The high incidence and bioethics of findings on magnetic resonance brain imaging of normal volunteers for neuroscience research

Incidental findings in MRI of the brain

This review investigates that there has been an increase in incidental brain MRI findings due to better technology and more scans. These unexpected, asymptomatic anomalies range from harmless to serious, requiring careful clinical and ethical handling. The prevalence of incidental findings with brain MRI is 4.2% and even higher when including white matter hyperintensities. There is a significant variation in this number dependent on the age of the person being scanned and the MRI quality.

An Equity and Justice-Informed Ethical Framework to Guide Incidental Findings in Brain Imaging Research

The handling of incidental findings (IFs) in brain imaging studies has been a source of contention among scientists and bioethicists. A conceptual framework informed by diversity, equity, and inclusion (DEI) and distributive justice approaches, namely EUSTICE, is proposed for the ethical handling and reporting of IFs in brain imaging research. I argue that EUSTICE provides a systematic and inclusive approach to addressing the ethical conundrum around IF disclosure and managing IFs proportionately and sensitively in brain imaging research. The EUSTICE framework may have implications for the field of neurosciences or human studies broadly in guiding ethics of IFs in research.

Prevalence of incidental intracranial findings on magnetic resonance imaging: a systematic review and meta-analysis

BACKGROUND

As the volume and fidelity of magnetic resonance imaging (MRI) of the brain increase, observation of incidental findings may also increase. We performed a systematic review and meta-analysis to determine the prevalence of various incidental findings.

METHODS

PubMed/MEDLINE, EMBASE and SCOPUS were searched from inception to May 24, 2021. We identified 6536 citations and included 35 reports of 34 studies, comprising 40,777 participants. A meta-analysis of proportions was performed, and age-stratified estimates for each finding were derived from age-adjusted non-linear models.

RESULTS

Vascular abnormalities were observed in 423/35,706 participants (9.1/1000 scans, 95%CI 5.2-14.2), ranging from 2/1000 scans (95%CI 0-7) in 1-year-olds to 16/1000 scans (95%CI 1-43) in 80-year-olds. Of these, 204/34,306 were aneurysms (3.1/1000 scans, 95%CI 1-6.3), which ranged from 0/1000 scans (95%CI 0-5) at 1 year of age to 6/1000 scans (95%CI 3-9) at 60 years. Neoplastic abnormalities were observed in 456/39,040 participants (11.9/1000 scans, 95%CI 7.5-17.2), ranging from 0.2/1000 scans (95%CI 0-10) in 1-year-olds to 34/1000 scans (95%CI 12-66) in 80-year-olds. Meningiomas were the most common, in 246/38,076 participants (5.3/1000 scans, 95%CI 2.3-9.5), ranging from 0/1000 scans (95%CI 0-2) in 1-year-olds to 17/1000 scans (95%CI 4-37) in 80-year-olds. Chiari malformations were observed in 109/27,408 participants (3.7/1000 scans, 95%CI 1.8-6.3), pineal cysts in 1176/32,170 (9/1000 scans, 95%CI 1.8-21.4) and arachnoid cysts in 414/36,367 (8.5/1000 scans, 95%CI 5.8-11.8).

CONCLUSION

Incidental findings are common on brain MRI and may result in substantial resource expenditure and patient anxiety but are often of little clinical significance.

Incidental Findings Among Youth Participating in Multimodal Imaging Research: Characteristics of Findings and Description of a Management Approach

Research imaging in healthy and clinical youth populations yields incidental findings that require a management strategy. Our primary objective was to document the frequency and nature of incidental findings within a research group integrating multiple imaging modalities. A second objective was to describe the evolution of an approach to handling incidental findings. A case example was included to display the intricacies of some of these scenarios. Youth, ages 13-20 years, with bipolar disorder, familial risk for bipolar disorder, or healthy controls, obtained one or a combination of neuroimaging, cardio-thoracic imaging, retinal imaging, and carotid imaging. All images were systematically reviewed for incidental findings. Overall, of 223 participants (n = 102 healthy controls), 59% (n = 131) had a brain magnetic resonance imaging (MRI) incidental finding and 27% (n = 60) had at least one incidental brain finding requiring non-urgent follow-up. In addition, of 109 participants with chest/cardiac MRI and carotid ultrasound, 3% (n = 3) had chest findings, 2% (n = 2) had cardiac findings, and 1% (n = 1) had a carotid finding. Of 165 youth with retinal imaging, 1% (n = 2) had incidental findings. While the vast majority of these incidental findings were of a non-serious, non-urgent nature, there were noteworthy exceptions. Imaging research groups need a system that emphasizes the value of clinical review of research images and one that is collaborative and responsive in order to inform follow-up plans. Rating systems that have been developed and used in neuroimaging for the classification of incidental findings can be adapted for use in areas other than the brain. Regardless of severity, incidental findings may raise anxiety in youth participants and their parents. The optimal threshold is one that balances transparency with utility.

