Battling COVID-19’s Mystifying Mental Fog
Nick Beebe
Illustrations by: Karen Mogami, Nick Beebe
When it struck me, I was one month in as a newly transferred Boston University sophomore, right out of spin class. Completely lost along Back Bay’s Newbury Street, I felt somehow frazzled in an area I already knew quite well; however, I dismissed my confusion as stemming from post-workout exhaustion and the adrenaline rush following my half-sister’s pregnancy announcement. I stared at Maps to get home. After that, things got worse. It was just a matter of time before my grades began to slip. BU’s notoriously merciless General Chemistry became more and more abstract and the entire process of DNA replication left my mind during a Biology exam, despite my love for genetics. Again, I dismissed my perplexing condition, attributing everything to the stress of a new school and city. Little did I know, something far bigger was transpiring. Ultimately, my experience with COVID-19’s “brain fog” is just one of the many brought by the persisting pandemic, though the experience that I, and many others, have faced has largely been ignored.
Contrary to popular belief, the signs of COVID-19 extend beyond the well-known symptoms of hypogeusia and anosmia, the loss of taste and smell, respectively. In fact, many don’t even experience these symptoms [1]. The lesser discussed burdens of memory loss, confusion, and delirium caused by COVID-19 form the mental “fog” I experienced for weeks, inhibiting my focus and abilities I had never even thought about, like coming up with everyday words. Other COVID-19 survivors detail this fog as debilitating, feeling similar to dementia and white static [2]. Unfortunately, the diverse and vague symptoms associated with COVID-19 have greatly complicated efforts to contain the pandemic. As physician Dr. Kari Nadeau of Stanford University remarks, a family with the same genetics that contracts the same strain of COVID-19 can experience widely different symptoms. Thankfully, there are several ongoing studies tracing which biomarkers influence the severity and symptoms of one’s infection [3]. Aside from death, the effects of COVID-19 on the brain are arguably the most malevolent repercussions of a COVID-19 infection.
Without memory, who would we be?
MEMORY AFTER AN IMMUNE RESPONSE
Following a strong immune response instigated by an infection, the strength of one’s memory often deteriorates. The hippocampus of the brain has the essential task of capturing experiences and transforming them into memories to be stored away; it reconsolidates memories by strengthening existing connections of the brain or establishing new ones on the brain’s outer surface. Any time you think of an experience from your past, whether it be from childhood or even just waking up this morning, your hippocampus is activated. The body, still overwhelmed by the fight it has endured post-infection, experiences amnesia due to hippocampal dysfunction [4]. Hippocampal dysfunction obstructs processes like spatial memory, fear conditioning (recalling certain stimuli as negative forewarnings), and novel object recognition (recognizing new items and/or changes in previously observed settings). For more than 1 in 4 patients with a major illness, alterations in memory processes are experienced, which can continue years after recovery [4]. Although this statistic isn’t specific to COVID-19, it can be used as a foundation for understanding how a major illness like COVID-19 links to the impairment of memory.
When it comes to COVID-19, inflammation can come, in large part, from cytokine storms. Cytokines are molecules that initiate an immune response through both the production of antibodies and killing of infected cells. A “storm” of cytokines occurs when the body overreacts to an initiated immune response and begins to kill its own cells instead of just antigens. Despite their similar spellings, antigens and antibodies are completely different things — antibodies are produced by the body’s immune system as a result of the presence of antigens, the invading particles that the immune system wants to eradicate. So, think of a cytokine “storm” as the same chaos that occurs when hypercompetitive WASP moms raid Vineyard Vines on Black Friday; it is very, very bad for nearly all parties involved. In fact, cytokine storms are so dangerous that upon hospital discharge following a COVID-19 infection, one can still die days later as a result of the inflammation inflicted [5]. The short-term brain inflammation that results from a strenuous immune response to viruses like SARS-CoV-2 (which causes COVID-19) can also trigger neurodegeneration, increasing the risk of conditions like Alzheimer’s [6]. Neurodegeneration doesn’t just impact future connections formed in the brain, but also pre-existing connections thought to be lifelong. To demystify the neural mechanisms underlying the progressive cognitive deficits following illness recovery, a better understanding of the interplay between neuroscience and pathology is crucial [4].
