Since the previous rapid review, a large amount of new research and reviews are available to draw upon. Many studies have addressed aspects previously identified as limitations such as the use of validated questionnaires, documenting pre-COVID health status, and control cohorts. Many studies now include only participants with RT-PCR verified infections and also focus on a range of disease severities from severe (hospitalized) to mild (managed in the community); PCR-validated infections ensure a higher quality of comparison between test-positive and control groups. Studies involving control groups contributed illuminating findings about prevalence and incidence of long COVID, which is lower than previous thought when compared to control groups. Comorbidities/factors that potentially indicate increased risk of developing long COVID-19 have been identified and widely agreed-upon, such as diabetes, cardiovascular diseases, obesity, and gender (female).
A case definition for long COVID has yet to be adopted but is commonly defined as COVID-related symptoms that persist or emerge beyond 4 weeks of infection with two subsequent phases: “ongoing symptomatic COVID-19 (OSC; signs and symptoms from 4 to 12 weeks from initial infection) and 2) post-COVID-19 syndrome (PCS; signs and symptoms beyond 12 weeks) with respect to symptomatology, abnormal functioning, psychological burden, and quality of life”. Long COVID includes both OSC and PCS.
It is estimated that 32% of non-hospitalized and 51% of hospitalized people experience Long COVID symptoms within 12 weeks of infection; however, higher estimates up to 92% have been reported in studies with a greater proportion in persons who were previously hospitalized. More than 200 symptoms affecting 10 organ systems have been identified in various reports or systematic reviews. Many patients (49%), experience at least one COVID-related symptom 12 months after infection (compared to 68% at 6 months).
For many persons, symptoms improve over time while others experience persistent and/or new symptoms. At 3 months post-infection the most frequently reported symptoms are fatigue (up to 98%), dyspnoea (up to 88%), headache (up to 91%) and taste/smell disorders (up to 58%).
Mechanism(s) leading to long COVID remain unclear, but these comorbidities/factors have been found to indicate potentially increased risk of developing long COVID:
o Age (60+)
o Greater number of symptoms during the acute phase of illness (typically 5+)
o Manifestation of specific symptoms
o Cardiovascular disease
o Obesity or high BMI
o Gender (female)
There is limited evidence to support the contention that vaccination lowers incidence of long COVID.
July 9, 2021
The frequency of Long COVID symptoms varies widely across studies based on populations studied, duration of follow up and methods of assessment of symptoms.
It is estimated that 1 in 50 persons experience Long COVID symptoms after 12 weeks; however, higher estimates up to 80% have been reported in studies with a greater proportion of persons who were previously hospitalized. A recent study of a mixed cohort of 96 persons found that only 22.9% had no symptoms at 12 months post diagnosis.
A wide range of symptoms affecting multiple organ systems has been reported. For many persons symptoms improve over time while others experience persistent and/or new symptoms. Among studies with the longest duration of follow up, the most frequently reported symptoms included fatigue (up to 65%), dyspnea (up to 50%), headache (up to 45%), anosmia/ageusia (up to 25%), cognitive memory/concentration (up to 39.6%) and sleep disorders (up to 26%).
Few studies estimated the duration of symptoms with estimates ranging from 2.2% for 6 months and 27% for 7-9 months.
The mechanism(s) leading to Long COVID remain unclear but those experiencing post acute sequelae tend to be older, have a greater number of symptoms during the acute phase of illness or manifest specific symptoms and live with multiple comorbid conditions such as obesity.
The lack of consensus on a definition of Long COVID contributes to marked variations in robust prevalence estimates.
A significant amount of evidence was produced since the previous review. This updated review was rewritten with extensive changes which have not been identified in red.
Hammond, B; Badea, A; Groot, G; Reeder, B; Howell-Spooner, B; Mueller, M. What is the incidence and duration of long COVID cases? 2022 Mar 31, Document no.: EPM210601v2 RR. In: COVID-19 Rapid Evidence Reviews [Internet]. SK: SK COVID Evidence Support Team, c2022. 18 p. (CEST rapid review report).