✓ Peer reviewedMatched observational cohortCardiovascular · Israel
Safety of the BNT162b2 mRNA Covid-19 Vaccine in a Nationwide Setting
Barda N, Dagan N, Ben-Shlomo Y, et al. · New England Journal of Medicine 385:1078–1090 · 2021
- Participants / data
- 884,828 vaccinated people matched to 884,828 unvaccinated people (Clalit Health Services, Israel).
- Key finding
- Vaccination was associated with an elevated risk of myocarditis (risk ratio 3.24; about 2.7 excess events per 100,000 people), as well as lymphadenopathy, appendicitis and herpes zoster. SARS-CoV-2 infection itself was associated with a substantially higher risk of myocarditis and several other serious events.
- Limitations
- Observational design; 42-day follow-up window; one health organization; wide confidence intervals for rare events.
✓ Peer reviewedNational surveillance analysisCardiovascular · Israel
Myocarditis after BNT162b2 mRNA Vaccine against Covid-19 in Israel
Mevorach D, Anis E, Cedar N, et al. · New England Journal of Medicine 385:2140–2149 · 2021
- Participants / data
- About 5.1 million people vaccinated in Israel (December 2020 – May 2021); 136 cases of definite or probable myocarditis.
- Key finding
- The highest incidence was after the second dose in males aged 16–19 (about 1 in 6,637). Most cases were clinically mild.
- Limitations
- Relies on reporting to a surveillance system, which may miss cases; early-period data.
✓ Peer reviewedCase series with laboratory analysisCoagulation
Thrombotic Thrombocytopenia after ChAdOx1 nCov-19 Vaccination
Greinacher A, Thiele T, Warkentin TE, et al. · New England Journal of Medicine 384:2092–2101 · 2021
- Participants / data
- 11 patients in Germany and Austria.
- Key finding
- Described vaccine-induced immune thrombotic thrombocytopenia (VITT): blood clots with low platelet counts, mediated by antibodies against platelet factor 4 (PF4), appearing 5–16 days after vaccination.
- Limitations
- Small case series; does not estimate how common the condition is.
✓ Peer reviewedMultinational observed-vs-expected cohortMultiple systems
COVID-19 vaccines and adverse events of special interest: A multinational Global Vaccine Data Network (GVDN) cohort study of 99 million vaccinated individuals
Faksova K, Walsh D, Jiang Y, et al. · Vaccine 42(9):2200–2211 · 2024
- Participants / data
- About 99 million vaccinated individuals across data sites in 8 countries.
- Key finding
- Confirmed previously identified safety signals — including myocarditis and pericarditis after mRNA vaccines, and Guillain-Barré syndrome and cerebral venous sinus thrombosis after the ChAdOx1 vaccine — and highlighted other potential signals for further investigation.
- Limitations
- Observed-vs-expected comparisons depend on background rates; a signal is a prompt for investigation, not proof of causation.
✓ Peer reviewedSelf-controlled case seriesNeurological · England
Neurological complications after first dose of COVID-19 vaccines and SARS-CoV-2 infection
Patone M, Handunnetthi L, Saatci D, et al. · Nature Medicine 27:2144–2153 · 2021
- Participants / data
- About 32 million people vaccinated in England (first dose).
- Key finding
- Found an increased risk of Guillain-Barré syndrome and Bell's palsy after the ChAdOx1 vaccine, and of hemorrhagic stroke after BNT162b2. The risk of these outcomes was higher after a positive SARS-CoV-2 test.
- Limitations
- First dose only; 28-day risk window; based on hospital admission records.
✓ Peer reviewedProspective app-based cohortMenstrual & hormonal
Association Between Menstrual Cycle Length and Coronavirus Disease 2019 (COVID-19) Vaccination: A U.S. Cohort
Edelman A, Boniface ER, Benhar E, et al. · Obstetrics & Gynecology 139(4):481–489 · 2022
- Participants / data
- 3,959 individuals in the U.S. (2,403 vaccinated, 1,556 unvaccinated).
- Key finding
- Vaccination was associated with a small increase in cycle length (less than one day on average), with no change in the length of menses. The change appeared to be temporary.
- Limitations
- App users in the U.S. may not represent all populations; self-tracked data.
Preprint — not peer reviewedExploratory immunology studyImmune · fatigue · autonomic
Immunological and Antigenic Signatures Associated with Chronic Illnesses after COVID-19 Vaccination
Bhattacharjee B, Lu P, Monteiro VS, et al. · medRxiv (preprint) · 2025
- Participants / data
- 42 people reporting post-vaccination syndrome and 22 healthy controls (Yale LISTEN study).
- Key finding
- Reported differences in immune cell populations, more frequent signs of Epstein-Barr virus reactivation, and elevated circulating spike protein in some participants with post-vaccination syndrome compared with controls.
- Limitations
- Not peer reviewed; small sample; the authors describe the findings as preliminary; cannot establish causation.