Gut-brain pathogenesis of post-acute COVID-19 neurocognitive symptoms

SARS-CoV-2 Spike Protein and Long COVID-Part 1: Impact of Spike Protein in Pathophysiological Mechanisms of Long COVID Syndrome

SARS-CoV-2 infection has resulted in more than 700 million cases and nearly 7 million deaths worldwide. Although vaccination efforts have effectively reduced mortality and transmission rates, a significant proportion of recovered patients-up to 40%-develop long COVID syndrome (LC) or post-acute sequelae of COVID-19 infection (PASC). LC is characterized by the persistence or emergence of new symptoms following initial SARS-CoV-2 infection, affecting the cardiovascular, neurological, respiratory, gastrointestinal, reproductive, and immune systems. Despite the broad range of clinical symptoms that have been described, the risk factors and pathogenic mechanisms behind LC remain unclear. This review, the first of a two-part series, is distinguished by the discussion of the role of the SARS-CoV-2 spike protein in the primary mechanisms underlying the pathophysiology of LC.

Pediatric SARS-CoV-2 infection and development of anxiety and depression

Objective

It remains unclear whether emerging mental health concerns in children infected with SARS-CoV-2 are a direct result of the infection or due to the indirect effects of the pandemic. Therefore, we sought to assess the frequency of new diagnoses of anxiety and/or depression among children diagnosed with and without SARS-CoV-2 infection who were tested in pediatric emergency departments.

Methods

A prospective cohort study with 6- and 12-month follow-ups was conducted across 14 Canadian tertiary-care pediatric emergency departments of the Pediatric Emergency Research Canada (PERC) network. The study included children aged <18 years who were tested for SARS-CoV-2 infection between August 2020 and February 2022. The primary outcome was the diagnosis of anxiety and/or depression reported during follow-up. The surveys incorporated a modified version of the International Severe Acute Respiratory and Emerging Infection Consortium (ISARIC) Long-COVID Pediatric Questionnaire.

Results

Among the participants who were eligible for 6- and 12-month follow-ups, 64.7% (268/414) of SARS-CoV-2-positive and 71.9% (743/1,033) of SARS-CoV-2-negative participants completed follow-up at these time points, respectively. The median age was 7.0 [inter-quartile range (IQR): 5.0-11.0] years, and 54.2% (548/1,011) were male. New diagnoses of anxiety and/or depression reported on either survey did not differ significantly between test-positive (4.1%, 11/268) and test-negative (2.8%; 21/743) participants [difference = 1.3% (95% CI: -1.3 to 4.2)]. There was a higher prevalence of new diagnoses of anxiety and/or depression among SARS-CoV-2-negative participants aged ≥12 years relative to those aged <12 years [8.7% (13/149) vs. 1.3% (8/594); difference = 7.4%; 95% CI of the difference = 3.0-12.5], but not among SARS-CoV-2-positive participants [4.4% (2/45) vs. 4.0% (9/223); difference = 0.4%; 95% CI of the difference = -5.6 to 9.4]. At 6 or 12 months, SARS-CoV-2-positive participants were more likely to experience confusion and/or lack of concentration, abdominal pain, and insomnia.

Conclusions

Although no association was found between SARS-CoV-2 infection and new diagnoses of anxiety and/or depression, SARS-CoV-2-positive participants were more likely to experience confusion/lack of concentration, abdominal pain, and insomnia. This finding, in the context of an increased prevalence of new diagnoses of anxiety and depression, underscores the impacts of societal changes on the mental health of children. Our finding that some non-specific symptoms were more frequently reported by SARS-CoV-2-positive participants emphasizes the need for further investigation of the underlying pathophysiologic mechanisms.

Animal Models for Long COVID: Current Advances, Limitations, and Future Directions

Long COVID (LC) represents a chronic, systemic, and often disabling condition that poses a significant ongoing threat to public health. Foundational scientific studies are needed to unravel the underlying mechanisms, with the ultimate goal of developing effective preventative and therapeutic strategies. Therefore, there is an urgent demand for animal models that can accurately replicate the clinical features of LC. This review integrates clinical epidemiological data to summarize the pathological changes in extrapulmonary systems involved in LC. Additionally, it critically examines the capacity of existing animal models, including nonhuman primates, genetically modified mice, and Syrian hamsters, to exhibit enduring postinfection symptoms that align with human clinical manifestations, and identifies key areas requiring further development. The objective is to offer insights that will aid in the development of next-generation animal models, thereby accelerating our understanding of how acute respiratory viral infections transition into chronic conditions, and ensuring preparedness for future pandemics.

