---
title: "Cohort vs SCCS for vaccine safety: Stroke incidence after COVID-19 vaccination may be underestimat"
id: "pubmed-42635361"
canonical_url: "https://medichelpline.com/clinical-feed/pubmed-42635361"
content_type: "clinical_feed_article"
specialty: "Critical Care"
source_name: "PubMed / NCBI"
source_url: "https://pubmed.ncbi.nlm.nih.gov/42635361/"
doi: "10.1080/14760584.2026.2720953"
published_at: "2026-08-24T12:00:00.000Z"
evidence_level: "Journal Article"
license: "CC-BY-NC-4.0 / Informational Use"
---
# Cohort vs SCCS for vaccine safety: Stroke incidence after COVID-19 vaccination may be underestimat
## Provenance & Clinical Metadata
- **Canonical URL:** https://medichelpline.com/clinical-feed/pubmed-42635361
- **Specialty:** [Critical Care](https://medichelpline.com/clinical-feed/critical-care.md)
- **Primary Source:** PubMed / NCBI
- **Source URL:** [Original Journal Publication](https://pubmed.ncbi.nlm.nih.gov/42635361/)
- **DOI:** [10.1080/14760584.2026.2720953](https://doi.org/10.1080%2F14760584.2026.2720953)
- **Published At:** 2026-08-24T12:00:00.000Z
- **Evidence Rating:** Journal Article
## Executive GIST (TL;DR)
- Study compared **cohort** and **self-controlled case series (SCCS)** approaches to assess excess **stroke** incidence after COVID-19 vaccination in a nationwide dataset from Qatar. - Design: retrospective nationwide matched cohort between 1 December 2020 and 11 April 2023, with 1,111,939 vaccinated persons matched 1:1 to unvaccinated controls. - Outcome window: stroke events occurring within 42 days after vaccination. - Findings: 8 strokes in the vaccinated cohort versus 19 in the matched unvaccinated cohort during follow-up. - Incidence rates: 0.2 (95% CI, 0.1–0.3) per 100,000 person-weeks in vaccinated and 0.4 (95% CI, 0.2–0.6) per 100,000 person-weeks in unvaccinated. - Adjusted hazard ratio for vaccination vs no vaccination: 0.42 (95% CI, 0.18–0.96), suggesting a lower observed risk in the between-person cohort comparison. - Contrast: These cohort results imply a spurious protective effect and diverge from previous **SCCS** analyses (including analyses in the same population) that found a small increased risk of stroke after vaccination. - Interpretation: The discrepancy may reflect **healthy vaccinee bias** and indicates cohort designs may underestimate rare adverse events when used alone to estimate absolute risk differences. - Methodological point: SCCS provides within-person estimates and is limited in directly estimating absolute risk differences; cohort designs provide absolute measures but are susceptible to between-person confounding. - Keywords and themes: bias, cerebrovascular disorders, healthy vaccinee effect, immunization, self-controlled case-series, vaccine safety assessment. - Note: The abstract does not report detailed stroke case definitions, the full set of covariates included in adjustment, or additional sensitivity analyses; those details were not reported in the provided source abstract.