Incidental Findings Identified on Head MRI for Investigation of Cognitive Impairment: A Retrospective Review

INTRODUCTION

Incidental findings are common in presumed healthy volunteers but are infrequently studied in patients in a clinical context.

OBJECTIVE

To determine the prevalence, nature, and management implications of incidental findings on head MRI in patients presenting with cognitive symptoms, and to quantify and describe unexpected MRI abnormalities that are of uncertain relevance to the patient's cognitive symptoms.

METHODS

A single-centre retrospective review of patients attending a regional early-onset cognitive disorders clinic between March 2012 and October 2018. Medical records of consecutive patients who underwent head MRI were reviewed. Unexpected MRI findings were classified according to their severity and likelihood of being incidental. Markers of small vessel disease and cerebral atrophy were excluded.

RESULTS

Records of 694 patients were reviewed (median age 60 years, 49.9% female), of whom 514 (74.1%) underwent head MRI. 54% of the patients received a diagnosis of a neurodegenerative disorder. Overall 111 incidental findings were identified in 100 patients of whom 18 patients (3.5%, 95% CI 2.2-5.6%) had 18 incidental findings classified as requiring additional medical evaluation. 82 patients (16%, 95% CI 13.0-19.5%) had 93 incidental findings without clearly defined diagnostic consequences. 17 patients (3.3%) underwent further investigations, 14 patients (2.7%) were referred to another specialist clinic and 3 patients (0.6%) were treated surgically. Two patients had MRI findings of uncertain relevance to their cognitive symptoms, necessitating prolonged clinic follow-up.

CONCLUSION

Incidental findings are common in patients with cognitive impairment from this large clinic-based series; however, few required additional medical evaluation. These data could help inform discussions between clinicians and people with cognitive symptoms regarding the likelihood and potential implications of incidental imaging findings.

Incidentaloma Discoveries in the Course of Neuroimaging Research

Among healthy volunteers in psychiatric brain functional magnetic resonance imaging (fMRI) research studies, the prevalence of incidentalomas can be as high as 34%, of which 10% show clinical significance. An incidentaloma is a lesion found by coincidence without clinical symptoms or suspicion. Like lesions and other types of accidental findings, it is found in healthy individuals recruited to take part in psychiatric studies. The prevalence of these accidental findings among specific psychiatric populations remains unknown. However, a precise understanding of cerebral neuroanatomy, neuroradiological expertise, and an appropriate choice of fMRI exploration sequences will increase the sensitivity of identifying these accidental findings and enable researchers to address their clinical relevance and nature. We present recommendations on how to appropriately inform patients or participants of the accidental findings. Additionally, we propose specific suggestions pertaining to the clinical research setting aimed for investigators and psychiatrists. Unlike current articles pertaining to incidentaloma, the current report provides a distinct focus on psychiatric issues and specific recommendations for studies involving psychiatric patients.

Two decades of SPECT/CT - the coming of age of a technology: An updated review of literature evidence

PURPOSE

Single-photon emission computed tomography (SPECT) combined with computed tomography (CT) was introduced as a hybrid SPECT/CT imaging modality two decades ago. The main advantage of SPECT/CT is the increased specificity achieved through a more precise localization and characterization of functional findings. The improved diagnostic accuracy is also associated with greater diagnostic confidence and better inter-specialty communication.

METHODS

This review presents a critical assessment of the relevant literature published so far on the role of SPECT/CT in a variety of clinical conditions. It also includes an update on the established evidence demonstrating both the advantages and limitations of this modality.

