The outbreak of the novel coronavirus, first identified in Wuhan, China was declared a Public Health Emergency of International Concern by the World Health Organisation (WHO) on the 31 st of January 2020. Subsequently named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), this virus has, as of the 5 th of November 2020, infected more than 47 million people, caused massive economic disruption worldwide and prompted a wide range of SARS-CoV-2 shares 79% genome sequence identity with SARS-CoV and targets cells via the same entry receptor, angiotensin-converting enzyme 2 (ACE2) which is displayed on airway epithelial cells, alveolar epithelial cells, vascular endothelial cells and macrophages in the lung. Host cellular entry is mediated by the viral Spike (S) protein, with the receptor binding domain on subunit 1 (S1), binding to ACE2, and proteolytic cleavage of S protein by the cellular serine protease TMPRSS2 facilitating viral activation (4) .
The first case series of 41 patients with COVID19 infection described an illness similar to SARS with most patients presenting with fever, dry cough, dyspnea and bilateral pneumonia, with characteristic ground glass opacities noted on computed tomography (CT) scan (5) .
Since this initial report it has emerged that a large proportion of COVID19 cases are asymptomatic or mildly symptomatic. Estimating the truly asymptomatic fraction is difficult, not least because of inconsistent symptom reporting between countries, with atypical symptoms sometimes excluded, but also the relative scarcity of follow up data on contacts of confirmed cases to identify transmission and differentiate between asymptomatic and presymptomatic infection. A systematic review of papers that account for a potential presymptomatic period estimated that 20% individuals affected by SARS-CoV-2 remain asymptomatic for the entire infection (6) .
Symptomatic COVID19 typically follows a triphasic course. Day one to seven comprise influenza like symptoms with headache, fatigue, dry cough and sore throat all being frequent. Gastrointestinal symptoms are relatively common and may occur in the absence of respiratory symptoms (7) . Diarrhoea, nausea/vomiting or abdominal pain were present in 19%, 11% and 7% respectively in a cohort of 370,000 confirmed cases reported to the Centre for Disease Control and prevention (CDC) in the United States (8) . Anosmia has emerged as a distinctive feature of COVID19 and is estimated to occur in 55% of infections (9) . Presence of anosmia in a cohort of healthcare workers tested for COVID19 gave an adjusted odds ratio of 7.21 of a positive test (10) . The pathophysiology of anosmia has not been fully elucidated, but post mortem samples have found an olfactory neuropathy (11) .
A proportion of patients will develop severe disease, with approximately 17% of those hospitalised requiring intensive care unit (ICU) admission (12) . In hospitalised patients, dyspnea develops 5-8 days post onset of symptoms. Progression to critical disease with development of acute respiratory distress syndrome (ARDS) or ventilation occurs J o u r n a l P r e -p r o o f approximately 10 days post symptom onset (5) . Age is strongly associated with risk for progression to severe disease, as are comorbidities such as obesity, hypertension and diabetes (12) . The severe / critical phase of disease is characterised by severe interstitial pneumonia with high oxygen requirements and elevations in inflammatory markers such as C-reactive protein, d-dimer, fibrinogen, lactate dehydrogenase and interleukin (IL)-6. Active viral replication in the lower respiratory tract, and viral particles are found in pneumocytes on post mortem examination (13) (14) . However these particles are sparse and the pathogenesis of this stage seems to be predominantly immune-mediated rather than due to direct viral cytoxicity, with a dysfunctional immune response leading to uncontrolled inflammation and end-organ damage. The mechanisms of immune dysregulation have not yet been fully elucidated, and studies to date have suggested heterogenous patterns of dysfunction in patients with severe disease (15) (16) . Patients with severe disease have a sustained elevation of a broader range of cytokines compared to those with moderate disease. Although viral RNA load by nasopharyngeal swab does not correlate with overall outcome, it does correlate with elevation of IFNα, IFNγ, tumour necrosis factor (TNF) and tumour necrosis factor-related apoptosis-inducing ligand (TRAIL), and patients with severe disease exhibit a slower decline in viral load, suggesting that viral persistence could drive ongoing inflammation. Lymphopenia is a common feature, with some studies showing a preferential effect on CD8+ T cells and others on CD4+ T cells, CD8+ T cells, NK cells and B cells, potentially reflecting recruitment to inflamed respiratory tissue (17) . CD4+ and CD8+ cells in patients with severe COVID19 display markers of activation or exhaustion but a significant minority show a minimal response (16) . Male sex is a risk factor for poor outcome in COVID19 and early work has identified sex differences in immune response, with female patients showing higher levels of activated and terminally differentiated T cells, particularly (19) , despite prophylactic low molecular weight heparin. Pulmonary embolism is particularly common, occurring twice as frequently compared to patients admitted to the ICU with influenza (19, 20) . Arterial thrombosis has also been observed. One centre reported 20 COVID-19 patients presenting with acute limb ischaemia in a 3 month period, an increase in incidence from 1.8% to 16.3% compared to the same period the prior year (21) . Coagulopathy plays a major role in the pathogenesis of COVID-19 with plateletfibrin thrombi shown on post mortem examination at a much greater frequency than those with influenza pneumonia (22) . Direct infection of the endothelium by SARS-CoV-2 has been demonstrated (23) , and endothelial disruption is postulated to activate the coagulation cascade leading to a prothrombotic state.