MEMORY & OTHER CORONAVIRUSES
Since there is less research on SARS-CoV-2, knowledge of other coronaviruses can be helpful in understanding this novel strain. SARS-CoV-2 is one of the seven known coronaviruses to infect human hosts. Of the remaining six, two (SARS-CoV and MERS-CoV) have remarkable similarities to SARS-CoV-2 [7].
SARS-CoV-2 doesn’t yet have ample trial results available, as it emerged a year ago and thorough studies are hindered by precautions like social distancing and travel restrictions. As a result, much initial research has looked to SARS-CoV and MERS-CoV in order to reach conclusions about SARS-CoV-2. In the presence of SARS-CoV and MERS-CoV, human memory is predicted to have been impaired in over a third of admitted patients [8]. From this knowledge, SARS-CoV-2 has been hypothesized to impact and kill neurons. This is problematic because unlike the common, natural death of neurons (apoptosis), the death of neurons due to inflammation (necrosis) often impacts fully functioning neurons. Since neurons pass messages from one to another, the demise of neurons that weren’t meant to die greatly affects the relaying of crucial informational signals, the success of the neuronal network, and thereby, memory overall.
However, every question about SARS-CoV-2 can’t be answered by investigating other strains of coronaviruses, as different strains have shown different potentials for infecting neurons. For instance, infections in the brain were not commonly reported in SARS-CoV patients. A preliminary report led by Yale University has confirmed the role of SARS-CoV-2 in neuronal death, noting changes in both infected and neighboring cells. However, since this research is so new, it has yet to undergo the scientific review process, meaning it hasn’t been entirely verified [9]. Further contemporary research will strengthen the validity of this finding, but definite answers will only be reached when comprehensive trials can be safely performed [10,1].
As infectious disease expert Dr. Jacomine Krijnse-Locker illustrates, SARS-CoV-2 acts like “a balloon on a string,” slithering along the surface of cells on the hunt for ACE2 receptors to attach to — which are highly concentrated on epithelial cells of the respiratory system [31]. These spike proteins are aided by protein dimers — two protein units clumped together — called hemagglutinin esterases, which assist in the invasion and breakdown of host cells [32]. Upon invading a host cell, the virus replicates its own genetic material (RNA) with an RNA template, which mostly codes for structural proteins that build the new virion (basic structure of the virus) along with non-structural proteins that help aid in the replication of RNA. Three more of SARS-CoV-2’s essential proteins are the N, E, and M proteins. The N protein helps protect RNA; if unprotected, there could be no replication of viral genetic material meaning the production process of viral particles to cause infection would be jeopardized. The E and M proteins of SARS-CoV-2 are responsible for managing the envelope encasing the particle and its progeny in all stages of development for new particles, from its capsulation to assembly to budding off of the host cell [33]. After budding off, newly made virus particles attack the rest of the body in the same process. However, this is not to say that COVID-19’s attack strategy is entirely understood as many questions still remain unanswered.
SARS-CoV-2 & THE BRAIN
Neurotropism
Substantial evidence suggests that COVID-19 affects systems other than the respiratory system (lungs, airways, and blood vessels), including the central nervous system, meaning it is neurotropic [11]. Neurotropic viruses can attack and ultimately obliterate neuronal cells. SARS-CoV-2, in turn, hinders the mental capabilities of an infected individual by attacking their neurons, within which the virus can mindlessly replicate [12,13,14]. Evidence supports the theory that SARS-CoV-2 has a negative impact on neurological function, as an early Chinese observational study associated COVID-19 infection with increased mental confusion, corroborating presence within the brain and its true danger [10]. In a study from northeastern France, 65% of observed COVID-19 patients reported confusion [15]. Of another study led in Germany, roughly 36% of COVID-19 cases were found to involve neurological symptoms — 25% of which involved the central nervous system — including dizziness, headaches, seizures, and brain fog [6]. Even after hospital discharge, one of the most persistent symptoms, as reported by over a third of COVID-19 patients, has been the loss of memory [16,6].