## Clinical Analysis & Structured Key Points
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Epub 2026 Aug 24. # Cohort studies may underestimate rare vaccine adverse events: evidence from stroke incidence following COVID-19 vaccination [Hiam Chemaitelly](https://pubmed.ncbi.nlm.nih.gov/?term=Chemaitelly+H&cauthor_id=42635361)[ 1 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-1 "Infectious Disease Epidemiology Group, Weill Cornell Medicine-Qatar, Cornell University, Doha, Qatar.")[ 2 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-2 "Department of Population Health Sciences, Weill Cornell Medicine, Cornell University, New York, NY, USA."), [Naveed Akhtar](https://pubmed.ncbi.nlm.nih.gov/?term=Akhtar+N&cauthor_id=42635361)[ 3 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-3 "Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar."), [Salman Al Jerdi](https://pubmed.ncbi.nlm.nih.gov/?term=Al+Jerdi+S&cauthor_id=42635361)[ 3 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-3 "Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar.")[ 4 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-4 "Department of Medical Education, Weill Cornell Medicine-Qatar, Cornell University, Doha, Qatar."), [Saadat Kamran](https://pubmed.ncbi.nlm.nih.gov/?term=Kamran+S&cauthor_id=42635361)[ 3 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-3 "Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar."), [Sujatha Joseph](https://pubmed.ncbi.nlm.nih.gov/?term=Joseph+S&cauthor_id=42635361)[ 3 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-3 "Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar."), [Deborah Morgan](https://pubmed.ncbi.nlm.nih.gov/?term=Morgan+D&cauthor_id=42635361)[ 3 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-3 "Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar."), [Ryan Uy](https://pubmed.ncbi.nlm.nih.gov/?term=Uy+R&cauthor_id=42635361)[ 3 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-3 "Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar."), [Fatma B Abid](https://pubmed.ncbi.nlm.nih.gov/?term=Abid+FB&cauthor_id=42635361)[ 4 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-4 "Department of Medical Education, Weill Cornell Medicine-Qatar, Cornell University, Doha, Qatar.")[ 5 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-5 "Infectious Disease Division, Hamad Medical Corporation, Doha, Qatar.")[ 6 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-6 "College of Medicine, QU Health, Qatar University, Doha, Qatar."), [Abdullatif Al-Khal](https://pubmed.ncbi.nlm.nih.gov/?term=Al-Khal+A&cauthor_id=42635361)[ 5 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-5 "Infectious Disease Division, Hamad Medical Corporation, Doha, Qatar."), [Abdul-Badi Abou-Samra](https://pubmed.ncbi.nlm.nih.gov/?term=Abou-Samra+AB&cauthor_id=42635361)[ 7 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-7 "Endocrine Division and Center for Diabetes and Endocrine Research \(CeDER\), University of Toledo, Toledo, OH, USA."), [Adeel A Butt](https://pubmed.ncbi.nlm.nih.gov/?term=Butt+AA&cauthor_id=42635361)[ 8 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-8 "Department of Medicine, Hackensack Meridian Health Network JFK University Medical Center, Edison, NJ, USA."), [Laith J Abu-Raddad](https://pubmed.ncbi.nlm.nih.gov/?term=Abu-Raddad+LJ&cauthor_id=42635361)[ 1 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-1 "Infectious Disease Epidemiology Group, Weill Cornell Medicine-Qatar, Cornell University, Doha, Qatar.")[ 2 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-2 "Department of Population Health Sciences, Weill Cornell Medicine, Cornell University, New York, NY, USA.")[ 9 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-9 "Department of Public Health, College of Health Sciences, QU Health, Qatar University, Doha, Qatar.")