Pathophysiological, Neuropsychological, and Psychosocial Influences on Neurological and Neuropsychiatric Symptoms of Post-Acute COVID-19 Syndrome: Impacts on Recovery and Symptom Persistence

Although the impact of post-acute COVID-19 syndrome (PACS) on patients and public health is undeniably significant, its etiology remains largely unclear. Much research has been conducted on the pathophysiology, shedding light on various aspects; however, due to the multitude of symptoms and clinical conditions that directly or indirectly define PACS, it is challenging to establish definitive causations. In this exploration, through systematically reviewing the latest pathophysiological findings related to the neurological symptoms of the syndrome, we aim to examine how psychosocial and neuropsychological symptoms may overlap with neurological ones, and how they may not only serve as risk factors but also contribute to the persistence of some primary symptoms of the disorder. Findings from our synthesis suggest that psychological and psychosocial factors, such as anxiety, depression, and loneliness, may interact with neurological symptoms in a self-reinforcing feedback loop. This cycle seems to be affecting both physical and psychological distress, potentially increasing the persistence and severity of PACS symptoms. By pointing out this interaction, in this review study, we attempt to offer a new perspective on the interconnected nature of psychological, psychosocial, and neurological factors, emphasizing the importance of integrated treatment approaches to disrupt this cycle and improve outcomes when possible.

Acute gastrointestinal and post-acute COVID-19 gastrointestinal syndrome assessment on the Gastrointestinal Symptom Rating Scale scoring system: A questionnaire-based survey

Background

Post-acute coronavirus disease 2019 (COVID-19) syndrome (PACS) is the persistence of sequel of acute SARS-COV-2 infection. Persistent/acquired gastrointestinal symptoms (GI-PACS) include loss of appetite, nausea, weight loss, abdominal pain, heartburn, dysphagia, altered bowel motility, dyspepsia, and irritable bowel syndrome. The study aimed to assess the short- and long-term GI-PACS syndrome on the GSRS scale.

Methods

A cross-sectional, retrospective record analysis and telephonic questionnaire-based survey were conducted at a tertiary referral center in northern India. The data incorporated patients treated from April 2021 to March 2023. Exclusion criteria were neurological disorders, dementia, inability to understand Hindi/English languages, and psychiatric problems. All patients who met the inclusion criteria were telephonically called from November 2023 to January 2024.

Results

The study population was 350 recovered patients from SARS-COVID-19 illness. Forty-three responses were removed during data cleaning and removal of duplication. The data analysis of 307 participants (ICU admissions=92, non-ICU admissions=123, and outdoor treatment =92) was done. The proportion of patients not having any GI symptoms, having at least one GI symptom, and having more than one GI symptom before SARS-COV-2 illness was 3%(3/307), 4.9% (15/307), and 3.6% (11/307), respectively. The four major GI symptoms analyzed in the study were vomiting, pain in the abdomen, diarrhea, and constipation. Overall, 13% (40/307) of the study population did not have any major GI symptoms before SARS-COV-2 diseases. During acute SARS-COV-2 illness, 86.97% (267/307) of patients develop new GI symptoms. Post SARS-COV-2 illness, the overall mean GSRS score for 15 items was 2.14 ± 0.829. The acquired GI-PACS was abdominal pain syndrome (mean score 2.5190 ± SD 0.86650), constipation syndrome (mean score 2.3844 ± 0.83840), reflux syndrome (mean score 2.2866 ± 1.31889), indigestion syndrome (mean score 1.8591 ± 0.93076), and diarrhea syndrome (mean score 1.8122 ± 0.90899). Overall, fever (95.1%, P = 0.007), anosmia (45.0%, P = 0.042), cough (80.1%, P = 0.032), and hospitalization (30.0%, P = 0.003) had a more significant association with one of the major four GI symptoms during the acute phase of SARS-COV-2 illness. Home-isolated patients having loss of appetite (95.4%, P = 0.0001) had a significant association with one of the major four GI symptoms during the acute phase of SARS-COV-2 illness. Hospitalized patients having fever (80.7%, P = 0.031), breathlessness (83.8%, P = 0.003), loss of smell (97.0%, P = 0.001), and cough (82.7%, P = 0.048) had a more significant association with one of the major four GI symptoms during the acute SARS-COV-2 illness. Abdominal pain, reflux, and constipation were considered severe GI symptoms (symptom GSRS score greater than total mean GSRS score). Diarrhea and indigestion were considered mild symptoms (symptom GSRS score was less than the total mean GSRS score). The GI symptoms during acute SARS-COV-2 illness recovered in 66.1% (203/307) patients within 3 months. The respondents taking medicines for more than 1 year following SARS-COV-2 illness were 19.2%. 12.0% (37/307) of respondents suffered from persistent GI symptoms on a mean follow-up of 20.1 ± 0.82 months.