Olfactory Bulb & Inside the Brain
SARS-CoV-2 is theorized to primarily travel through the nostrils and into the olfactory bulb (the part of the brain where receptor cells transform odorants into neural stimuli to indicate what one is smelling) [7]. When the olfactory bulb fails to control SARS-CoV-2, symptoms of COVID-19 often arise after five days of the virus’ incubation within the body, but may remain veiled for up to two weeks [17]. Though the olfactory bulb has been regarded as highly efficient at controlling neuroinvasion, many viruses, like SARS-CoV-2, can successfully enter the body this way, leading to inflammation and demyelination of neurons [18,19]. Demyelination refers to the unraveling of the myelin sheath within neurons, which proves to be extremely detrimental. Myelin sheath operates like one of those moving walkways, or travelators, found in airports. Just like when people get onto a travelator, receptors called dendrites that branch off from a neuron’s cell body obtain signals from neighboring neurons. As this communication from other cells is processed, “action potentials” result which, in this analogy, are the travelers heading down the moving walkway. When travelers exit the travelator, they head in varying directions with different destinations. Neurons, similarly, release a myriad of neurotransmitters that target specific receptors after achieving an action potential, going on to mediate different bodily functions. SARS-CoV-2, when in the brain, rips this travelator away from neurons and its dendrites, leaving its “passengers” (action potentials) completely stranded. There is, inevitably, no other way for the signals within the neuron to travel than by the myelin sheath.
COVID-19’s extension into the brain from the olfactory bulb has been supported by the detection of SARS-CoV-2 genetic code in the cerebral-spinal fluid (CSF) of admitted patients [20,19,17]. CSF is meant to protect the brain and spinal cord from injury and also acts as a garbage disposal for unwanted matter housed in the brain (i.e. SARS-CoV-2). However, the detectability of SARS-CoV-2 in CSF can be influenced by the severity of infection, the time between initial infection and sample collection, and even the sensitivity of the test administered [21]. The discovery that SARS-CoV-2 invasion is greatly restricted by removing the olfactory bulb in mice also heavily suggests the involvement of this structure in infection [22].
UNDERSTANDING “BRAIN FOG”
Simply put, scientists aren’t in agreement about what causes brain fog. It remains especially boggling as it even affects those who became only slightly ill with COVID-19 and those with no pre-existing brain conditions [6]. Current theories of the origin of COVID-19’s fog range from the prolonged activation of the immune system after COVID-19 to inflammation within blood vessels and cells to the possibility of antibodies attacking neurons [2].
Medical literature is often intentionally impersonal, but listening to the experiences of individuals that have faced COVID-19’s fog can lead researchers and their studies in cogent directions, and even warn of what those fighting COVID-19 may face. For one 59-year-old woman, cognitive impairment was noted by her attending medical personnel, as she expressed difficulty with memory and executive reasoning [23]. Prior to hospitalization, she was forgetting her children’s names, her work schedule, and the names of grocery stores near her home. A CT scan of the woman revealed a lower density of the brain in the left frontal lobe, which is responsible for attention, concentration, and memory [23]. Though this particular patient was in her late 50s, a number of sources have wrongly associated issues of memory loss and confusion from COVID-19 with older age. Pre-existing conditions like Alzheimer’s and the deterioration of the blood-brain barrier in older adults can leave them more susceptible to neuroinvasion, but old age is certainly not a prerequisite for experiencing mental fog [24,6,4].
In fact, a recent study has found a prevalence of COVID-related confusion in groups with mean ages around 40 [8]. To claim that the fog mainly affects older and more vulnerable populations does a great disservice to the public as they navigate this pandemic, minimizing the struggles faced by younger people like me, 19 at the time, and 16-year-old Natalia Ruspini, a Californian who also endured brain fog for months [3]. Another young victim, 32-year-old Hanna Lockman, asserts that COVID-19 has “eaten” her brain. The disease has made it difficult to remember words and to take her medications [25]. Lockman even feels like there’s a physical fog housed in her head [25]. Michael Reagan, 50, remembers “nothing at all” about his twelve-day Parisian vacation with his boyfriend after contracting coronavirus just weeks later. Julia Donahue, 61, details her failure to call to mind the simple word “toothbrush,” needing to explain its function instead to get the point across. Erica Taylor, 31, couldn’t recall that the only Toyota Prius in her apartment building’s parking lot belonged to her [2]. As these various experiences show, COVID-19 can bring about the most bewildering of new-onset cognitive impairment in nearly anyone [23]. The inflammation caused by the brain’s response to SARS-CoV-2 means there is a strong chance neurodegeneration and cognitive decline can progress, even years after contracting the disease. Recent findings strongly suggest a positive correlation between COVID-19 infection and the risk for developing neurological disease later in life, though this hypothesis lacks an ironclad research backbone [6].