CONCLUSIONS

For the majority of applications, SPECT/CT should be a routine imaging technique, fully integrated into the clinical decision-making process, including oncology, endocrinology, orthopaedics, paediatrics, and cardiology. Large-scale prospective studies are lacking, however, on the use of SPECT/CT in certain clinical domains such as neurology and lung disorders. The review also presents data on the complementary role of SPECT/CT with other imaging modalities and a comparative analysis, where available.

Potentially serious incidental findings on brain and body magnetic resonance imaging of apparently asymptomatic adults: systematic review and meta-analysis

OBJECTIVES

To determine prevalence and types of potentially serious incidental findings on magnetic resonance imaging (MRI) in apparently asymptomatic adults, describe factors associated with potentially serious incidental findings, and summarise information on follow-up and final diagnoses.

DESIGN

Systematic review and meta-analyses.

DATA SOURCES

Citation searches of relevant articles and authors' files in Medline and Embase (from inception to 25 April 2017).

REVIEW METHODS

Eligible studies included prevalence and types of incidental findings detected among apparently asymptomatic adults undergoing MRI of the brain, thorax, abdomen, or brain and body. Data on study population and methods, prevalence and types of incidental findings, and final diagnoses were extracted. Pooled prevalence was estimated by random effects meta-analysis, and heterogeneity by τ2 statistics.

MAIN OUTCOME MEASURES

Prevalence of potentially serious incidental findings on MRI of the brain, thorax, abdomen, and brain and body.

RESULTS

Of 5905 retrieved studies, 32 (0.5%) met the inclusion criteria (n=27 643 participants). Pooled prevalence of potentially serious incidental findings was 3.9% (95% confidence interval 0.4% to 27.1%) on brain and body MRI, 1.4% (1.0% to 2.1%) on brain MRI, 1.3% (0.2% to 8.1%) on thoracic MRI, and 1.9% (0.3% to 12.0%) on abdominal MRI. Pooled prevalence rose after including incidental findings of uncertain potential seriousness (12.8% (3.9% to 34.3%), 1.7% (1.1% to 2.6%), 3.0% (0.8% to 11.3%), and 4.5% (1.5% to 12.9%), respectively). There was generally substantial heterogeneity among included studies. About half the potentially serious incidental findings were suspected malignancies (brain, 0.6% (95% confidence interval 0.4% to 0.9%); thorax, 0.6% (0.1% to 3.1%); abdomen, 1.3% (0.2% to 9.3%); brain and body, 2.3% (0.3% to 15.4%)). There were few informative data on potential sources of between-study variation or factors associated with potentially serious incidental findings. Limited data suggested that relatively few potentially serious incidental findings had serious final diagnoses (48/234, 20.5%).

CONCLUSIONS

A substantial proportion of apparently asymptomatic adults will have potentially serious incidental findings on MRI, but little is known of their health consequences. Systematic, long term follow-up studies are needed to better inform on these consequences and the implications for policies on feedback of potentially serious incidental findings.

SYSTEMATIC REVIEW REGISTRATION

Prospero CRD42016029472.

Discovery and informing research participants of incidental findings detected in brain magnetic resonance imaging studies: Review and multi-institutional study

BACKGROUND

Brain imaging studies using magnetic resonance imaging (MRI) sometimes reveal incidental findings (IFs) that might be relevant to some of the health issues in research participants. Although professional communities have discussed how to manage these IFs, there is no global consensus on the concrete handling procedures including how to inform participants of IFs.

METHODS

First, this study reviewed previous studies for the number of IFs discovered in brain imaging studies using MEDLINE. Second, a multi-institutional study determined the number of IF discoveries and evaluated the method of informing participants at multiple institutions, which participated in a national brain science project in Japan.

RESULTS

Both the review and multi-institutional study showed that IFs with a high urgency level were discovered in 0-2.0% of participants, including healthy volunteers, and that the rate of IF discovery in general was higher in studies conducted in elderly population. Moreover, multi-institutional study suggested the criteria used to judge whether or not to inform participants of IFs may differ by institution.

CONCLUSIONS

Our results suggest that in order to ensure informing the participants of high urgency IFs, physicians who are capable of interpreting brain images clinically should review all brain images, and the establishment of a support system is required for brain imaging studies at nonmedical institutions. Since the method of informing participants of IFs might affect their understanding and acceptance of IFs, which are related to managing risks of false "clean bill of health" or psychological impacts of informing IFs, further research focusing on communication of IFs is needed.

