Lab Update 26 – COVID-19 Vaccine Side Effects: The Essential Facts

July 2021
Dr Piet Wessels & Dr Cathy van Rooyen
Ampath | ampath.co.za

Overview

Social media is flooded with misinformation about vaccine risks.
This summary addresses two of the most common medical concerns:

1. Can a patient with previous thrombosis receive a COVID-19 vaccine?
2. Can a patient with allergies receive a COVID-19 vaccine?

Blood Clots, COVID-19 & Vaccination

✅ Key Points:

• COVID-19 infection itself carries a high risk of venous and arterial thrombosis.
• Hospitalised COVID-19 patients are routinely prescribed anticoagulants.
• Outpatient anticoagulant use is still being researched for high-risk groups.
• Post-discharge prophylaxis is recommended while D-dimer remains elevated.

Vaccine-Associated Clotting (VITT)

• Some patients may develop antibodies to platelet factor 4 (PF4) post-vaccination.
• This can lead to Vaccine-Induced Thrombocytopenia with Thrombosis (VITT).
• Timing: Occurs 4–28 days after vaccination
• Risk:
  • 1.7 events per 100,000 (South African Sisonke trial)
  • All cases had known thrombosis risk factors
• Risk of thrombosis is much higher with COVID-19 infection (up to 30% in hospitalised patients).
• A previous history of thrombosis is not a contraindication for vaccination.

Bleeding Risks Post-Vaccination

• Rarely, patients may develop immune thrombocytopenia (ITP).
• Bleeding is uncommon, even with low platelets.
• No evidence of platelet dysfunction due to vaccination.

Managing Suspected VITT

Clinical signs > 4 days post-vaccination:

• Severe or persistent headache
• Blurred vision
• Seizures
• Focal neurological signs
• Severe abdominal pain
• Shortness of breath
• Chest pain
• Leg pain or swelling

Management:

• Consult a clinical haematologist immediately.
• Avoid: Heparin, warfarin, LMWH, and platelet transfusions.
• Manage in units experienced with heparin-induced thrombocytopenia (HITT).

4T Score for VITT Probability

Thrombocytopenia

50% fall: 2

• 30–50% fall: 1
• <30% fall: 0

Timing of Onset

• 4–16 days after vaccination: 2

2 weeks/unclear: 1

• No vaccination: 0

Thrombosis

• New thrombosis: 2
• Progressive/recurrent: 1
• None: 0

Other Causes of Thrombocytopenia

• None: 2
• Possible: 1
• Definite: 0

Interpretation

• 0–3: Low probability
• 4–5: Intermediate probability
• 6–8: High probability

Diagnostics in Suspected VITT

• Anti-PF4 antibody (ELISA)
• Full blood count with peripheral smear
• D-dimer
• Imaging based on symptoms, e.g.:
  • MR venography
  • CT angiography (chest/abdomen)
  • Compression ultrasound (limbs)

Severe Allergic Reactions to Vaccines

Key facts:

• Anaphylaxis is rare.
  • mRNA vaccines: 2.4–4.7 per million
  • Adenoviral vector vaccines: <1 per million
• PEG (in Pfizer) and Polysorbate 80 (in J&J) are likely allergens.
• Vaccines do not contain gelatine or egg proteins.
• Most reactions occur within 5–30 minutes and are treatable.

Screening Questions for Allergy Risk

1. Severe allergy/anaphylaxis to PEG or Polysorbate 80?
2. Anaphylaxis to injectable meds or other vaccines?
3. Anaphylaxis to food, venom, or environmental allergens?
4. Diagnosed with mastocytosis or mast cell activation syndrome?
5. Do you have asthma, hay fever, or mild allergies?

Interpreting Answers

• Yes to 1 or 2:
  → Refer to allergy clinic for testing
• Yes to 3 or 4:
  → Vaccinate in hospital with 30-minute observation
• Yes to 5 only (mild allergy):
  → Vaccinate in any setting with 15-minute observation

Testing & Vaccination Approach

• Allergy clinics use skin prick tests with PEG or Polysorbate-containing substances
• Commercial allergy tests for these excipients are being developed

Approach to COVID-19 Vaccine Allergy

High-Risk:

• Allergy to PEG or Polysorbate 80
  → Refer to allergy clinic

Medium-Risk:

• Previous anaphylaxis to injectable/vaccine
  → Refer to allergy clinic
  • If PEG-positive: consider J&J or AZ
  • If Polysorbate-positive: consider Pfizer
  • If positive to both: vaccine contraindicated

Low-Risk:

• Asthma or mild allergy
  → Vaccinate in any setting with observation

Anaphylaxis after vaccination:

• Draw SST sample for tryptase 30 minutes to 4 hours post-reaction
• Also collect baseline tryptase ≥24 hours later

Conclusion

COVID-19 vaccination is essential and should proceed with minimal barriers.
Doctors and patients need access to accurate information to promote safe, confident uptake.

References

1. Takuva et al., NEJM, June 3, 2021
2. Leentjens et al., Lancet Haematol, 2021
3. Turner PJ et al., WAO Journal, Feb 2021
4. Banjeri A et al., JACI: In Practice, Dec 2020

📌 For further guidance, contact your local Ampath pathologist.