MOVING FORWARD
Confronting Eurocentrism
After all of this, you may wonder: Why haven’t I heard anything about this COVID-19 “brain fog”? When they haven’t been discussing the skyrocketing numbers of COVID-19 cases and deaths or the U.S. government’s blatant failure of a pandemic response, media outlets frequently have emphasized the symptoms of loss of smell and loss of taste, stemming from a German virologist’s conclusion in early March [26]. It is now known that cisgender females of European descent are the most likely to lose both senses. Admittedly, anosmia (loss of smell) and ageusia (loss of taste) are very obvious indicators that one may have COVID-19, but not everyone experiences these symptoms. Comparatively, it was sensations of delirium that were found in 31.2% of COVID-19 patients upon hospital admission in France, while anosmia affected only 3.2% [27]. Furthermore, over one-third of patients infected with SARS-CoV-2 in Wuhan, China were regarded as experiencing a range of neurological impairment from a very early study [28]. So why was decreased mental clarity and function ignored in early COVID-19 patients for anosmia and ageusia? The answer likely lies in Eurocentrism.
There are stark differences between the reported rates of chemosensory dysfunction (loss of smell and/or taste) in countries with majority East Asian populations and countries with majority White populations [26,29]. This disparity was noted by one study, which, at the time of its writing, noted there were only 5 published studies conducted on East Asians compared to the 33 on Whites pertaining to COVID-19-induced smell loss. For taste loss, there were only 4 conducted on East Asians compared to the 26 conducted on Whites. How are researchers going to understand the differences in symptoms across different populations if they are primarily studying Whites? And how does COVID-19 affect those of other ancestral backgrounds? These questions are in dire need of being addressed. Though I am entirely of European descent, it was the simple fact that I didn’t lose my senses of taste or smell — alongside the U.S. government’s downplaying of the virus until after I beat COVID-19 — that led me to dismiss the possibility of actually having the illness. With diversity in research participants, answers about this Earth-stopping pathogen will finally begin to reflect all of whom it has affected. This same call for diversity is needed to best comprehend the inner workings of COVID-19’s brain fog as well.
The Necessity for Additional Studies
As the literature pertaining to COVID-19 continues to expand as time progresses, it is my hope that the scientific community can come to an understanding about what links COVID-19 to confusion, memory loss, and delirium — the infrastructure of this brain fog. Large-scale studies that span countries, and even continents, will have the opportunity to greatly diversify public knowledge pertaining to COVID-19 and ancestral background, particularly with regard to its impact on the brain. With the information currently available, researchers are now advocating the need for neurological assessment and timely evaluation for those that have been afflicted by COVID-19 [23]. Roughly 91% of 640 survey respondents who have recovered from COVID-19 expressed several ongoing symptoms, one being its relentless brain fog [25]. Additionally, “long haulers,” individuals who have been battling COVID-19 for a prolonged period of time, often describe their most debilitating and persistent symptom to be the impairment of memory and concentration [25].
Moving forward, collaboration between researchers is desperately needed around the globe to better understand how COVID-19 impacts memory and causes mental fog in patients [30,1]. In order to directly combat COVID-19, we must work together in unity — in collaboration — for the sake of our country and the world. There are many people dismissing this science that absolutely does exist, or failing to understand how their ignorance to social distancing and safety protocols negatively impacts the population at large. Everyone must grasp that SARS-CoV-2 is a self-serving anarchist on its own team, targeting any human it comes in contact with. In the end, it is imperative to wear a mask and social distance; although few of us might want to remember 2020, experiencing the mental fog of COVID-19 is truly not worth it.
REFERENCES
1. Alam, S., Willows, S., Kulka, M., & Sandhu, J. (2020). Severe acute respiratory syndrome coronavirus 2 may be an underappreciated pathogen of the central nervous system. European Journal of Neurology, 27(11), 1–13. doi:10.1111/ene.14442.
2. Belluck, P. (2020, October 14). ‘I feel like I have dementia’: brain fog plagues COVID survivors. Retrieved October 18, 2020, from www.nytimes.com/2020/10/11/health/ covid-survivors.html.
