{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,10]],"date-time":"2026-08-10T12:57:12Z","timestamp":1786366632382,"version":"build-2736575974"},"reference-count":17,"publisher":"American Society of Hematology","issue":"4","content-domain":{"domain":["ashpublications.org"],"crossmark-restriction":true},"short-container-title":[],"published-print":{"date-parts":[[2021,7,29]]},"abstract":"<jats:p>Vaccination using the adenoviral vector COVID-19 vaccine ChAdOx1 nCoV-19 (AstraZeneca) has been associated with rare vaccine-induced immune thrombotic thrombocytopenia (VITT). Affected patients test strongly positive in platelet factor 4 (PF4)\/polyanion enzyme immunoassays (EIAs), and serum-induced platelet activation is maximal in the presence of PF4. We determined the frequency of anti-PF4\/polyanion antibodies in healthy vaccinees and assessed whether PF4\/polyanion EIA+ sera exhibit platelet-activating properties after vaccination with ChAdOx1 nCoV-19 (n = 138) or BNT162b2 (BioNTech\/Pfizer; n = 143). In total, 19 of 281 participants tested positive for anti-PF4\/polyanion antibodies postvaccination (All: 6.8% [95% confidence interval (CI), 4.4-10.3]; BNT162b2: 5.6% [95% CI, 2.9-10.7]; ChAdOx1 nCoV-19: 8.0% [95% CI, 4.5% to 13.7%]). Optical densities were mostly low (between 0.5 and 1.0 units; reference range, &amp;lt;0.50), and none of the PF4\/polyanion EIA+ samples induced platelet activation in the presence of PF4. We conclude that positive PF4\/polyanion EIAs can occur after severe acute respiratory syndrome coronavirus 2 vaccination with both messenger RNA- and adenoviral vector-based vaccines, but many of these antibodies likely have minor (if any) clinical relevance. Accordingly, low-titer positive PF4\/polyanion EIA results should be interpreted with caution when screening asymptomatic individuals after vaccination against COVID-19. Pathogenic platelet-activating antibodies that cause VITT do not occur commonly following vaccination.<\/jats:p>","DOI":"10.1182\/blood.2021012217","type":"journal-article","created":{"date-parts":[[2021,5,14]],"date-time":"2021-05-14T15:55:48Z","timestamp":1621007748000},"page":"299-303","update-policy":"https:\/\/doi.org\/10.1182\/blood.2019cm0000","source":"Crossref","is-referenced-by-count":134,"title":["Frequency of positive anti-PF4\/polyanion antibody tests after COVID-19 vaccination with ChAdOx1 nCoV-19 and BNT162b2"],"prefix":"10.1182","volume":"138","author":[{"given":"Thomas","family":"Thiele","sequence":"first","affiliation":[{"name":"Department of Transfusion Medicine, Institute of Immunology and Transfusion Medicine,"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3388-8785","authenticated-orcid":false,"given":"Lena","family":"Ulm","sequence":"additional","affiliation":[{"name":"Friedrich-Loeffler Institute of Medical Microbiology,"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2672-8230","authenticated-orcid":false,"given":"Silva","family":"Holtfreter","sequence":"additional","affiliation":[{"name":"Department of Immunology, Institute of Immunology and Transfusion Medicine, and"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6660-9949","authenticated-orcid":false,"given":"Linda","family":"Sch\u00f6nborn","sequence":"additional","affiliation":[{"name":"Department of Transfusion Medicine, Institute of Immunology and Transfusion Medicine,"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sven Olaf","family":"Kuhn","sequence":"additional","affiliation":[{"name":"Department of Anaesthesiology, University Medicine Greifswald, Greifswald, Germany;"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6171-8874","authenticated-orcid":false,"given":"Christian","family":"Scheer","sequence":"additional","affiliation":[{"name":"Department of Anaesthesiology, University Medicine Greifswald, Greifswald, Germany;"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8046-7588","authenticated-orcid":false,"given":"Theodore E.","family":"Warkentin","sequence":"additional","affiliation":[{"name":"Department of Pathology and Molecular Medicine and"},{"name":"Department of Medicine, McMaster University, Hamilton, ON, Canada; and"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-5020-8542","authenticated-orcid":false,"given":"Barbara M.","family":"Br\u00f6ker","sequence":"additional","affiliation":[{"name":"Department of Immunology, Institute of Immunology and Transfusion Medicine, and"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6391-1341","authenticated-orcid":false,"given":"Karsten","family":"Becker","sequence":"additional","affiliation":[{"name":"Friedrich-Loeffler Institute of Medical Microbiology,"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3365-0168","authenticated-orcid":false,"given":"Konstanze","family":"Aurich","sequence":"additional","affiliation":[{"name":"Department of Transfusion Medicine, Institute of Immunology and Transfusion Medicine,"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kathleen","family":"Selleng","sequence":"additional","affiliation":[{"name":"Department of Transfusion Medicine, Institute of Immunology and Transfusion Medicine,"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Nils-Olaf","family":"H\u00fcbner","sequence":"additional","affiliation":[{"name":"Central Unit for Infection Prevention and Control, University Medicine Greifswald, Greifswald, Germany"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8343-7336","authenticated-orcid":false,"given":"Andreas","family":"Greinacher","sequence":"additional","affiliation":[{"name":"Department of Transfusion Medicine, Institute of Immunology and Transfusion Medicine,"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"234","published-online":{"date-parts":[[2021,5,14]]},"reference":[{"issue":"10277","key":"2021072815240997500_B1","doi-asserted-by":"crossref","first-page":"881","DOI":"10.1016\/S0140-6736(21)00432-3","article-title":"Oxford COVID Vaccine Trial Group. 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