Ethical framework for the detection, management and communication of incidental findings in imaging studies, building on an interview study of researchers' practices and perspectives

BACKGROUND

As thousands of healthy research participants are being included in small and large imaging studies, it is essential that dilemmas raised by the detection of incidental findings are adequately handled. Current ethical guidance indicates that pathways for dealing with incidental findings should be in place, but does not specify what such pathways should look like. Building on an interview study of researchers' practices and perspectives, we identified key considerations for the set-up of pathways for the detection, management and communication of incidental findings in imaging research.

METHODS

We conducted an interview study with a purposive sample of researchers (n = 20) at research facilities across the Netherlands. Based on a qualitative analysis of these interviews and on existing guidelines found in the literature, we developed a prototype ethical framework, which was critically assessed and fine-tuned during a two-day international expert meeting with bioethicists and representatives from large population-based imaging studies from the United Kingdom, Germany, Sweden and Belgium (n = 14).

RESULTS

Practices and policies for the handling of incidental findings vary strongly across the Netherlands, ranging from no review of research scans and limited feedback to research participants, to routine review of scans and the arrangement of clinical follow-up. Respondents felt that researchers do not have a duty to actively look for incidental findings, but they do have a duty to act on findings, when detected. The principle of reciprocity featured prominently in our interviews and expert meeting.

CONCLUSION

We present an ethical framework that may guide researchers and research ethics committees in the design and/or evaluation of appropriate pathways for the handling of incidental findings in imaging studies. The framework consists of seven steps: anticipation of findings, information provision and informed consent, scan acquisition, review of scans, consultation on detected abnormalities, communication of the finding, and further clinical management and follow-up of the research participant. Each of these steps represents a key decision to be made by researchers, which should be justified not only with reference to costs and/or logistical considerations, but also with reference to researchers' moral obligations and the principle of reciprocity.

Neurological Fallacies Leading to Malpractice: A Case Studies Approach

A young woman presents with an intracranial arachnoid cyst. Another is diagnosed with migraine headache. An elderly man awakens with a stroke. And a baby delivered vaginally after 2 hours of questionable electronic fetal monitoring patterns grows up to have cerebral palsy. These seemingly disparate cases share a common underlying theme: medical myths. Myths that may lead not only to misdiagnosis and treatment harms but to seemingly never-ending medical malpractice lawsuits, potentially culminating in a settlement or judgment against an unsuspecting neurologist. This article provides a case studies approach exposing the fallacies and highlighting proper management of these common neurologic presentations.

Communicating results in post-Belmont era biomonitoring studies: lessons from genetics and neuroimaging research

BACKGROUND

Biomonitoring is a critical tool to assess the effects of chemicals on health, as scientists seek to better characterize life-course exposures from diverse environments. This trend, coupled with increased institutional support for community-engaged environmental health research, challenge established ethical norms related to biomonitoring results communication and data sharing between scientists, study participants, and their wider communities.

METHODS

Through a literature review, participant observation at workshops, and interviews, we examine ethical tensions related to reporting individual data from chemical biomonitoring studies by drawing relevant lessons from the genetics and neuroimaging fields.

RESULTS

In all three fields ethical debates about whether/how to report-back results to study participants are precipitated by two trends. First, changes in analytical methods have made more data accessible to stakeholders. For biomonitoring, improved techniques enable detection of more chemicals at lower levels, and diverse groups of scientists and health advocates now conduct exposure studies. Similarly, innovations in genetics have catalyzed large-scale projects and broadened the scope of who has access to genetic information. Second, increasing public interest in personal medical information has compelled imaging researchers to address demands by participants to know their personal data, despite uncertainties about their clinical significance. Four ethical arenas relevant to biomonitoring results communication emerged from our review: tensions between participants' right-to-know their personal results versus their ability or right-to-act to protect their health; whether and how to report incidental findings; informed consent in biobanking; and open-access data sharing.

CONCLUSION

Ethically engaging participants in biomonitoring studies requires consideration of several issues, including scientific uncertainty about health implications and exposure sources, the ability of participants to follow up on potentially problematic results, tensions between individual and community research protections, governance and consent regarding secondary use of tissue samples, and privacy challenges in open access data sharing.