3. Reddy, S. (2020, September 28). Four different family members. Four different COVID-19 outcomes. Retrieved October 13, 2020, from www.wsj.com/articles/four-different-family-members-four-different-covid-19-outcomes-11601299859.
4. Tchessalova, D. & Tronson, N. (2019). Memory deficits in males and females long after subchronic immune challenge. Neurobiology of Learning and Memory, 158, 60–72. doi:10.1016/j.nlm.2019.01.003.
5. Isacson, O. (2020). The consequences of coronavirus-induced cytokine storm are associated with neurological diseases, which may be preventable. Frontiers in Neuroscience, 11, 1–2. doi:10.3389/fneur.2020.00745.
6. Heneka, M., Golenbock, D., Latz, E., Morgan, D., & Brown, R. (2020). Immediate and long-term consequences of COVID-19 infections for the development of neurological. disease. Alzheimer’s Research & Therapy, 12(1). doi:10.1186/s13195-020-00640-3.
7. Diaz, A. & Baweja, R. (2020). The role of neurotropism in psychiatric patients with COVID‐19. European Archives of Psychiatry and Clinical Neuroscience. doi:10.1007/s00406-020-01197-w.
8. Rogers, J., Chesney, E., Oliver, D., Pollak, T.A., McGuire, P., Fusar-Poli, P., … David, A.S. (2020). Psychiatric and neuropsychiatric presentations associated with severe coronavirus infections: a systematic review and meta-analysis with comparison to the COVID-19 pandemic. Lancet Psychiatry, 7(7), 611–627. doi:10.1016/s2215-0366(20)30203-0.
9. Song, E., Zhang, C., Israelow, B., Lu, P., Weizman, O., Liu, F., … Iwasaki, A. (2020). Neuroinvasion of SARS-CoV-2 in human and mouse brain. bioRxiv. doi:10.1101/ 2020.06.25.169946.
10. Chu, H., Chan, J.F., Yuen, T.T., Shuai, H., Yuan, S., Wang, Y., … Yuen, K. (2020). Comparative tropism, replication kinetics, and cell damage profiling of SARS-CoV-2 and SARS-CoV with implications for clinical manifestations, transmissibility, and laboratory studies of COVID-19: an observational study. Lancet Microbe, 1(1). doi:10.1016/s2666-5247(20)30004-5.
11. Beghi, E., Feigin, V., Caso, V., Santalucia, P., & Logroscino, G. (2020). COVID-19 infection and neurological complications: present findings and future predictions. Neuroepidemiology, 54(5), 364–369. doi:10.1159/000508991.
12. Debnath, M., Berk, M., & Maes, M. (2020). Changing dynamics of psychoneuroimmunology during the COVID-19 pandemic. Brain, Behavior, and Immunity - Health, 5, 1–3. doi:10.1016/j.bbih.2020.100096.
13. Scoppettuolo, P., Borrelli, S., & Naeije, G. (2020). Neurological involvement in SARS-CoV-2 infection: A clinical systematic review. Brain, Behavior, and Immunity - Health, 5. doi:10.1016/j.bbih.2020.100094.
14. Bougakov, D., Podell, K., & Goldberg, E. (2020). Multiple neuroinvasive pathways in COVID-19. Molecular Neurobiology, 1–12. doi:10.1007/s12035-020-02152-5.
15. Aghagoli, G., Marin, B.G., Katchur, N.J., Chaves-Sell, F., Asaad, W.F., & Murphy, S.A. (2020). Neurological involvement in COVID-19 and potential mechanisms: a review. Neurocritical Care, 1–10. doi:10.1007/s12028-020-01049-4.
16. Garrigues, E., Janvier, P., Kherabi, Y., Le Bot, A., Hamon, A., Gouze, H., … Nguyen, Y. (2020). Post-discharge persistent symptoms and health-related quality of life after hospitalization for COVID-19. Journal of Infection. doi:10.1016/j.jinf.2020.08.029.
17. Asadi-Pooya, A. & Simani, L. (2020). Central nervous system manifestations of COVID-19: a systematic review. Journal of the Neurological Sciences, 413. doi:10.1016/j.jns.2020.116832.
18. Desforges, M., Le Coupanec, A., Dubeau, P., Bourgouin, A., Lajoie, L., Dubé, M., & Talbot, P.J. (2019). Human coronaviruses and other respiratory viruses: underestimated opportunistic pathogens of the central nervous system? Viruses, 12(1). doi:10.3390/v12010014.
