In February of 2021, a disorder resembling heparin-induced thrombocytopenia (HIT) was described in a cohort of 11 patients approximately 1-2 weeks after vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) with ChAdOx1 nCOV-19 (AstraZeneca). A similar disorder was also noted in a group of patients who received vaccination with Ad26.COV2.S (Johnson & Johnson [Janssen]). The disorder is characterized by thrombocytopenia, thrombosis, and the presence of immunoglobulin G (IgG) antibodies against platelet factor 4 (PF4). The syndrome is referred to as vaccine-induced immune thrombotic thrombocytopenia, or VITT. It is also known as thrombosis with thrombocytopenia syndrome (TTS).

The exact incidence of this syndrome is unknown; however, by the spring of 2021 within the United States, approximately 15 cases had been identified among 8 million individuals vaccinated with As26.COV2.S (incidence of 1 in 533 333). In Norway, the incidence is higher (approximately 1 in 26 000). Initial studies showed a female predominance; however, a later study from the United Kingdom showed no sex predilection. Regardless, the incidence is felt to be extremely low, and the risk of death and serious outcomes of COVID-19, including thrombosis, far outweigh the risk of TTS possibly associated with these highly efficacious vaccines.

VITT is distinct from HIT in that the antibodies are not heparin dependent in VITT. Triggers other than heparin previously have been described as causing a HIT-like syndrome. These include pentosan polysulfate, antiangiogenic agent PI-88, and hypersulfated chondroitin sulfate. Other known triggers include bacterial and viral infection as well as joint replacement surgery. Such cases with no exposure to heparin are known as "spontaneous HIT."

IgG antibodies in the serum of patients with VITT strongly activate platelets via PF4, both in the presence and absence of heparin. These antibodies may also cause widespread activation of coagulation, endothelial cells, and neutrophils, leading to higher risk of thrombosis.

Related topics: cutaneous reactions after COVID-19 vaccination, COVID-19