COVID-19

StatPearls: "SARS-CoV-2 gains entry into host cells by binding the SARS-CoV-2 S protein (S1) to the ACE2 receptors in the respiratory epithelium." "ACE2 receptors are also expressed in other organs," and "SARS-CoV-2–induced organ dysfunction likely results from a combination of mechanisms, including direct viral toxicity, ischemic injury from vasculitis, thrombosis, immune dysregulation, and RAAS dysregulation."

Clotting. "COVID-19 also produces a hypercoagulable state, reflected by the high prevalence of venous thromboembolic events. Laboratory findings associated with this hypercoagulability include markedly elevated D-dimer and fibrinogen levels, along with prolonged prothrombin time (PT) and partial thromboplastin time (aPTT)" (StatPearls).

Heart. "Myocardial injury, including myocardial ischemia or infarction and myocarditis, represents a well-recognized cardiac manifestation of COVID-19" (StatPearls).

Blood pressure medicines are safe to continue. "Inhibition of the renin-angiotensin-aldosterone system (RAAS) does not increase the risk of COVID-19 hospitalization or severe disease" (StatPearls).

"The primary mode of transmission of SARS-CoV-2 is via exposure to respiratory droplets carrying the infectious virus from close contact or direct transmission from presymptomatic, asymptomatic, or symptomatic individuals harboring the virus." "Airborne transmission with aerosol-generating procedures has also been implicated in the spread of COVID-19" (StatPearls). CDC: "Infected people can transmit SARS-CoV-2, the virus that causes COVID-19, before symptom onset," and "People infected with SARS-CoV-2 can transmit the virus if they are vaccinated or unvaccinated."

Timing. "Peak transmissibility occurs from prior to symptom onset to a few days after, but most people can shed virus up to 10 days following infection" (CDC). Incubation: a pooled mean of 6.5 days in 2020 studies, and a median of 3–4 days during Omicron (CDC clinical page); the CDC patient page says "Symptoms may appear 2-14 days after exposure to the virus."

Risk of severe disease. "Age is the strongest risk factor for severe COVID-19, with risk increasing substantially as age increases above 75 years" (CDC clinical considerations). The CDC outpatient page lists age over 65, not being up to date on vaccination, multiple medical conditions, and "Immunocompromising conditions or use of immunosuppressive medications, such as chemotherapy." Long-term care residence is another factor.

Severity bands (CDC, from IDSA definitions). "Mild to Moderate Illness: Individuals who have any of the various signs and symptoms of COVID-19 and do not need supplemental oxygen who have an oxygen saturation (SpO2) ≥94% on room air at sea level." "Severe Illness: Individuals who have SpO2 <94% on room air at sea level or needing supplemental oxygen."

Outpatients at high risk get an antiviral: nirmatrelvir/ritonavir within 5 days of symptom onset is the first option; a 3-day course of IV remdesivir within 7 days is second; molnupiravir only when neither is accessible or appropriate (CDC, IDSA 10/14/2025, labels).

Hospitalized patients. Dexamethasone is for patients who need oxygen — IDSA (9/25/2020) "suggests against the use of glucocorticoids" without hypoxemia requiring supplemental oxygen. Remdesivir is suggested for severe disease (IDSA 2/7/2022) but not routinely started on invasive ventilation or ECMO. Baricitinib or tocilizumab may be added to corticosteroids for severe or critical illness (IDSA; baricitinib section 4/29/2022, tocilizumab section 9/14/2021). Prophylactic-dose heparin is the default (NIH guidelines, archived 2024).

Definition (CDC, Mar. 9, 2026). "Long COVID, also known as Post-COVID Conditions (PCC), is an IACC that can occur after SARS-CoV-2 infection, and is present for at least 3 months as a continuous, relapsing and remitting, or progressive disease state that affects one or more organ systems." IACC means an infection-associated chronic condition. CDC adds that "The definition for Long COVID will continue to be evaluated as data are collected, analyzed, and reported and our understanding of the IACC improves." By contrast, "Most patients with acute COVID-19 appear to recover from their initial acute COVID-19 illness within 4 weeks, and many patients continue to recover between 4 and 12 weeks."

What it looks like. CDC links long COVID with "Development of new or recurrent symptoms and conditions after the symptoms of initial acute COVID-19 illness have resolved," "Symptoms that can emerge, persist, resolve, and reemerge over varying lengths of time," "A spectrum of physical, social, and psychological consequences," and "Functional limitations that can affect patient wellness and quality of life and may cause disability." Onset patterns include symptoms that "Develop following asymptomatic disease or a period of acute symptom relief or remission" and that "Worsen pre-existing symptoms or conditions." Post-exertional malaise (PEM) "is the worsening of symptoms following even minor physical or mental exertion, with symptoms typically worsening 12 to 48 hours after activity and lasting for days or even weeks."

Why it happens — possible, not proven. "Long COVID is heterogeneous and may be attributed to different underlying pathophysiologic processes." CDC lists possible etiologies: organ damage from the acute infection, a dysregulated inflammatory state, microvascular dysfunction, "Ongoing viral activity associated with an intra-host viral reservoir," autoimmunity and an inadequate antibody response.

Who is at higher risk (CDC). Women; Hispanic people; people who had more severe COVID-19, "including hospitalization or being admitted to the intensive care"; people with underlying health conditions; and "People who did not get a COVID-19 vaccine." Severity raises the risk but is not required — symptoms can develop after asymptomatic infection. "Preventing severe outcomes of COVID-19 illness reduces people's risk of developing Long COVID," and "Providers should promote COVID-19 vaccines as the best tool available to prevent Long COVID."