There is a large set of symptoms associated with long COVID, impacting many different organs and body systems. Long COVID symptoms can differ significantly from person to person. Symptom severity ranges from mild to incapacitating.

Common symptoms reported in studies include fatigue, muscle pain, shortness of breath, chest pain, cognitive dysfunction ("brain fog") and post-exertional malaise (symptoms worsen after activity). This symptom worsening typically occurs 12 to 48 hours after activity and can be triggered by either mental or physical effort. It lasts between days and weeks.

Children and adolescents can also experience serious symptoms and long-term adverse health effects, including serious mental health impacts related to persistent COVID-19 symptoms. The most common symptoms in children are persistent fever, sore throat, problems with sleep, headaches, shortness of breath, muscle weakness, fatigue, loss of smell or distorted smell, and anxiety. Most children with long COVID experience three or more symptoms.
Common neurological symptoms in long COVID are difficulty concentrating, cognitive impairment and headaches. People also frequently experience loss of taste and loss of smell. Likewise children and young people may also experience cognitive impairment.

Some people with long COVID experience dysautonomia, a malfunction of the central nervous system. People with dysautonomia may experience palpitations and tachycardia (raised heart rate) after minor effort or upon standing up. This can be associated with dizziness and nausea. If the heart rate is raised by 30 beats per minute or more after continuous standing; this is described as postural orthostatic tachycardia syndrome.

In terms of mental health, people with long COVID often experience sleep difficulties. Depression and anxiety levels are raised in the first two months after infection, but return to normal afterwards. This was in contrast to other neurological symptoms, such as brain fog and seizures, which lasted at least two years.
Difficulty breathing is the second-most common symptom of long COVID. Shortness of breath is among the most common symptoms in children and young people as well. People can also experience a persistent cough.

Less frequently, people with long COVID experience diarrhea and nausea.

In the cardiovascular system, effort intolerance and chest pain occur often in people with long COVID. People are at increas…

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The causes of long COVID are not yet fully understood. It is likely that there is no single cause, but instead multiple, and possibly overlapping, mechanisms that all contribute to the development of long COVID. Organ damage from the acute infection can explain a part of the symptoms, but long COVID is also observed in people where organ damage seems to be absent. Several hypotheses have been put forward explaining long COVID, including:
• blood clotting and endothelial dysfunction in the blood vessels
• neurological issues: problems with signalling from the brainstem and the vagus nerve
• immune system dysregulation, including the reactivation of viruses like the Epstein–Barr virus
• impacts of the virus on the microbiota, including viral persistence
• autoimmunity
Further hypotheses include a dysfunction of the mitochondria and the cellular energy system, persistent systemic inflammation, and the persistence of SARS-COV-19 antigens.
Organ damage from the acute infection may explain symptoms in some people with long COVID. Radiological tests such as lung MRIs often show up as normal even for people who show clear desaturation (lowered blood oxygen level) after mild exercise. Other tests, such as a dual-energy CT scan, do show perfusion defects in a subset of people with respiratory symptoms. Imaging of the heart show contradictory results. Imaging of brains show changes after COVID infection, even if this has not been studied in relation to long COVID. For instance, some show a smaller olfactory bulb, a brain region associated with smell.

In a subset of people with long COVID, there is evidence that SARS-COV-2 remains in the body after the acute infection. This evidence comes from biopsies, studies of blood plasma, and by the indirect immune effects of persistent virus. Viral DNA or proteins have been found months to a year after acute infection in various studies. A small study demonstrated viral RNA up to nearly two years after an acute infection in people with long COVID. Persistent virus has also been found in people without long COVID, but at a lower rate. Persistent virus could lead to symptoms via possible effects on coagulation and via microbiome and neuroimmune abnormalities.

During or after acute COVID infection, various dormant viruses can become reactivated. For instance, SARS-COV-2 can reactivate the Epstein-Barr virus, the virus that is responsible for infectious mononucleosis. This virus lies dormant in most people. There is some evidence of a relationship between its …

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There are no standardised tests to determine if symptoms persisting after COVID-19 infection are due to long COVID. Diagnosis is based on a history of suspected or confirmed COVID-19 symptoms, and by considering and ruling out alternative diagnoses. Diagnosis of long COVID can be challenging because of the wide range of symptoms people with long COVID may display.

Early diagnostic criteria of long COVID required a laboratory-confirmed COVID-19 infection, but current criteria do not require this anymore, given that people may not get tested during the acute infection. For instance, people who develop long COVID after asymptomatic infection would have little reason to get tested. Furthermore, tests for COVID are not foolproof, and can come back negative. False negatives are more common for children, women and people with a low viral load.

There are diagnostic tools available for some elements of long COVID, such as the tilt table test or a NASA lean test for POTS and MRI scans to test for cardiovascular impairment. Routine tests offered in standard care often come back normal.