A national multicentre retrospective study conducted in China revealed that the incidence of thrombocytopenia (< 150 * 10
9/L) on admission in COVID-19 was 36.2% [
3], which is similar to that in SARS (40–45%%) and MERS (36%) [
4]. It has been widely accepted that thrombocytopenia is indicative of disease severity, and a progressive decline of platelet counts was significantly associated with increased mortality [
1]. Although thrombocytopenia is often believed to be an indicator of bleeding, the frequency of bleeding events was reportedly significantly lower in COVID-19 than in Ebola and other infections caused by haemorrhagic fever viruses. Liao et al
. found that only three out of 55 nonsurvivors experienced nonlethal haemorrhagic events [
4]. Bowles et al
. also showed that no clinically significant haemorrhage was found in 35 COVID-19 patients who suffered from a prolonged activated partial-thromboplastin time (aPTT) [
5]. The possible reason for this might be that the pattern of coagulation disorder in COVID-19 is of a more severe hypercoagulative state rather than a hypocoagulative one [
6]. Currently, the mechanisms by which SARS-CoV-2 causes thrombocytopenia are speculated to involve the following: (1) an impaired haematopoietic microenvironment caused by systemic inflammation or cytokine storm, for example, elevated IL-6, which is a common phenomenon in SARS-CoV-2 infection [
4], could suppress haematopoiesis [
7]. (2) SARS-CoV-2 might directly infect haematopoietic stem cells or megakaryocytes through angiotensin-converting enzyme 2 (ACE2), CD13 or CD66a, as in other coronavirus infections that elicit thrombocytopenia [
8]. (3) antiviral antibodies cross-reacting with haematopoietic cells and (or) platelets, e.g. anti-adenovirus antibodies, can cross-react with platelet integrin GPIIb/IIIa [
8]. Indeed, Chen et al
. showed that delayed-phase thrombocytopenia was the result of impaired maturation of megakaryocytes in COVID-19 patients [
9]. (4) An autopsy of nonsurvivors revealed thrombotic microangiopathy and disseminated intravascular coagulation, which lead to the increased consumption of platelets [
6]. (5) Activated platelets can be scavenged via splenic/hepatic macrophages. In fact, two separate teams independently provided evidence that platelets are hyperactivated in COVID-19 patients [
10,
11]. The activation of the Mitogen-activated protein kinase pathway could partially explain this platelet hyperreactivity [
10,
11].
Similar to the influenza virus, rhinovirus and some other viruses, SARS-CoV-2 may also directly interact with platelets, thereby altering their quantity and/or function. ACE2 has been shown to be a receptor of both SARS-CoV-2 and SARS-CoV. SARS-CoV-2 possesses an affinity for ACE2 that is at least 10 times greater than that of SARS-CoV [
12]. Transmembrane protease serine 2 (TMPRSS2), a serine protease, mediates the SARS-CoV-2 virus cell membrane fusions by proteolytically cleaving and activating the spike protein. Recently, Zhang et al
. discovered that platelets expressed abundant ACE2 and TMPRSS2 [
11]. Here, SARS-CoV-2 was shown to directly induce platelet activation and aggregation and promote thrombosis [
11].