[ 10 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#full-view-affiliation-10 "College of Health and Life Sciences, Hamad Bin Khalifa University, Doha, Qatar.") Affiliations Expand ### Affiliations * 1 Infectious Disease Epidemiology Group, Weill Cornell Medicine-Qatar, Cornell University, Doha, Qatar. * 2 Department of Population Health Sciences, Weill Cornell Medicine, Cornell University, New York, NY, USA. * 3 Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar. * 4 Department of Medical Education, Weill Cornell Medicine-Qatar, Cornell University, Doha, Qatar. * 5 Infectious Disease Division, Hamad Medical Corporation, Doha, Qatar. * 6 College of Medicine, QU Health, Qatar University, Doha, Qatar. * 7 Endocrine Division and Center for Diabetes and Endocrine Research (CeDER), University of Toledo, Toledo, OH, USA. * 8 Department of Medicine, Hackensack Meridian Health Network JFK University Medical Center, Edison, NJ, USA. * 9 Department of Public Health, College of Health Sciences, QU Health, Qatar University, Doha, Qatar. * 10 College of Health and Life Sciences, Hamad Bin Khalifa University, Doha, Qatar. * PMID: **42635361** * DOI: [ 10.1080/14760584.2026.2720953 ](https://doi.org/10.1080/14760584.2026.2720953) Item in Clipboard # Cohort studies may underestimate rare vaccine adverse events: evidence from stroke incidence following COVID-19 vaccination Hiam Chemaitelly et al. Expert Rev Vaccines. 2026 Dec. Show details Display options Display options Format Abstract PubMed PMID Expert Rev Vaccines Actions * [ Search in PubMed ](https://pubmed.ncbi.nlm.nih.gov/?term=%22Expert+Rev+Vaccines%22%5Bjour%5D&sort=date&sort_order=desc) * [ Search in NLM Catalog ](https://www.ncbi.nlm.nih.gov/nlmcatalog?term=%22Expert+Rev+Vaccines%22%5BTitle+Abbreviation%5D) * [ Add to Search ](https://pubmed.ncbi.nlm.nih.gov/42635361/) . 2026 Dec;25(1):2720953. doi: 10.1080/14760584.2026.2720953. Epub 2026 Aug 24. ### Authors [Hiam Chemaitelly](https://pubmed.ncbi.nlm.nih.gov/?term=Chemaitelly+H&cauthor_id=42635361)[ 1 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-1 "Infectious Disease Epidemiology Group, Weill Cornell Medicine-Qatar, Cornell University, Doha, Qatar.")[ 2 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-2 "Department of Population Health Sciences, Weill Cornell Medicine, Cornell University, New York, NY, USA."), [Naveed Akhtar](https://pubmed.ncbi.nlm.nih.gov/?term=Akhtar+N&cauthor_id=42635361)[ 3 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-3 "Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar."), [Salman Al Jerdi](https://pubmed.ncbi.nlm.nih.gov/?term=Al+Jerdi+S&cauthor_id=42635361)[ 3 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-3 "Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar.")[ 4 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-4 "Department of Medical Education, Weill Cornell Medicine-Qatar, Cornell University, Doha, Qatar."), [Saadat Kamran](https://pubmed.ncbi.nlm.nih.gov/?term=Kamran+S&cauthor_id=42635361)[ 3 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-3 "Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar."), [Sujatha Joseph](https://pubmed.ncbi.nlm.nih.gov/?term=Joseph+S&cauthor_id=42635361)[ 3 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-3 "Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar."), [Deborah Morgan](https://pubmed.ncbi.nlm.nih.gov/?term=Morgan+D&cauthor_id=42635361)[ 3 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-3 "Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar."), [Ryan Uy](https://pubmed.ncbi.nlm.nih.gov/?term=Uy+R&cauthor_id=42635361)[ 3 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-3 "Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar."), [Fatma B Abid](https://pubmed.ncbi.nlm.nih.gov/?term=Abid+FB&cauthor_id=42635361)[ 4 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-4 "Department of Medical Education, Weill Cornell Medicine-Qatar, Cornell University, Doha, Qatar.")[ 5 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-5 "Infectious Disease Division, Hamad Medical Corporation, Doha, Qatar.")[ 6 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-6 "College of Medicine, QU Health, Qatar University, Doha, Qatar."), [Abdullatif Al-Khal](https://pubmed.ncbi.nlm.nih.gov/?term=Al-Khal+A&cauthor_id=42635361)[ 5 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-5 "Infectious Disease Division, Hamad Medical Corporation, Doha, Qatar."), [Abdul-Badi Abou-Samra](https://pubmed.ncbi.nlm.nih.gov/?term=Abou-Samra+AB&cauthor_id=42635361)[ 7 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-7 "Endocrine Division and Center for Diabetes and Endocrine Research \(CeDER\), University of Toledo, Toledo, OH, USA."), [Adeel A Butt](https://pubmed.ncbi.nlm.nih.gov/?term=Butt+AA&cauthor_id=42635361)[ 8 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-8 "Department of Medicine, Hackensack Meridian Health Network JFK University Medical Center, Edison, NJ, USA."), [Laith J Abu-Raddad](https://pubmed.ncbi.nlm.nih.gov/?term=Abu-Raddad+LJ&cauthor_id=42635361)[ 1 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-1 "Infectious Disease Epidemiology Group, Weill Cornell Medicine-Qatar, Cornell University, Doha, Qatar.")