Two basic ethical problems of incidental findings in population-based, non-intervening magnetic resonance imaging (MRI) research

Population-based magnetic resonance imaging (MRI) generates many beneficial consequences for future generations of patients. But there are also some unintended implications for present patients and research subjects. The most important of these implications is the occurrence of incidental findings. In what follows, I will explicate two basic ethical problems concerning incidental findings. I will first argue that the main cause of these two problems lies in the conflicted nature of the medical researcher's role. I will then clarify the two problems, which involves appealing to two basic ethical requirements. The first problem originates from specific empirical conditions of epidemiological MRI research, which indicate shortcomings in the process of obtaining informed consent. The second problem is due to a conflict between obligations of beneficence for the research subjects and the researcher's duty to produce valid and generalizable study results. Finally, I will develop some preliminary perspectives for the solution of these problems.

The role of MRI and CT of the brain in first episodes of psychosis

AIM

To investigate whether imaging is associated with early detection of the organic causes of the first episode of psychosis (FEP).

MATERIALS AND METHODS

Individuals with FEP but no neurological signs referred to a tertiary centre for cerebral magnetic resonance imaging (MRI) or computed tomography (CT) were reviewed retrospectively. Two groups were evaluated with either CT or MRI; the two groups were independent and no individual underwent both CT and MRI.

RESULTS

One hundred and twelve consecutive cerebral MRI and 204 consecutive CT examinations were identified. Three (2.7%) individuals had brain lesions [brain tumour and human immunodeficiency virus (HIV) encephalopathy] potentially accountable for the psychosis at MRI. Seventy patients (62.5%) had incidental brain lesions, such as cerebral atrophy, small vessel ischaemic changes, unruptured Circle of Willis aneurysm, cavernoma, and arachnoid cysts at MRI. Three patients (1.5%) had focal brain lesions (primary or secondary tumours) potentially accountable for the psychosis at CT. One hundred and thirty-three patients (65.2%) had incidental brain lesions unrelated to the psychosis on CT scan. There was no significant difference between MRI and CT imaging in detecting organic disease potentially responsible for FEP (p < 0.001).

CONCLUSION

Routine MRI or CT imaging of the brain is unlikely to reveal disease leading to a significant change in management. MRI was comparable with CT in terms of diagnosis of both pathological and incidental cerebral lesions. Therefore, routine brain structural imaging of FEP in patients without focal neurology may not be routinely required and if imaging is requested then CT may function equally as well as MRI as the first-line investigation.

3.0 T MRI of 2000 consecutive patients with localisation-related epilepsy

OBJECTIVES

Clinical guidelines suggest that all patients diagnosed with localised seizures should be investigated with MRI to identify any epileptogenic structural lesions, as these patients may benefit from surgical resection. There is growing impetus to use higher field strength scanners to image such patients, as some evidence suggests that they improve detection rates. We set out to review the detection rate of radiological abnormalities found by imaging patients with localised seizures using a high-resolution 3.0 T epilepsy protocol.

METHODS

Data were reviewed from 2000 consecutive adult patients with localisation-related epilepsy referred between January 2005 and February 2011, and imaged at 3.0 T using a standard epilepsy protocol.

RESULTS

An abnormality likely to be related to seizure activity was identified in 403/2000 (20.2%) patients, with mesial temporal sclerosis diagnosed in 211 patients. 313/2000 (15.6%) had lesions potentially amenable to surgery. Abnormalities thought unrelated to seizure activity were found in 324/2000 (16.1%), with 8.9% having evidence of ischaemic disease.

CONCLUSIONS

Since the introduction of the then National Institute for Clinical Excellence guidelines in 2004, the detection rate of significant pathology using a dedicated 3.0 T epilepsy protocol has not fallen, despite the increased numbers of patients being imaged. This is the largest study of epilepsy imaging at 3.0 T to date and highlights the detection rates of significant pathology in a clinical setting using a high-strength magnet. The prevalence of ischaemic disease in this population is significantly higher than first thought, and may not be incidental, as is often reported.

Bad news about bad news: the disclosure of risks to insurability in research consent processes

One of the phenomena associated with research is "incidental findings," that is, unexpected findings made during the research, and outside the scope of the research, which have potential health importance. One underappreciated risk of incidental findings is the potential loss of the research subject's insurability; or if a research subject fails to disclose incidental findings when applying for insurance, the insurance contract may be voidable by the insurer. In this article, we seek to explain the insurability risks associated with incidental findings and to make recommendations for how researchers and research ethics committees should address the issue of disclosure of these risks.

