal.pone.0118283 [DOI] [PMC free article] [PubMed] [Google Scholar]

[zld250224r6] 6.Basso C. Myocarditis. N Engl J Med. 2022;387(16):1488-1500. doi: 10.1056/NEJMra2114478 [DOI] [PubMed] [Google Scholar]

PERMALINK

Risk of Myocarditis or Pericarditis With High-Dose vs Standard-Dose Influenza Vaccine

Manan Pareek, MD, MSc, PhD

Niklas Dyrby Johansen, MD, PhD

Daniel Modin, MD

Matthew M Loiacono, PhD

Rebecca C Harris, MBioch, MSc, PhD

Marine Dufournet, PhD

Carsten Schade Larsen, MD, DMSc

Lykke Larsen, MD, PhD

Lothar Wiese, MD, PhD

Michael Dalager-Pedersen, MD, PhD

Brian L Claggett, PhD

Kira Hyldekær Janstrup, PhD

Katja Vu Bartholdy, MD

Katrine Feldballe Bernholm, MD

Julie Inge-Marie Helene Borchsenius, MD

Filip Søskov Davidovski, MD

Lise Witten Davodian, MD

Maria Dons, MD

Lisa Steen Duus, MD

Caroline Espersen, MD

Frederik Holme Fussing, MD

Anne Marie Reimer Jensen, MD

Nino Emanuel Landler, MD, PhD

Adam Cadovius Femerling Langhoff, MD

Mats Christian Højbjerg Lassen, MD

Anne Bjerg Nielsen, MD

Camilla Ikast Ottosen, MD

Morten Sengeløv, MD

Kristoffer Grundtvig Skaarup, MD

Scott D Solomon, MD

Martin J Landray, MBChB, PhD

Gunnar H Gislason, MD, PhD

Lars Køber, MD, DMSc

Pradeesh Sivapalan, MD, PhD

Cyril Jean-Marie Martel, PhD

Jens Ulrik Stæhr Jensen, MD, PhD

Tor Biering-Sørensen, MD, MSc, MPH, PhD

Abstract

Introduction

Methods

Results

Table. Participant Baseline Characteristics.

Figure. Risk of Myocarditis or Pericarditis Among Patients Receiving High-Dose vs Standard-Dose Influenza Vaccine.

Discussion

References

Associated Data

Supplementary Materials

ACTIONS

PERMALINK

RESOURCES

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