[ 2 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-2 "Department of Population Health Sciences, Weill Cornell Medicine, Cornell University, New York, NY, USA.")[ 9 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-9 "Department of Public Health, College of Health Sciences, QU Health, Qatar University, Doha, Qatar.")[ 10 ](https://pubmed.ncbi.nlm.nih.gov/42635361/#short-view-affiliation-10 "College of Health and Life Sciences, Hamad Bin Khalifa University, Doha, Qatar.") ### Affiliations * 1 Infectious Disease Epidemiology Group, Weill Cornell Medicine-Qatar, Cornell University, Doha, Qatar. * 2 Department of Population Health Sciences, Weill Cornell Medicine, Cornell University, New York, NY, USA. * 3 Neurosciences Institute, Hamad Medical Corporation, Doha, Qatar. * 4 Department of Medical Education, Weill Cornell Medicine-Qatar, Cornell University, Doha, Qatar. * 5 Infectious Disease Division, Hamad Medical Corporation, Doha, Qatar. * 6 College of Medicine, QU Health, Qatar University, Doha, Qatar. * 7 Endocrine Division and Center for Diabetes and Endocrine Research (CeDER), University of Toledo, Toledo, OH, USA. * 8 Department of Medicine, Hackensack Meridian Health Network JFK University Medical Center, Edison, NJ, USA. * 9 Department of Public Health, College of Health Sciences, QU Health, Qatar University, Doha, Qatar. * 10 College of Health and Life Sciences, Hamad Bin Khalifa University, Doha, Qatar. * PMID: **42635361** * DOI: [ 10.1080/14760584.2026.2720953 ](https://doi.org/10.1080/14760584.2026.2720953) Item in Clipboard Cite Display options Display options Format Abstract PubMed PMID ## Abstract **Background:** The self-controlled case series (SCCS) design is widely used for short-term vaccine safety assessment but allows only indirect estimation of absolute risk differences. This study evaluated whether absolute measures should be estimated exclusively using cohort designs by comparing excess stroke incidence following COVID-19 vaccination in a nationwide cohort with previous estimates from an SCCS analysis. **Methods:** A nationwide retrospective cohort study was conducted in Qatar between 1 December 2020 and 11 April 2023. Stroke incidence during the 42 days following vaccination was compared between 1,111,939 vaccinated persons and an equal number of individually matched unvaccinated persons. **Results:** During follow-up, 8 strokes occurred in the vaccinated cohort and 19 in the unvaccinated cohort, corresponding to incidence rates of 0.2 (95% CI, 0.1-0.3) and 0.4 (95% CI, 0.2-0.6) per 100,000 person-weeks, respectively. The adjusted hazard ratio comparing vaccinated with unvaccinated individuals was 0.42 (95% CI, 0.18-0.96). **Conclusion:** These cohort findings suggesting a spurious protective effect contrast with SCCS results from the same and other populations, which indicate a small increase in post-vaccination stroke risk. This divergence may reflect healthy vaccinee bias in between-person comparisons, challenging the exclusive use of cohort designs for estimating absolute risk differences. **Keywords:** Bias; cerebrovascular disorders; healthy vaccinee effect; immunity; immunization; self-controlled case-series; vaccine. [PubMed Disclaimer](https://pubmed.ncbi.nlm.nih.gov/disclaimer/) ## Similar articles * [ Incidence of stroke following COVID-19 vaccination: a nationwide self‑controlled risk interval study in Qatar. ](https://pubmed.ncbi.nlm.nih.gov/41299818/) Chemaitelly H, Akhtar N, Al Jerdi S, Kamran S, Joseph S, Morgan D, Uy R, Abid FB, Al-Khal A, Abou-Samra AB, Butt AA, Abu-Raddad LJ.Chemaitelly H, et al.Expert Rev Vaccines. 2025 Dec;24(1):1101-1110. doi: 10.1080/14760584.2025.2596676. Epub 2025 Dec 2.Expert Rev Vaccines. 2025.PMID: 41299818 * [ All-cause mortality within seven days of COVID-19 vaccination: A matched cohort and self-controlled case series study in Lombardy, Italy. ](https://pubmed.ncbi.nlm.nih.gov/42184895/) Corrao G, Sguazzini E, Gori A, Monti E, Cereda D, Melazzini M, Franchi M.Corrao G, et al.J Infect. 2026 Jul;93(1):106769. doi: 10.1016/j.jinf.2026.106769. Epub 2026 May 24.J Infect. 2026.PMID: 42184895 * [ Effects of human papillomavirus (HPV) vaccination programmes on community rates of HPV-related disease and harms from vaccination. ](https://pubmed.ncbi.nlm.nih.gov/41276264/) Henschke N, Bergman H, Buckley BS, Crosbie EJ, Dwan K, Golder SP, Kyrgiou M, Loke YK, McIntosh HM, Prob
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