Incidental findings found in "healthy" volunteers during imaging performed for research: current legal and ethical implications

Incidental findings found in "healthy" volunteers during research imaging are common and have important implications for study design and performance, particularly in the areas of informed consent, subjects' rights, clinical image analysis and disclosure. In this study, we aimed to determine current practice and regulations concerning information that should be given to research subjects when obtaining consent, reporting of research images, who should be informed about any incidental findings and the method of disclosure. We reviewed all UK, European and international humanitarian, legal and ethical agencies' guidance. We found that the guidance on what constitutes incidental pathology, how to recognise it and what to do about it is inconsistent between agencies, difficult to find and less complete in the UK than elsewhere. Where given, guidance states that volunteers should be informed during the consent process about how research images will be managed, whether a mechanism exists for identifying incidental findings, arrangements for their disclosure, the potential benefit or harm and therapeutic options. The effects of incidentally discovered pathology on the individual can be complex and far-reaching. Radiologist involvement in analysis of research images varies widely; many incidental findings might therefore go unrecognised. In conclusion, guidance on the management of research imaging is inconsistent, limited and does not address the interests of volunteers. Improved standards to guide management of research images and incidental findings are urgently required.

Perspectives of Canadian Researchers on Ethics Review of Neuroimaging Research

The current and potential uses of neuroimaging in healthcare and beyond have spurred discussion about the ethical issues related to neuroimaging and neuroimaging research. This study examined the perspectives of neuroimagers on ethical issues in their research and on the ethics review process. One hundred neuroimagers from 13 Canadian neuroscience centers completed an online survey and 35 semi-structured interviews were conducted. Neuroimagers felt that most ethical and social issues identified in the literature were dealt with adequately, well, and even very well by research ethics boards (REBs), but some issues such as incidental findings and transfer of knowledge were problematic. Neuroimagers reported a range of practical problems in the ethics review process. We aimed to gather perspectives from REB on the ethics review process, but insufficient participation by REBs prevented us from reporting their perspectives. Given shortcomings identified by neuroimagers as well as longstanding issues in Canadian ethics governance, we believe that substantial challenges exist in Canadian research ethics governance that jeopardize trust, communication, and the overall soundness of research ethics governance. Neuroimagers and REBs should consider their shared responsibilities in developing guidance to handle issues such as incidental findings, risk assessment, and knowledge transfer.

Incidental findings on brain magnetic resonance imaging: systematic review and meta-analysis

OBJECTIVE

To quantify the prevalence of incidental findings on magnetic resonance imaging (MRI) of the brain.

DESIGN

Systematic review and meta-analysis of observational studies.

DATA SOURCES

Ovid Medline (1950 to May 2008), Embase (1980 to May 2008), and bibliographies of relevant articles. Review methods Two reviewers sought and assessed studies of people without neurological symptoms who underwent MRI of the brain with or without intravenous contrast for research purposes or for occupational, clinical, or commercial screening.

MAIN OUTCOME MEASURES

Overall disease specific and age specific prevalence of incidental brain findings, calculated by meta-analysis of pooled proportions using DerSimonian-Laird weights in a random effects model.

RESULTS

In 16 studies, 135 of 19 559 people had neoplastic incidental brain findings (prevalence 0.70%, 95% confidence interval 0.47% to 0.98%), and prevalence increased with age (chi(2) for linear trend, P=0.003). In 15 studies, 375 of 15 559 people had non-neoplastic incidental brain findings (prevalence 2.0%, 1.1% to 3.1%, excluding white matter hyperintensities, silent infarcts, and microbleeds). The number of asymptomatic people needed to scan to detect any incidental brain finding was 37. The prevalence of incidental brain findings was higher in studies using high resolution MRI sequences than in those using standard resolution sequences (4.3% v 1.7%, P<0.001). The prevalence of neoplastic incidental brain findings increased with age.

CONCLUSIONS

Incidental findings on brain MRI are common, prevalence increases with age, and detection is more likely using high resolution MRI sequences than standard resolution sequences. These findings deserve to be mentioned when obtaining informed consent for brain MRI in research and clinical practice but are not sufficient to justify screening healthy asymptomatic people.