Conclusion

Long-term COVID-19 syndrome frequently manifested as GI symptoms, whereas most symptoms subsided with time.

Human microbiome in post-acute COVID-19 syndrome (PACS)

The global COVID-19 pandemic, which began in 2019, is still ongoing. SARS-CoV-2, also known as the severe acute respiratory syndrome coronavirus 2, is the causative agent. Diarrhea, nausea, and vomiting are common GI symptoms observed in a significant number of COVID-19 patients. Additionally, the respiratory and GI tracts express high level of transmembrane protease serine 2 (TMPRSS2) and angiotensin-converting enzyme-2 (ACE2), making them primary sites for human microbiota and targets for SARS-CoV-2 infection. A growing body of research indicates that individuals with COVID-19 and post-acute COVID-19 syndrome (PACS) exhibit considerable alterations in their microbiome. In various human disorders, including diabetes, obesity, cancer, ulcerative colitis, Crohn's disease, and several viral infections, the microbiota play a significant immunomodulatory role. In this review, we investigate the potential therapeutic implications of the interactions between host microbiota and COVID-19. Microbiota-derived metabolites and components serve as primary mediators of microbiota-host interactions, influencing host immunity. We discuss the various mechanisms through which these metabolites or components produced by the microbiota impact the host's immune response to SARS-CoV-2 infection. Additionally, we address confounding factors in microbiome studies. Finally, we examine and discuss about a range of potential microbiota-based prophylactic measures and treatments for COVID-19 and PACS, as well as their effects on clinical outcomes and disease severity.

Neuroinflammation in Post COVID-19 Sequelae: Neuroinvasion and Neuroimmune Crosstalk

The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in December 2019 triggered a swift global spread, leading to a devastating pandemic. Alarmingly, approximately one in four individuals diagnosed with coronavirus disease 2019 (COVID-19) experience varying degrees of cognitive impairment, raising concerns about a potential increase in neurological sequelae cases. Neuroinflammation seems to be the key pathophysiological hallmark linking mild respiratory COVID-19 to cognitive impairment, fatigue, and neurological sequelae in COVID-19 patients, highlighting the interaction between the nervous and immune systems following SARS-CoV-2 infection. Several hypotheses have been proposed to explain how the virus disrupts physiological pathways to trigger inflammation within the CNS, potentially leading to neuronal damage. These include neuroinvasion, systemic inflammation, disruption of the lung and gut-brain axes, and reactivation of latent viruses. This review explores the potential origins of neuroinflammation and the underlying neuroimmune cross-talk, highlighting important unanswered questions in the field. Addressing these fundamental issues could enhance our understanding of the virus's impact on the CNS and inform strategies to mitigate its detrimental effects.

Diffusion magnetic resonance spectroscopy captures microglial reactivity related to gut-derived systemic lipopolysaccharide: A preliminary study

Neuroinflammation is a key component underlying multiple neurological disorders, yet non-invasive and cost-effective assessment of in vivo neuroinflammatory processes in the central nervous system remains challenging. Diffusion weighted magnetic resonance spectroscopy (dMRS) has shown promise in addressing these challenges by measuring diffusivity properties of different neurometabolites, which can reflect cell-specific morphologies. Prior work has demonstrated dMRS utility in capturing microglial reactivity in the context of lipopolysaccharide (LPS) challenges and serious neurological disorders, detected as changes of microglial metabolite diffusivity properties. However, the extent to which such dMRS metrics are capable of detecting subtler and more nuanced levels of neuroinflammation in populations without overt neuropathology is unknown. Here we examined the relationship between intrinsic, gut-derived levels of systemic LPS and dMRS-based apparent diffusion coefficients (ADC) of choline, creatine, and N-acetylaspartate (NAA) in two brain regions: the thalamus and the corona radiata. Higher plasma LPS concentrations were significantly associated with increased ADC of choline and NAA in the thalamic region, with no such relationships observed in the corona radiata for any of the metabolites examined. As such, dMRS may have the sensitivity to measure microglial reactivity across populations with highly variable levels of neuroinflammation, and holds promising potential for widespread applications in both research and clinical settings.