SARS-CoV-2 causes direct and indirect cardiac complications. Myocardial injury, manifested by elevation of cardiac biomarkers were reported in an initial case series from Wuhan, China (5) . Echocardiographic abnormalities were observed in 55% of patients in a global observational study of COVID-19 patients who underwent routine echocardiography, 46% of whom had no pre-existing cardiovascular disease (24) . Right ventricular dysfunction may be a result of pulmonary embolism or pneumonia, and myocardial infarction related to the prothrombotic state. Direct infection of the coronary vasculature may account for some of the observed cardiac complications. A study of cardiac magnetic resonance imaging in convalescent COVID-19 patients, the majority of whom had not been hospitalised, demonstrated changes in more than three quarters, suggestive of cardiac inflammatory involvement (25) , and viral RNA has been found in the myocardium of patients who died of COVID-19. Dramatic increases in the rate of out of hospital cardiac arrests was observed in New York, Paris and Lombardy, Italy during the early peak of COVID-19 diagnoses (26)(27) (28) . While some of the observed increase was likely related to disruptions to normal medical care, COVID-19 was considered directly responsible for a proportion of cases.
Diverse neurological presentations have been observed. Most common are non-specific symptoms such as headache, fatigue and dizziness, while confusion or impaired consciousness is seen in 15% of those with severe disease (29) . Acute stroke has been observed, again more commonly in severe infection (29) and COVID-19 has been demonstrated to be an independent risk factor for acute ischaemic stroke (30) . Cases of acute inflammatory demyelinating polyneuropathy, meningoencephalitis, haemorrhagic posterior reversible encephalopathy syndrome and acute necrotizing encephalopathy have all been reported in association with COVID-19 (31) . Although multiple routes of entry to the central nervous system (CNS) have been proposed, including trans-synaptic spread by the olfactory nerve and an increase in blood brain barrier permeability through endothelial disruption, cerebrospinal fluid and autopsy studies do not provide consistent evidence of direct CNS invasion (32) .
Increased understanding of the pathophysiology of COVID-19 has revealed a range of therapeutic targets. In addition to preventative therapeutics and vaccines, which are not the focus of this review, additional therapeutic targets can be broadly divided into three categories. Firstly anti-viral therapies that inhibit viral replication, shorten the infectious period and inhibit progression to severe disease, targeted mainly at the pre-symptomatic or early symptomatic period. Secondly immunomodulatory agents that target the maladaptive host immune response observed in moderate and severe COVID19 that aim to interrupt proinflammatory feedback loops to hasten recovery and prevent transition to critical severity and ICU admission. Lastly supportive care to manage the consequences of COVID-19, including end organ damage, such as ARDS and VTE. Respiratory support is commonly needed in hospitalized patients. The WHO recommends an oxygenation target of 94% or above during initial resuscitation and 90% or above thereafter.
An early intubation strategy was used by many centres early in the pandemic, due to the observation that patients often deteriorate precipitously and may be more safely intubated while clinically stable, and to minimise infection risk by avoiding aerosol generation due to the use of high flow oxygen or non-invasive ventilation (NIV). However mechanical ventilation carries its own risks and is a limited resource. This and some limited evidence that NIV may avoid ventilation (33) has led to a shift towards trialling NIV prior to a decision to proceed to invasive ventilation.
Prone positioning is used in both ventilated and non-ventilated patients due to its proven benefit in ARDS due to causes other than COVID-19, and the observation that it improves oxygenation in COVID-19 patients (34), although it is unclear if this influences overall clinical outcome.
All patients without contraindications should receive thromboprophylaxis and a there should be a low threshold to assess for venous thromboembolism. There are ongoing studies to assess the correct type and optimal dose of anticoagulation to maximise clinical outcomes (35) . However there was no difference in clinical status distribution at day 11 in the group randomized to a ten day course compared to standard of care, and no difference in any of the secondary end-points including all-cause mortality, duration of hospitalization or time to clinical improvement (39) . Thus despite some evidence of benefit, results are conflicting, and optimal duration of therapy and ideal time along the disease course to introduce therapy has not been fully elucidated. 
