Pan American Health Organization · 2021
Interim guidance on pharmacovigilance and risk assessment for thrombotic events following Oxford-AstraZeneca COVID-19 vaccination for clinicians and public health professionals.
- Issuer
- Pan American Health Organization
- Edition
- 2021
- Evidence
- Expert consensus based on EMA PRAC and WHO GACVS pharmacovigilance assessment
- Reviewed
- 19 Oct 2026 · GMJ
What this guideline says
GMJ summary
This communiqué from the Pan American Health Organization addresses reports of thrombotic events following Oxford-AstraZeneca (Vaxzevria) COVID-19 vaccination in Europe as of April 2021. Based on assessments by WHO’s Global Advisory Committee on Vaccine Safety (GACVS) and the European Medicines Agency’s Pharmacovigilance Risk Assessment Committee (PRAC), the document confirms that whilst very rare coagulation events—particularly cerebral venous sinus thrombosis (CVST) and splanchnic vein thrombosis (SVT)—may be linked to the vaccine, the overall benefit-risk balance remains favourable.
As of 7 April 2021, PRAC evaluated 62 CVST and 24 SVT events from approximately 25 million vaccinated persons; by 4 April, 169 CVST and 53 SVT cases had been reported across 34 million vaccinated individuals in Europe. Most cases occurred in women under 60 years of age within two weeks of vaccination. Despite these signals, both WHO and EMA recommend continuation of vaccination campaigns given the risk of COVID-19 morbidity and mortality.
The guidance provides specific clinical recommendations for patient education on warning signs, emphasises the importance of specialist treatment for rapid recovery, and outlines risk-benefit considerations for individuals at elevated thrombotic risk. Countries are advised to maintain pharmacovigilance systems and continue reporting adverse events whilst investigations proceed.
Key points
For clinicians — doses and thresholds as stated in the guideline
- As of 7 April 2021, 62 cases of cerebral venous sinus thrombosis (CVST) and 24 cases of splanchnic vein thrombosis (SVT) were reported in approximately 25 million vaccinated individuals.
- By 4 April 2021, 169 CVST and 53 SVT cases had been reported across 34 million vaccinated persons in Europe.
- Most thrombotic events occurred in women under 60 years of age, within two weeks after vaccination.
- Patients should seek immediate medical care within 4–20 days post-vaccination if experiencing difficulty breathing, chest pain, leg inflammation, persistent abdominal pain, neurological symptoms (persistent severe headaches or blurred vision), or petechiae beyond the injection site.
- Pre-vaccination risk-benefit analysis is recommended for persons at risk for thromboembolic events (e.g. oral contraceptive users, history of previous thrombotic events).
- COVID-19 infection should be ruled out as a causal event when investigating thrombotic complications.
- Rapid specialist treatment is essential to facilitate recovery and prevent complications from these very rare coagulation events.
- WHO and EMA conclude the benefits of Oxford-AstraZeneca vaccination outweigh risks and recommend continuation of vaccination campaigns with enhanced pharmacovigilance.
From the guideline
Verbatim
“Patients should seek immediate medical care if (especially within 4 to 20 days following vaccination) they have difficulty breathing, chest pain, inflammation in the legs, persistent abdominal pain, neurological symptoms (including persistent and severe headaches or blurred vision), or small spots of blood under the skin beyond the injection site.”
Editor’s perspective
Why this matters for Georgia and the region
Prof. Giorgi Pkhakadze, MD, MPH, PhD
Editor-in-Chief, Georgian Medical Journal · Chair, Public Health Institute of Georgia · WHO consultant, health and migration
As Georgia's national COVID-19 vaccination programme expanded in 2021, this PAHO guidance arrived at a critical juncture when public confidence required transparent communication about vaccine safety signals. The document's emphasis on maintaining robust pharmacovigilance whilst continuing vaccination reflects the essential balance we must strike in our region, where COVID-19 transmission has severely impacted health systems. I would stress to Georgian clinicians the specific 4–20 day post-vaccination window for thrombotic symptoms and the importance of rapid specialist referral, as timely recognition can prevent severe complications. The practical challenge for Georgia will be ensuring that primary care physicians and rural practitioners have clear pathways for urgent haematology consultation and that public messaging about warning signs reaches vaccinated individuals without undermining overall vaccine confidence in a population already facing substantial COVID-19 mortality risk.
ORCID 0000-0001-7609-4515 · Perspective written for the GMJ Guidelines Library, 19 Oct 2026
Applicability
Georgia and the South Caucasus
For Georgia and the South Caucasus, this guidance is directly applicable to national COVID-19 vaccination programmes using Oxford-AstraZeneca (Vaxzevria) vaccine. National regulatory authorities and immunisation programmes should ensure robust pharmacovigilance systems are in place to detect and report thrombotic events, particularly CVST and SVT occurring within two weeks of vaccination. Clinicians should be trained to recognise warning signs in the 4–20 day post-vaccination window and to conduct pre-vaccination risk assessments for individuals with thrombotic risk factors, including those using oral contraceptives. Health communication strategies must inform vaccinated persons about symptoms requiring urgent medical attention whilst maintaining public confidence in the overall favourable benefit-risk profile of the vaccine in the context of COVID-19 transmission dynamics in the region.
Questions this guideline answers
Quick answers
What thrombotic events have been associated with Oxford-AstraZeneca COVID-19 vaccine?
Very rare cases of cerebral venous sinus thrombosis (CVST) and splanchnic vein thrombosis (SVT) have been reported, occurring primarily in women under 60 years of age within two weeks of vaccination. As of 4 April 2021, 169 CVST and 53 SVT cases were reported in 34 million vaccinated individuals in Europe.
When should patients seek medical care after receiving Oxford-AstraZeneca vaccine?
Patients should seek immediate medical care within 4–20 days post-vaccination if they experience difficulty breathing, chest pain, leg inflammation, persistent abdominal pain, neurological symptoms including persistent severe headaches or blurred vision, or small spots of blood under the skin beyond the injection site.
Should vaccination with Oxford-AstraZeneca continue despite thrombotic event reports?
Yes. Both WHO and the European Medicines Agency consider the benefits of the Oxford-AstraZeneca vaccine to outweigh its risks, given the risk of COVID-19 morbidity and mortality, and recommend that vaccinations continue with enhanced pharmacovigilance.
What pre-vaccination assessment is recommended for individuals at thrombotic risk?
A pre-vaccination risk-benefit analysis is recommended for persons at risk for thromboembolic events, including those using oral contraceptives or with a history of previous thrombotic events. COVID-19 infection should also be ruled out as a causal event.
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- Influenza and Other Respiratory Viruses: Surveillance in the Americas 2021
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- Issuer
- Pan American Health Organization
- Type
- Procedures, manuals, guidelines
- Status
- Current
- Published
- 2021-01-01
- Licence
- CC BY-NC-SA 3.0 IGO
- Evidence
- Expert consensus based on EMA PRAC and WHO GACVS pharmacovigilance assessment
- Record history
- Created 19 Oct 2026 · updated 19 Oct 2026 · next check Apr 2027
Pan American Health Organization. Considerations on Thrombotic Events Following Administration of the Oxford-AstraZeneca COVID-19 Vaccine in Europe, 7 April 2021. Washington, D.C.: PAHO; 2021. (PAHO/HSS/MT/COVID-19/21-0004).GMJ Guidelines Library records are original summaries written by the GMJ editorial team from the issuer’s published document and reviewed by a physician. They do not replace the full guideline. The document is the copyright of its issuer and is linked, not hosted. Spotted an error or a newer edition? Tell the Guidelines Desk.
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