An observational study to identify the prevalence of thrombocytopenia and anti-PF4/polyanion antibodies in Norwegian health care workers after COVID-19 vaccination

J Thromb Haemost. 2021 Jul;19(7):1813-1818. doi: 10.1111/jth.15352. Epub 2021 May 18.


Background: The COVID-19 vaccine from AstraZeneca (AZD1222) is one of several vaccines introduced to provide immunity against SARS-CoV-2. Recently, more than 50 cases have been reported presenting a combination of thrombosis, thrombocytopenia, and remarkably high levels of anti-platelet factor 4 (PF4)/polyanion antibodies post-AZD1222 vaccination. Now linked to the vaccine, the condition is referred to as vaccine-induced immune thrombotic thrombocytopenia. The European Medicines Agency still recommends vaccination with AZD1222, but several European countries have temporally paused and/or restricted its use because of the perceived risk of this severe side effect. Because there is no description of PF4/polyanion antibody testing in the clinical trials, knowledge about the prevalence of such antibodies in a vaccinated cohort is needed.

Objectives: To investigate prevalence of thrombocytopenia and anti-PF4/polyanion antibodies in a population recently vaccinated with AZD1222.

Patients/methods: Four hundred and ninety-two health care workers recently vaccinated with the first dose of AZD1222 were recruited from two hospitals in Norway. Study individuals were screened for thrombocytopenia and the presence of anti-PF4/polyanion antibodies with a PF4/PVS immunoassay. Side effects after vaccination were registered.

Results: The majority of study participants had normal platelet counts and negative immunoassay. Anti-PF4/polyanion antibodies without platelet activating properties were only detected in six individuals (optical density ≥0.4, range 0.58-1.16), all with normal platelet counts. No subjects had severe thrombocytopenia.

Conclusions: We found low prevalence of both thrombocytopenia and antibodies to PF4/polyanion-complexes among Norwegian health care workers after vaccination with AZD1222.

Keywords: COVID-19 vaccines; COVID-19 vaccines/adverse effects; drug related side effects and adverse reactions; platelet factor 4; thrombocytopenia.

Publication types

  • Observational Study

MeSH terms

  • COVID-19 Vaccines
  • COVID-19*
  • ChAdOx1 nCoV-19
  • Europe
  • Health Personnel
  • Heparin
  • Humans
  • Norway / epidemiology
  • Platelet Factor 4
  • Polyelectrolytes
  • Prevalence
  • SARS-CoV-2
  • Thrombocytopenia* / chemically induced
  • Thrombocytopenia* / diagnosis
  • Thrombocytopenia* / epidemiology
  • Vaccination


  • COVID-19 Vaccines
  • Polyelectrolytes
  • polyanions
  • Platelet Factor 4
  • Heparin
  • ChAdOx1 nCoV-19