Mechanisms of Gut-Related Viral Persistence in Long COVID

Long COVID (post-acute sequelae of COVID-19-PASC) is a consequence of infection by SARS-CoV-2 that continues to disrupt the well-being of millions of affected individuals for many months beyond their first infection. While the exact mechanisms underlying PASC remain to be defined, hypotheses regarding the pathogenesis of long COVID are varied and include (but are not limited to) dysregulated local or systemic inflammatory responses, autoimmune mechanisms, viral-induced hormonal imbalances, skeletal muscle abnormalities, complement dysregulation, novel abzymes, and long-term persistence of virus and/or fragments of viral RNA or proteins. This review article is based on a comprehensive review of the wide range of symptoms most often observed in long COVID and an attempt to integrate that information into a plausible hypothesis for the pathogenesis of PASC. In particular, it is proposed that long-term dysregulation of the gut in response to viral persistence could lead to the myriad of symptoms observed in PASC.

Exploring the Complexities of Long COVID

Since the emergence of the SARS-CoV-2 virus in 2019, nearly 700 million COVID-19 cases and 7 million deaths have been reported globally. Despite most individuals recovering within four weeks, the Center for Disease Control (CDC) estimates that 7.5% to 41% develop post-acute infection syndrome (PAIS), known as 'Long COVID'. This review provides current statistics on Long COVID's prevalence, explores hypotheses concerning epidemiological factors, such as age, gender, comorbidities, initial COVID-19 severity, and vaccine interactions, and delves into potential mechanisms, including immune responses, viral persistence, and gut dysbiosis. Moreover, we conclude that women, advanced age, comorbidities, non-vaccination, and low socioeconomic status all appear to be risk factors. The reasons for these differences are still not fully understood and likely involve a complex relationship between social, genetic, hormonal, and other factors. Furthermore, individuals with Long COVID-19 seem more likely to endure economic hardship due to persistent symptoms. In summary, our findings further illustrate the multifaceted nature of Long COVID and underscore the importance of understanding the epidemiological factors and potential mechanisms needed to develop effective therapeutic strategies and interventions.

COVID-19-associated encephalopathy: connection between neuroinflammation and microbiota-gut-brain axis

While neurological complications of COVID-19, such as encephalopathy, are relatively rare, their potential significant impact on long-term morbidity is substantial, especially given the large number of infected patients. Two proposed hypotheses for the pathogenesis of this condition are hypoxia and the uncontrolled release of proinflammatory cytokines. The gut microbiota plays an important role in regulating immune homeostasis and overall gut health, including its effects on brain health through various pathways collectively termed the gut-brain axis. Recent studies have shown that COVID-19 patients exhibit gut dysbiosis, but how this dysbiosis can affect inflammation in the central nervous system (CNS) remains unclear. In this context, we discuss how dysbiosis could contribute to neuroinflammation and provide recent data on the features of neuroinflammation in COVID-19 patients.

Long COVID in Children, Adults, and Vulnerable Populations: A Comprehensive Overview for an Integrated Approach

Long COVID affects both children and adults, including subjects who experienced severe, mild, or even asymptomatic SARS-CoV-2 infection. We have provided a comprehensive overview of the incidence, clinical characteristics, risk factors, and outcomes of persistent COVID-19 symptoms in both children and adults, encompassing vulnerable populations, such as pregnant women and oncological patients. Our objective is to emphasize the critical significance of adopting an integrated approach for the early detection and appropriate management of long COVID. The incidence and severity of long COVID symptoms can have a significant impact on the quality of life of patients and the course of disease in the case of pre-existing pathologies. Particularly, in fragile and vulnerable patients, the presence of PASC is related to significantly worse survival, independent from pre-existing vulnerabilities and treatment. It is important try to achieve an early recognition and management. Various mechanisms are implicated, resulting in a wide range of clinical presentations. Understanding the specific mechanisms and risk factors involved in long COVID is crucial for tailoring effective interventions and support strategies. Management approaches involve comprehensive biopsychosocial assessments and treatment of symptoms and comorbidities, such as autonomic dysfunction, as well as multidisciplinary rehabilitation. The overall course of long COVID is one of gradual improvement, with recovery observed in the majority, though not all, of patients. As the research on long-COVID continues to evolve, ongoing studies are likely to shed more light on the intricate relationship between chronic diseases, such as oncological status, cardiovascular diseases, psychiatric disorders, and the persistent effects of SARS-CoV-2 infection. This information could guide healthcare providers, researchers, and policymakers in developing targeted interventions.

