Systematic Review Finds Higher Risk of Thromboembolic Events Following First Dose of ChAdOx1-S Vaccine

Research published in the Indian Journal of Community Medicine showcases the increased risk of certain events such as CVS, PVT, and thrombocytopenia by 2-3 times at the first dose of the ChAdOx1-S vaccine. However, for secondary outcomes like any venous, arterial thromboembolic, and hemorrhagic events, heart attack, or coronary artery disease (CAD), no significant link was found, or the evidence was low to very low.

ChAdOx1-S (AstraZeneca COVID-19 vaccine AZD1222, Vaxzevria™, SII COVISHIELD™) is an adenoviral vector vaccine administered in two doses, with a recommended interval of 4 to 12 weeks between doses. In early 2021, cases of venous thrombosis with thrombocytopenia were reported within 3 to 4 weeks after receiving the ChAdOx1-S vaccine.

Numerous case reports and case studies have been published mentioning CVS thrombosis, deep vein thrombosis, and pulmonary embolism but none of the studies reported any epidemiological measure of the strength of association between ChAdOx1-S vaccination and the incidence of thromboembolic, thrombocytopenic, and hemorrhagic events.

To determine the same, a systematic review and meta-analysis of analytical studies were conducted by Raman S Vaman, Scholar, ICMR-School of Public Health, ICMR- National Institute of Epidemiology, Chennai, Tamil Nadu, India and colleagues.

Two independent reviewers screened for eligible studies, extracted data, and assessed the risk of bias. The DerSimonian-Laird random effects model was used to pool the incidence rate ratios (IRRs) separately for the first and second doses. Heterogeneity was measured using I2 statistics. A total of 20 studies were included, comprising 11 self-controlled case series and nine cohort studies, with a combined participant pool of 254 million.

Research indicated the following findings:

• A meta-analysis of 17 studies revealed an elevated risk of cerebrovascular thrombosis (IRR = 3.5), portal vein thrombosis (PVT) (IRR = 2.0), and thrombocytopenia (IRR = 1.6) in individuals who received the ChAdOx1-S vaccine compared to controls.
• No increased risk was observed after the second dose or for secondary outcomes.

“Since most studies were conducted in Europe, it is crucial to develop national-level databases in low- and middle-income countries to monitor rare adverse events in the post-marketing phase,” researchers concluded.

Reference: Vaman, Raman S.^*; Kumar, Muthusamy S.^1,*; Jeyashree, Kathiresan²; Periasami, Ashok; Abdulkader, Rizwan S.³; Murhekar, Manoj⁴. Association between COVID-19 Vaccination (ChAdOx1-S) and Thromboembolic, Thrombocytopenic, Hemorrhagic Events: A Systematic Review and Meta-analysis of Analytical Epidemiological Studies. Indian Journal of Community Medicine 49(4):p 571-578, Jul–Aug 2024. | DOI: 10.4103/ijcm.ijcm_676_23.