Increased platelet activation in SARS‐CoV‐2 infected non‐hospitalised children and adults, and their household contacts

Abstract

COVID-19 is associated with haemostatic dysregulation, with thromboembolism occurring in 25% of hospitalised COVID-19 patients and microvascular thrombi reported at autopsy. Platelets are activated in COVID-19 patients requiring intensive care, while limited data in mild COVID19 shows no changes in platelet phenotype. Children have low-risk of severe COVID-19 and thrombosis. If haemostasis is fundamental to COVID-19 pathogenesis, then age-related haemostatic differences may protect children from COVID-19. While there is evidence of changes in leucocyte populations of non-hospitalised children and adults infected and exposed to SARS-CoV-2, the effect on paediatric platelets is unknown. We investigated platelet surface-markers in adults and children who were SARS-CoV-2-positive and their household contacts. We aimed to establish whether SARS-CoV-2 induced changes in platelet phenotype in individuals with mild COVID-19. Participants were recruited from the Respiratory Infection Clinic at The Royal Children’s Hospital (RCH), Melbourne, using the enrollment protocol and participant pool previously described. Upon a positive SARS-CoV-2 polymerase chain reaction (PCR) test, blood collection was arranged for family members at two time points: ‘acute’, within two weeks of post-test and ‘convalescent’, 4–7 weeks post-test. Individuals were classified ‘SARS-CoV-2-positive’ if they tested positive, and ‘SARS-CoV-2-exposed’ if they tested negative on repeated tests but remained in close household contact with individuals who tested positive. All participants recovered at home. This study was approved by the RCH Human Research Ethics Committee (QA/63666/RCHM-2020). Healthy samples were collected prior to the COVID-19 pandemic, under the Royal Children’s Hospital Human Research and Ethics Committee 34183/32287. Controls were treated identically to COVID-19-related samples. Blood was collected into tubes containing one part sodium citrate to nine parts blood (Sarstedt, US). Samples from COVID-19 families and healthy adults were collected by venepuncture, while samples from healthy children were collected by cannula during elective day surgery. The platelet count was determined using ADVIA 2120i (Siemens, Germany). Platelet surface-markers were determined by the International Society of Thrombosis and Haemostasis guidelines for platelet function disorders (Table SI). Platelet count was standardised to 4 9 10 platelets/μl in 1% BSA-HEPES. Samples were stained and fixed with 1% formaldehyde. An isotype control included 20 lM