Hippocampal region metabolites and cognitive impairment in patients with general paresis: based on 1H-proton magnetic resonance spectroscopy

BACKGROUND

This study utilizes Hydrogen proton magnetic resonance spectroscopy (1H-MRS) to investigate metabolite concentrations in the bilateral hippocampus of general paresis (GP) patients.

METHODS

A total of 80 GP patients and 57 normal controls (NCs) were enrolled. Metabolite ratios in the bilateral hippocampus were measured using 1H-MRS. Cognitive function was assessed using the Mini-Mental State Examination (MMSE). Based on MMSE scores, participants were categorized into normal control, mild cognitive impairment, and moderate-severe dementia groups. Metabolite ratios (N-acetylaspartate (NAA)/creatine (Cr), choline (Cho)/creatine (Cr), N-acetylaspartate (NAA)/choline (Cho), myoinositol (MI)/creatine (Cr), choline (Cho)/N-acetylaspartate (NAA)) were compared between groups, and correlations between metabolite ratios and cognitive performance were examined.

RESULTS

MMSE scores progressively decreased in the normal, mild cognitive impairment, and moderate-severe dementia groups (p < 0.001). The moderate-severe dementia group showed significantly lower NAA/Cr ratios in the left hippocampus region (L-NAA/Cr ratios) (p < 0.001) and higher Cho/NAA ratios in the left hippocampus region (L-Cho/NAA ratios) (p < 0.05) compared to the other groups. However, differences in L-NAA/Cr and L-Cho/NAA ratios between the mild cognitive impairment group and the NC group were not significant in the hippocampus region (p > 0.05). NAA/Cho and NAA/Cr ratios in the right hippocampus region (R-NAA/Cho and R-NAA/Cr ratios) in the moderate-severe dementia group were lower than those in the control group (p < 0.05). No correlation was found between metabolite ratios and MMSE scores in bilateral hippocampus regions.

CONCLUSION

There are distinctive metabolic characteristics in the hippocampus of GP patients. GP patients exhibited lower NAA/Cr and NAA/Cho ratios in the bilateral hippocampus, indicating neuron loss in these areas, which may become more pronounced as the disease progresses.

Alterations in microbiota of patients with COVID-19: implications for therapeutic interventions

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) recently caused a global pandemic, resulting in more than 702 million people being infected and over 6.9 million deaths. Patients with coronavirus disease (COVID-19) may suffer from diarrhea, sleep disorders, depression, and even cognitive impairment, which is associated with long COVID during recovery. However, there remains no consensus on effective treatment methods. Studies have found that patients with COVID-19 have alterations in microbiota and their metabolites, particularly in the gut, which may be involved in the regulation of immune responses. Consumption of probiotics may alleviate the discomfort caused by inflammation and oxidative stress. However, the pathophysiological process underlying the alleviation of COVID-19-related symptoms and complications by targeting the microbiota remains unclear. In the current study, we summarize the latest research and evidence on the COVID-19 pandemic, together with symptoms of SARS-CoV-2 and vaccine use, with a focus on the relationship between microbiota alterations and COVID-19-related symptoms and vaccine use. This work provides evidence that probiotic-based interventions may improve COVID-19 symptoms by regulating gut microbiota and systemic immunity. Probiotics may also be used as adjuvants to improve vaccine efficacy.

Exploring the Influence of Gut-Brain Axis Modulation on Cognitive Health: A Comprehensive Review of Prebiotics, Probiotics, and Symbiotics

Recent research exploring the relationship between the gut and the brain suggests that the condition of the gut microbiota can influence cognitive health. A well-balanced gut microbiota may help reduce inflammation, which is linked to neurodegenerative conditions. Prebiotics, probiotics, and symbiotics are nutritional supplements and functional food components associated with gastrointestinal well-being. The bidirectional communication of the gut-brain axis is essential for maintaining homeostasis, with pre-, pro-, and symbiotics potentially affecting various cognitive functions such as attention, perception, and memory. Numerous studies have consistently shown that incorporating pre-, pro-, and symbiotics into a healthy diet can lead to improvements in cognitive functions and mood. Maintaining a healthy gut microbiota can support optimal cognitive function, which is crucial for disease prevention in our fast-paced, Westernized society. Our results indicate cognitive benefits in healthy older individuals with probiotic supplementation but not in healthy older individuals who have good and adequate levels of physical activity. Additionally, it appears that there are cognitive benefits in patients with mild cognitive impairment and Alzheimer's disease, while mixed results seem to arise in younger and healthier individuals. However, it is important to acknowledge that individual responses may vary, and the use of these dietary supplements should be tailored to each individual's unique health circumstances and needs.

