More than five years after the start of the COVID-19 pandemic, SARS-CoV-2 continues to profoundly affect the lives of millions of people worldwide - not because of the acute infection, but because of its lingering consequences. Long-standing COVID, also referred to as acute post-COVID condition (PACS) or post-COVID syndrome in the scientific literature, refers to a set of symptoms that persist or appear after the initial infection, beyond four to twelve weeks, without any other identified medical explanation.
The World Health Organization estimates that 10 to 20 % of people who contract COVID-19 develop prolonged symptoms. In Canada, this potentially represents several hundred thousand people. In Quebec, many patients find themselves without structured care, not knowing where to consult or how to name their condition. Clinique Omicron offers medical assessments for patients with post-COVID symptoms at several of its branches in Quebec.
What is long COVID: definition and diagnostic criteria
WHO defines long-onset COVID as a condition occurring in people with a history of probable or confirmed SARS-CoV-2 infection, presenting with symptoms that persist beyond three months after initial infection, last at least two months and cannot be explained by any other diagnosis. Symptoms may be continuous since the acute infection, or may disappear and then reappear, or may first appear several weeks after apparent recovery. This variability contributes to the complexity of the diagnosis.
It's important to note that long-onset COVID can occur after a mild, moderate or severe infection. Initially, it was thought that only hospitalized patients were at risk - epidemiological data have shown that many patients with long-onset COVID have only had a mild infection, managed at home, without hospitalization. Vaccination reduces the risk of developing long COVID, but does not completely eliminate it.
The most frequent symptoms of COVID long
Long COVID presents with an extremely varied spectrum of symptoms, affecting multiple organ systems. Post-exertional fatigue is the most common symptom, and one of the most incapacitating. It differs from ordinary fatigue in that it is disproportionate to the effort expended, worsens after even mild physical or cognitive activity - post-exertional malaise - and is persistent, not improving with rest. Brain fog« is the second most commonly reported symptom: difficulty concentrating, short-term memory problems, slowness in processing information, the sensation of thinking through absorbent cotton.
Respiratory symptoms - shortness of breath on exertion, chest tightness, persistent cough - are common even in the absence of lung lesions detectable on imaging. Cardiovascular manifestations may include palpitations, tachycardia at rest or on exertion, and orthostatic intolerance - postural orthostatic tachycardia syndrome (POTS) is found in a significant proportion of patients with long COVID. Diffuse joint and muscle pain, persistent headaches, sleep disorders, tinnitus, cognitive dysfunction and anxiety-depressive symptoms complete the frequently encountered clinical picture.
The biological mechanisms behind long COVID
Research in recent years has identified several biological mechanisms potentially involved in long COVID, although the full pathophysiology remains to be elucidated. Viral persistence - the presence of RNA fragments or viral proteins in certain tissues months after infection - is the most widely studied hypothesis at present. Persistent immune dysfunction, with chronic activation of certain inflammatory pathways, has been documented in many patients. Reactivation of latent viruses such as Epstein-Barr virus or cytomegalovirus has also been observed.
Alterations in coagulation - microthrombi in small vessels - and autonomic nervous system dysfunction partly explain cardiovascular symptoms and POTS. Disturbances in the intestinal microbiome and abnormalities in serotonin production have been reported in recent studies published in Nature. These multiple mechanisms explain the diversity of clinical presentations and the complexity of management.
Diagnostic approach: how to make the diagnosis
The diagnosis of long COVID is primarily clinical - based on the history, characterization of symptoms and exclusion of other causes. There is no single laboratory test that can confirm the diagnosis. The medical approach aims to document symptoms, assess their functional impact, look for treatable biological abnormalities and rule out differential diagnoses. A basic work-up usually includes a complete blood count, metabolic panel, thyroid evaluation, iron panel, blood glucose, cardiovascular and pulmonary evaluation, depending on symptoms.
Functional assessment tools such as the post-exertion fatigue score, quality of life questionnaires and orthostatic tests can complete the evaluation. Some patients benefit from cardiovascular investigation - electrocardiogram, holter, echocardiography - or neurological assessment, depending on the clinical picture. Keeping a symptom diary is useful for identifying fluctuations and aggravating factors.
Care and treatment available in 2026
There is as yet no validated curative treatment specific to long COVID. Management is based on a multidisciplinary, symptomatic approach, adapted to the profile of each patient. Energy management - pacing - is the most important approach for patients with post-exertion fatigue: it consists in strictly adhering to one's exertion limits to avoid post-exertion malaise and enable gradual recovery. Physical reconditioning, when tolerated, must be gradual and supervised.
Targeted symptomatic treatments can be proposed depending on the manifestations: low-dose melatonin for sleep disorders, low-dose beta-blockers for POTS and tachycardia, antihistamines for certain profiles suggestive of mast cell activation, cognitive neurorehabilitaton for mental fog. Clinical trials are underway to assess the efficacy of low-dose anticoagulation, immunomodulators and antivirals in specific patient subgroups.
Frequently asked questions about COVID long in Quebec
Is it possible to develop long COVID after a mild infection?
Yes, long-onset COVID can occur after any form of initial infection, including cases considered mild and managed entirely at home. Several large-scale studies have confirmed that there is no direct correlation between the severity of acute infection and the risk of developing long-onset COVID. People who have never been hospitalized, or who have had very few initial symptoms, may present disabling post-COVID symptoms several months later.
How long does COVID long last?
The duration varies greatly from one individual to another. Some people see their symptoms resolve within a few months - often between six and twelve months after infection. Others experience persistent symptoms for two, three or four years, with no significant improvement. Current data suggest that around half of patients improve within a year of infection, but a significant proportion remain symptomatic over the long term. Vaccination before or after infection appears to be associated with a slightly better prognosis.
Can vaccination against COVID aggravate long-standing COVID?
The data available in 2026 do not confirm that vaccination worsens long-onset COVID. On the contrary, several studies have observed a partial improvement in symptoms in some patients with long-onset COVID after vaccination. That said, a subgroup of patients report a transient worsening after vaccination - raising questions about individual mechanisms. The decision to vaccinate in the presence of active long-onset COVID should ideally be discussed with the treating physician on a case-by-case basis.
Where to go for COVID long in Quebec without a waiting list?
Clinique Omicron offers medical consultations for the evaluation of post-COVID symptoms at several of its branches in Quebec. A consultation can be obtained without a family doctor or prior referral. The doctor will carry out a complete evaluation, propose a work-up adapted to your clinical picture and refer you to specialized resources if necessary. Early, structured care improves long-term functional prognosis.
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