{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,31]],"date-time":"2026-10-31T13:25:45Z","timestamp":1793453145539,"version":"4.5.0"},"reference-count":12,"publisher":"MDPI AG","issue":"9","license":[{"start":{"date-parts":[[2021,8,31]],"date-time":"2021-08-31T00:00:00Z","timestamp":1630368000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"National Centre for Infectious Disease Catalyst grant","award":["#FY2021LXR"],"award-info":[{"award-number":["#FY2021LXR"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Vaccines"],"abstract":"<jats:p>Anaphylactic reactions were observed after Singapore\u2019s national coronavirus disease 2019 (COVID-19) vaccination programme started in December 2020. We report the clinical and laboratory features of three patients in our institution who developed anaphylactic reactions after receiving the Pifzer BNT162b2 vaccine. IgM and IgG antibodies, but not IgE antibodies to the Pfizer BNT162b2 vaccine, were detected in all subjects. Similarly, mild to high elevated levels of anti-polyethylene glycol (PEG) IgG (1035\u201319709 U\/mL, vs. vaccine-naive &lt; 265 U\/mL, vaccine-tolerant &lt; 785 U\/mL) and IgM (1682\u20135310 U\/mL, vs. vaccine-naive &lt; 1011 U\/mL, vaccine-tolerant &lt; 1007 U\/mL) were detected in two out of three patients via commercial ELISA. High levels of serum anaphylatoxin C3a (79.0 \u00b1 6.3 \u03bcg\/mL, mean \u00b1 SD, vs. normal &lt; 10 \u03bcg\/mL) were observed in all three patients during the acute phase of the reaction, while tryptase levels, a marker of mast cell activation, were not elevated. Finally, one patient with the highest levels of anti-PEG IgG, IgM, and anti-Pfizer BNT162b2 IgG and IgM exhibited an enhanced Th2 cytokine serum profile during an acute reaction, with high levels of IL-4 (45.7 pg\/mL, vs. vaccine-naive\/tolerant &lt; 2.30 pg\/mL), IL-33 (86.4 pg\/mL, vs. vaccine-naive\/tolerant &lt; 5.51 pg\/mL) and IL-10 (22.9 pg\/mL, vs. vaccine-naive\/tolerant &lt; 12.49 pg\/mL) diminishing over time following corticosteroid treatment. Taken together, we propose these cases of anaphylaxis described are driven by a complement activation-related pseudoallergy (CAPRA), rather than classical IgE-mediated mechanisms.<\/jats:p>","DOI":"10.3390\/vaccines9090974","type":"journal-article","created":{"date-parts":[[2021,9,1]],"date-time":"2021-09-01T03:13:17Z","timestamp":1630465997000},"page":"974","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":34,"title":["Pseudo-Anaphylactic Reactions to Pfizer BNT162b2 Vaccine: Report of 3 Cases of Anaphylaxis Post Pfizer BNT162b2 Vaccination"],"prefix":"10.3390","volume":"9","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-1167-353X","authenticated-orcid":false,"given":"Xin Rong","family":"Lim","sequence":"first","affiliation":[{"name":"Department of Rheumatology, Allergy and Immunology, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1119-4934","authenticated-orcid":false,"given":"Bernard Pui","family":"Leung","sequence":"additional","affiliation":[{"name":"Department of Rheumatology, Allergy and Immunology, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore"},{"name":"Health and Social Sciences, Singapore Institute of Technology, Singapore 138683, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Carol Yee Leng","family":"Ng","sequence":"additional","affiliation":[{"name":"Clinical Immunology Laboratory, Tan Tock Seng Hospital, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Justina Wei Lynn","family":"Tan","sequence":"additional","affiliation":[{"name":"Department of Rheumatology, Allergy and Immunology, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Grace Yin Lai","family":"Chan","sequence":"additional","affiliation":[{"name":"Department of Rheumatology, Allergy and Immunology, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Chien Mei","family":"Loh","sequence":"additional","affiliation":[{"name":"Clinical Immunology Laboratory, Tan Tock Seng Hospital, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Gwendolyn Li Xuan","family":"Tan","sequence":"additional","affiliation":[{"name":"Clinical Immunology Laboratory, Tan Tock Seng Hospital, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Valerie Hui Hian","family":"Goh","sequence":"additional","affiliation":[{"name":"Clinical Immunology Laboratory, Tan Tock Seng Hospital, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Lok To","family":"Wong","sequence":"additional","affiliation":[{"name":"Clinical Immunology Laboratory, Tan Tock Seng Hospital, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Chong Rui","family":"Chua","sequence":"additional","affiliation":[{"name":"Clinical Immunology Laboratory, Tan Tock Seng Hospital, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Sze Chin","family":"Tan","sequence":"additional","affiliation":[{"name":"Department of Rheumatology, Allergy and Immunology, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Samuel Shang Ming","family":"Lee","sequence":"additional","affiliation":[{"name":"Department of Rheumatology, Allergy and Immunology, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Hwee Siew","family":"Howe","sequence":"additional","affiliation":[{"name":"Department of Rheumatology, Allergy and Immunology, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Bernard Yu Hor","family":"Thong","sequence":"additional","affiliation":[{"name":"Department of Rheumatology, Allergy and Immunology, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Khai Pang","family":"Leong","sequence":"additional","affiliation":[{"name":"Department of Rheumatology, Allergy and Immunology, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433, Singapore"},{"name":"Clinical Immunology Laboratory, Tan Tock Seng Hospital, Singapore 308433, Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2021,8,31]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"CDC COVID-19 Response Team, and Food and Drug Administration (2021). 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Methods"}],"container-title":["Vaccines"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2076-393X\/9\/9\/974\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T06:52:57Z","timestamp":1760165577000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2076-393X\/9\/9\/974"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2021,8,31]]},"references-count":12,"journal-issue":{"issue":"9","published-online":{"date-parts":[[2021,9]]}},"alternative-id":["vaccines9090974"],"URL":"https:\/\/doi.org\/10.3390\/vaccines9090974","relation":{},"ISSN":["2076-393X"],"issn-type":[{"value":"2076-393X","type":"electronic"}],"subject":[],"published":{"date-parts":[[2021,8,31]]}}}