Neurovascular coupling impairment as a mechanism for cognitive deficits in COVID-19

Components that comprise our brain parenchymal and cerebrovascular structures provide a homeostatic environment for proper neuronal function to ensure normal cognition. Cerebral insults (e.g. ischaemia, microbleeds and infection) alter cellular structures and physiologic processes within the neurovascular unit and contribute to cognitive dysfunction. COVID-19 has posed significant complications during acute and convalescent stages in multiple organ systems, including the brain. Cognitive impairment is a prevalent complication in COVID-19 patients, irrespective of severity of acute SARS-CoV-2 infection. Moreover, overwhelming evidence from in vitro, preclinical and clinical studies has reported SARS-CoV-2-induced pathologies in components of the neurovascular unit that are associated with cognitive impairment. Neurovascular unit disruption alters the neurovascular coupling response, a critical mechanism that regulates cerebromicrovascular blood flow to meet the energetic demands of locally active neurons. Normal cognitive processing is achieved through the neurovascular coupling response and involves the coordinated action of brain parenchymal cells (i.e. neurons and glia) and cerebrovascular cell types (i.e. endothelia, smooth muscle cells and pericytes). However, current work on COVID-19-induced cognitive impairment has yet to investigate disruption of neurovascular coupling as a causal factor. Hence, in this review, we aim to describe SARS-CoV-2's effects on the neurovascular unit and how they can impact neurovascular coupling and contribute to cognitive decline in acute and convalescent stages of the disease. Additionally, we explore potential therapeutic interventions to mitigate COVID-19-induced cognitive impairment. Given the great impact of cognitive impairment associated with COVID-19 on both individuals and public health, the necessity for a coordinated effort from fundamental scientific research to clinical application becomes imperative. This integrated endeavour is crucial for mitigating the cognitive deficits induced by COVID-19 and its subsequent burden in this especially vulnerable population.

Gut microbe-host interactions in post-COVID syndrome: a debilitating or restorative partnership?

Post-COVID syndrome (PCS) patients have reported a wide range of symptoms, including fatigue, shortness of breath, and diarrhea. Particularly, the presence of gastrointestinal symptoms has led to the hypothesis that the gut microbiome is involved in the development and severity of PCS. The objective of this review is to provide an overview of the role of the gut microbiome in PCS by describing the microbial composition and microbial metabolites in COVID-19 and PCS. Moreover, host-microbe interactions via the microbiota-gut-brain (MGB) and the microbiota-gut-lung (MGL) axes are described. Furthermore, we explore the potential of therapeutically targeting the gut microbiome to support the recovery of PCS by reviewing preclinical model systems and clinical studies. Overall, current studies provide evidence that the gut microbiota is affected in PCS; however, diversity in symptoms and highly individual microbiota compositions suggest the need for personalized medicine. Gut-targeted therapies, including treatments with pre- and probiotics, have the potential to improve the quality of life of affected individuals.

Long COVID or Post-COVID-19 Condition: Past, Present and Future Research Directions

The presence of symptoms after an acute SARS-CoV-2 infection (long-COVID) has become a worldwide healthcare emergency but remains underestimated and undertreated due to a lack of recognition of the condition and knowledge of the underlying mechanisms. In fact, the prevalence of post-COVID symptoms ranges from 50% during the first months after the infection up to 20% two-years after. This perspective review aimed to map the existing literature on post-COVID symptoms and to identify gaps in the literature to guide the global effort toward an improved understanding of long-COVID and suggest future research directions. There is a plethora of symptomatology that can be due to COVID-19; however, today, there is no clear classification and definition of this condition, termed long-COVID or post-COVID-19 condition. The heterogeneity in the symptomatology has led to the presence of groups/clusters of patients, which could exhibit different risk factors and different mechanisms. Viral persistence, long-lasting inflammation, immune dysregulation, autoimmune reactions, reactivation of latent infections, endothelial dysfunction and alteration in gut microbiota have been proposed as potential mechanisms explaining the complexity of long-COVID. In such an equation, viral biology (e.g., re-infections, SARS-CoV-2 variants), host biology (e.g., genetics, epigenetics) and external factors (e.g., vaccination) should be also considered. These various factors will be discussed in the current perspective review and future directions suggested.