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

Abstract

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.

FIGURE 1

In total, only eight of the 492 vaccinated individuals (1.6%) had thrombocytopenia, with platelet count above 100 × 10⁹/L. Line represents median, error bars 95% confidence interval

FIGURE 2

Anti‐PF4/PVS reactive antibodies were detectable in six individuals (1.2%) in the vaccine cohort by ELISA. The highest OD value was 1.16, OD ≥0​.4 indicate presence of antibodies. OD, Optical density; PF4, platelet factor 4