Vaccination against COVID-19 using adenovirus vector vaccines causes vaccine-induced thrombotic thrombocytopenia (VITT) (ChAdOx1 nCoV-19; Ad26.COV2-S). For an observational study, researchers followed VITT patients for changes in their reactivity of platelet-activating antiplatelet factor 4 (PF4) immunoglobulin G (IgG) antibodies using an anti-PF4/heparin IgG enzyme immunoassay (EIA) and a functional test for PF4-dependent, platelet-activating antibodies and new thrombotic complications. About 65 VITT patients (41 females; median age, 51 years; range, 18-80 years) were followed for 25 weeks on average (range, 3-36 weeks). The functional test became negative in 48/65 individuals (73.8%; CI, 62.0% to 83.0%). 

The median period from a negative functional test result to a positive functional test result was 15.5 weeks (range, 5-28 weeks). Parallel to it, EIA optical density (OD) values reduced from a median of 3.12 to 1.52 (P<.0001), however only 14 (21.5%) individuals showed seroreversion to a negative result. About 5 patients (7.5%) had chronic platelet-activating antibodies and high EIA ODs for more than 11 weeks. Regardless of whether PF4-dependent platelet-activating antibodies were still present, none of the 29 VITT patients who received a second immunization dose with an mRNA COVID-19 vaccine had new thromboses or a significant rise in anti-PF4/heparin IgG EIA OD. Most VITT patients had transitory PF4-dependent platelet-activating antibodies. VITT patients can get a second COVID-19 mRNA-vaccine injection without risk.