Optical densities of the anti-PF4/heparin IgG EIA (antigen assay) before and after second vaccination shot against COVID-19. Of 28/29 patients with the second vaccination against COVID-19, serum samples before and after the second vaccination shot were available (median time, 13 days after second vaccination [range, 5-79 days]). The optical density of the anti-PF4/heparin IgG EIA decreased in 21 of the patients, while in 6 patients the OD did not increase beyond the interassay variability (±0.1 OD), overall P = .00043. Only 1 patient showed a slight increase in OD of the anti-PF4/heparin IgG EIA (2.47 to 2.82) after the second vaccination.