19. Ogier, M., Andéol, G., Sagui, E., & Dal Bo, G. (2020). How to detect and track chronic neurologic sequelae of COVID-19? Use of auditory brainstem responses and neuroimaging for long-term patient follow-up. Brain, Behavior, and Immunity - Health, 5. doi:10.1016/j.bbih.2020.100081.
20. Lu, Y., Li, X., Geng, D., Mei, N., Wu, P., Huang, C., … Yin, B. (2020). Cerebral micro-structural changes in COVID-19 patients - an MRI-based 3-month follow-up study. EClinicalMedicine, 25. doi:10.1016/j.eclinm.2020.100484.
21. Espíndola, O., Siqueira, M., Soares, C.N., de Lima, M., Leite, A., Araujo, A., … Silva, M. (2020). Patients with COVID-19 and neurological manifestations show undetectable SARS-CoV-2 RNA levels in the cerebrospinal fluid. International Journal of Infectious Diseases, 96, 567–569. doi:10.1016/j.ijid.2020.05.123.
22. Wu, Y., Xu, X., Chen, Z., Duan, J., Hashimoto, K., Yang, L., … Yang, C. (2020). Nervous system involvement after infection with COVID-19 and other coronaviruses. Brain, Behavior, and Immunity, 87, 18–22. doi:10.1016/j.bbi.2020.03.031.
23. Garg, A., Marji, A., Goyal, S., & Ismail, R. (2020). A case of COVID-19 with memory impairment and delayed presentation as stroke. Cureus, 12(8), 1–9. doi:10.7759/ cureus.10025.
24. Hascup, E. & Hascup, K. (2020). Does SARS-CoV-2 infection cause chronic neurological complications? GeroScience, 42(4), 1083–1087. doi:10.1007/s11357-020-00207-y.
25. Rubin, R. (2020). As their numbers grow, COVID-19 “long haulers” stump experts. JAMA, 324(14). doi:10.1001/jama.2020.17709.
26. von Bartheld, C., Hagen, M., & Butowt, R. (2020). Prevalence of chemosensory dysfunction in COVID-19 patients: a systematic review and meta-analysis reveals significant ethnic differences. ACS Chemistry, 11(19), 2944–2961. doi:10.1101/2020.06.15.20132134.
27. Steinmeyer, Z., Vienne-Noyes, S., Bernard, M., Steinmeyer, A., Balardy, L., Piau, A., & Sourdet, S. (2020). Acute care of older patients with COVID-19: clinical characteristics and outcomes. Geriatrics, 5(4). doi:10.3390/geriatrics5040065.
28. Mao, L., Jin, H., Wang, M., Hu, Y., Chen, S., He, Q., … Hu, B. (2020). Neurologic manifestations of hospitalized patients with coronavirus disease 2019 in Wuhan, China. JAMA Neurology, 77(6), 683–690. doi:10.1001/jamaneurol.2020.1127.
29. Azhideh, A., Menbari-Oskouie, I., & Yousefi-Asl, M. (2020). Neurological manifestation of COVID-19: a literature review. International Clinical Neuroscience Journal 7(4), 164–170. doi:10.1016/j.jocn.2020.05.017.
30. Varatharaj, A., Thomas, N., Ellul, M., Davies, N., Pollak, T., Tenorio, E.L., … Michael, B. (2020). Neurological and neuropsychiatric complications of COVID-19 in 153 patients: a UK-wide surveillance study. Lancet Psychiatry, 7(10), 875–882. doi:10.2139/ssrn.3601761.
31. Beck, M. & Hummer, G. (2020, August 18) Spike protein of the new corona virus is more flexible than expected. Retrieved October 21, 2020 from www.mpg.de/15269546/sars-cov-2-spike-protein.
32. Zeng, Q., Langereis, M., van Vliet, A., Huizinga, E., & de Groot, R. (2008). Structure of coronavirus hemagglutinin-esterase offers insight into corona and influenza virus evolution. Microbiology, 105(26), 9065–9069. doi:10.1073/pnas.0800502105.
33. Schoeman, D. & Fielding, B. (2019). Coronavirus envelope protein: current knowledge.Virology Journal, 16(1). doi:10.1186/s12985-019-1182-0.