Lopinavir/ritonavir is a protease inhibitor used to treat HIV infection, which has in vitro activity against SARS-CoV-2, however multiple RCTs failed to show benefit (50)(41). Camostat mesylate is a serine protease protease inhibitor that has been shown to inhibit SARS-CoV-2 interaction with TMPRSS2 in vitro (4) 
Dexamethasone is the only medication to date that has been shown to reduce mortality in COVID-19 infection in a large RCT. The Randomised Evaluation of COVID-19 therapy (RECOVERY) multicentre trial which included 6425 patients showed a significant reduction in mortality at 28 days in patients receiving oxygen or invasive mechanical ventilation when administered 6 milligrams of dexamethasone daily (or equivalent) for up to ten days (52) .
This contradicted initial WHO guidance to avoid corticosteroids, a recommendation based on lack of benefit observed in SARS, MERS and influenza pneumonia. The benefit observed in the RECOVERY trial was greater in those recruited more than seven days after onset of illness, and the authors hypothesise that the benefit of corticosteroids is due to the prominent immune mediated pathology at this phase of the disease. The benefit of corticosteroids has since been further supported by other RCTs and meta-analyses (53) (54) .
The rationale for use of tocilizumab is derived from its approved use in cytokine release syndrome associated with chimeric antigen receptor (CAR) T cell therapy, an IL-6 mediated process that resembles the systemic inflammatory response in severe COVID-19.
Tocilizumab is a humanised monoclonal antibody (mAb) against the IL-6 receptor, also approved for the treatment of rheumatoid arthritis and giant cell arteritis. Benefit has been suggested in case series and retrospective cohort studies of tocilizumab (55, 56) , however the Roche funded RCT COVACTA reported no benefit in the primary endpoint of difference in clinical endpoint at four weeks in a population of patients hospitalised with or in the secondary endpoint of overall mortality (57) . However, a secondary endpoint analysis showed significantly shorter stays in ICU and quicker hospital discharge in those who received tocilizumab, with adverse events being equal between the two groups. Importantly, J o u r n a l P r e -p r o o f a post hoc analysis also showed significantly less clinical failure in patients not mechanically ventilated at baseline in the tocilizumab arm compared to placebo, with a 40% reduction in progression to ICU within this subgroup of patients. Further trials targeting this pre-ICU group are underway (58) . Additionally, the COVACTA trial recruited patients based on oxygenation requirements and did not specifically target patients with evidence a systemic inflammatory syndrome, and it is possible that a greater benefit may be observed in this specific group. Other IL-6 inhibitors, sarilumab, another anti-IL-6 receptor mAb and siltuximab, an antI-IL-6 mAb are also under investigation.
The use of passive antibody therapy predates modern antimicrobial therapy with the use of antibody preparations derived from immune donors known as "serum therapy" dating back to In addition, VIR-7831, developed by Vir Biotechnology, Inc and GlaxoSmithKline plc was developed using peripheral blood mononuclear cells from a donor recovered from SARS, and targets an epitope of RBD that is conserved within the Sarbecovirus genus (69) . VIR-7831 is undergoing a phase two/phase three trial in outpatients with early COVID-19.
Granulocyte-macrophage colony-stimulating factor (GM-CSF), a myelpoietic growth factor is a pro-inflammatory cytokine involved in alveolar homeostasis. Human recombinant GM-CSF (sargramostim) is being trialled in hypoxic respiratory failure with the aim of stabilising alveolar macrophage and epithelial cell function and preventing against secondary infection (70) . Conversely inhibition of GM-CSF is effective in a number of inflammatory conditions that resemble the systemic inflammatory response in COVID-19, including ARDS, haemophagocytic lymphohistiocytosis (HLH) and rheumatoid arthritis, and anti GM-CSF mAbs could be useful in COVID-19 (70) . Research into use of agents targeting GM-CSF is ongoing.
Janus Kinase (JAK) inhibitors such as Barcitinib and Ruxolitinib inhibit transmembrane proteins that mediate and amplify extracellular signals from growth factors and cytokines.
They are licensed for rheumatoid arthritis and myeloproliferative disorders respectively but have also been used for HLH. A recent meta-analysis of JAK inhibitors in COVID-19 showed a significantly reduced odds ratio of mortality or ICU admission, although only two small RCTs were included in the analysis with 41 and 17 patients recruited in each (71) .
Despite the wealth of data generated on potential therapies for COVID-19 many questions remain. The early course of disease has been well described but the frequency and duration of long term effects is unknown. The treatments described above only represent a fraction of all those under investigation, but treatment options remain limited. With global transmission remaining high, the search for effective therapies targeted at each stage of disease must remain a priority. 