The CDC states that avoiding crowded indoor spaces reduces the risk of COVID-19 infection. [227] When indoors, increasing the rate of air change, decreasing recirculation of air and increasing the use of outdoor air can reduce transmission. [227] [228] The WHO recommends ventilation and air filtration in public spaces to help clear out infectious aerosols. [229] [230] [231] A high incidence of thrombosis and venous thromboembolism occurs in people transferred to intensive care units with COVID‑19 infections, and may be related to poor prognosis. [138] Blood vessel dysfunction and clot formation (as suggested by high D-dimer levels caused by blood clots) may have a significant role in mortality, incidents of clots leading to pulmonary embolisms, and ischaemic events (strokes) within the brain found as complications leading to death in people infected with COVID‑19. [139] Infection may initiate a chain of vasoconstrictive responses within the body, including pulmonary vasoconstriction – a possible mechanism in which oxygenation decreases during pneumonia. [139] Furthermore, damage of arterioles and capillaries was found in brain tissue samples of people who died from COVID‑19. [140] [141]

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Chan JF, Yuan S, Kok KH, To KK, Chu H, Yang J, etal. (February 2020). "A familial cluster of pneumonia associated with the 2019 novel coronavirus indicating person-to-person transmission: a study of a family cluster". Lancet. 395 (10223): 514–523. doi: 10.1016/S0140-6736(20)30154-9. PMC 7159286. PMID 31986261. COVID-19 is mainly transmitted when people breathe in air contaminated by droplets/ aerosols and small airborne particles containing the virus. Infected people exhale those particles as they breathe, talk, cough, sneeze, or sing. [78] [79] [80] [81] Transmission is more likely the closer people are. However, infection can occur over longer distances, particularly indoors. [78] [82] It is unknown whether different persons use similar antibody genes in response to COVID‑19. [155] Host cytokine response Mild versus severe immune response during virus infectionSeveral notable variants of SARS-CoV-2 emerged throughout 2020. [108] [109] Cluster 5 emerged among minks and mink farmers in Denmark. [110] After strict quarantines and a mink euthanasia campaign, the cluster was assessed to no longer be circulating among humans in Denmark as of 1 February 2021. [111] Repurposed antiviral drugs make up most of the research into COVID‑19 treatments. [483] [484] Other candidates in trials include vasodilators, corticosteroids, immune therapies, lipoic acid, bevacizumab, and recombinant angiotensin-converting enzyme 2. [484] The likelihood of new-onset, self-reported long COVID after a second coronavirus (COVID-19) infection compared with a first infection, using data from the COVID-19 Infection Survey. Experimental Statistics.

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In November 2020, the US Food and Drug Administration (FDA) issued an emergency use authorisation for the investigational monoclonal antibody therapy bamlanivimab for the treatment of mild-to-moderate COVID‑19. [501] Bamlanivimab is authorised for people with positive results of direct SARS-CoV-2 viral testing who are twelve years of age and older weighing at least 40 kilograms (88lb), and who are at high risk for progressing to severe COVID‑19 or hospitalisation. [501] This includes those who are 65 years of age or older, or who have chronic medical conditions. [501] Nogrady B (November 2020). "What the data say about asymptomatic COVID infections". Nature. 587 (7835): 534–535. Bibcode: 2020Natur.587..534N. doi: 10.1038/d41586-020-03141-3. PMID 33214725.Half of young adults with COVID-19 had persistent symptoms after 6 months". medicalxpress.com . Retrieved 10 July 2021. Tocilizumab has been included in treatment guidelines by China's National Health Commission after a small study was completed. [506] [507] It is undergoing a Phase II non-randomised trial at the national level in Italy after showing positive results in people with severe disease. [508] [509] Combined with a serum ferritin blood test to identify a cytokine storm (also called cytokine storm syndrome, not to be confused with cytokine release syndrome), it is meant to counter such developments, which are thought to be the cause of death in some affected people. [510] The interleukin-6 receptor (IL-6R) antagonist was approved by the FDA to undergo a Phase III clinical trial assessing its effectiveness on COVID‑19 based on retrospective case studies for the treatment of steroid-refractory cytokine release syndrome induced by a different cause, CAR T cell therapy, in 2017. [511] There is no randomised, controlled evidence that tocilizumab is an efficacious treatment for CRS. Prophylactic tocilizumab has been shown to increase serum IL-6 levels by saturating the IL-6R, driving IL-6 across the blood–brain barrier, and exacerbating neurotoxicity while having no effect on the incidence of CRS. [512] a b c d e Li C, Zhao C, Bao J, Tang B, Wang Y, Gu B (November 2020). "Laboratory diagnosis of coronavirus disease-2019 (COVID-19)". Clinica Chimica Acta; International Journal of Clinical Chemistry. 510: 35–46. doi: 10.1016/j.cca.2020.06.045. PMC 7329657. PMID 32621814. COVID‑19 may also cause substantial structural changes to blood cells, sometimes persisting for months after hospital discharge. [142] A low level of blood lymphocytes may result from the virus acting through ACE2-related entry into lymphocytes. [143] Kidneys