Covid-19 is a new illness on a scale not seen in living memory. Every article and webcast about covid-19 has the same message: “We are still learning; we don’t know enough yet.” However, out of this sea of information, protean patterns of illness are emerging. Doctors, governments, and countries have made huge efforts to treat patients and prevent disease spread, but given the novelty of this disease, we should consider what we can learn from the information we have so far, and what more can be done to help in the supportive management and prevention of new cases.
So far, there has been much emphasis on the treatment of acute life-threatening manifestation of covid-19. However, we are seeing increasing evidence of a “long-tail” of covid-19 related illness, and we need to consider how to support patients with prolonged illness from covid-19. As a group of authors, we have become interested in this area from a personal and professional perspective. CR and AUL have both experienced a long-tail of covid-19 symptoms and MM through her experience treating people with these symptoms in general practice.
Many individuals not admitted to hospital are reporting a prolonged and debilitating course of illness. Doctors have highlighted their experience of prolonged symptoms.    Patient led-research is already underway with the publication of data on protracted symptoms, such as the data from the Body Politic Covid-19 Slack support group.  After sharing our recent BMJ rapid response on social media, many other people shared their experience of symptoms that have lasting up to 13 weeks or more.  Among 61 reported symptoms, the most noted persistent symptoms were fatigue and shortness of breath, especially with exertion, chest tightness, cough, skin rash, elevated temperatures, and neurological disturbances.  The Covid-19 symptom tracking app developed at King’s College London, which has been downloaded by 3.9 million people globally to date, has reported that 10% people had symptoms at 25 days and 5% were still ill one month later.  The patterns within prolonged symptoms may not be noticed during acute hospital admissions, nor in the community if longitudinal patient symptom tracking and testing is not carried out. More formal evaluation is necessary and would be useful for clinicians and patients. Actively engaging patients and carers in research design and evaluation is crucial to ensure that people’s experiences are being adequately captured.
Some patients have observed a periodic “tertian” or “quartan” pattern, where symptoms recur every 3rd or 4th day.  As patients recover from covid-19, individuals say that they experience less intense recurrences of their original symptom “clusters,” predominantly at longer intervals. It is unclear if some of these recurrences represent re-activation viraemias or are a consequence of immune response. Current guidelines for self-isolation of “7 days after start of symptoms” in the UK may not be adequate in many individuals,  particularly with reports of prolonged faecal viral shedding up to 1 month for some patients.   However, longitudinal assessments of serial PCR testing currently undertaken largely through oropharyngeal swabs, (or stool samples) are not routine community practice, apart from some selected settings such as care homes.
Despite covid-19 testing being widely available, concerns remain around current limited sensitivity and specificity of Covid-19 PCR swab tests due to the timing and technique in collecting the swab samples.  Highly specific IgG antibody tests have been developed, however uncertainty remains about the proportion of covid-19 patients that seroconvert and produce IgG antibodies after infection. Further uncertainty exists around level and duration of immunity and carrier risk after covid-19 recovery. 
UK prevalence studies suggest that 5% of the national population and 17% of London’s population have developed IgG antibodies.   For individuals with confirmed covid-19 infections, high initial antibody conversion rates have been found in small numbers of hospital patients.  WHO guidance currently recommends return to work after 2 weeks of absence of symptoms.  However, some staff who have returned to work during a quiescent phase are unable to continue work because of the resurgence of symptoms.
Prolonged covid-19 illness follows a distinct pattern, reflecting a cluster of symptoms of longer duration and of unknown cause. The presentation of this periodic pattern of symptoms could help improve understanding of the nature of the virus-host interaction in covid-19 pathogenesis, and guide future targeted assessment, management, and treatment interventions. It is now apparent that this is a multi-system disorder with a far wider range of symptoms than currently identified on the NHS website.  Rather than solely relying on diagnostic tests, clinical acumen is crucial to unravel the covid-19 puzzle. Those with less severe relapsing symptoms may not present to general practice or may present at varying timepoints.
The dual hallmarks of prolonged illness with relapsing and remitting pattern of recurrence have significant implications for the individual, who needs care and advice. Consequences may include a prolonged sickness absence and multidisciplinary health needs. A successful recovery requires a gradual rehabilitation and an individualised return to work plan. As our understanding develops on the length of time that symptoms persist, there may be further health implications relevant to immunity, infectivity, and return to work. Individuals will need monitoring and follow-up, with understanding and acceptance shown by managers and colleagues. 
Clare Rayner is a retired occupational health physician.
Amali U Lokugamage is a consultant in obstetrics and gynaecology at Whittington Health NHS Trust, London, UK, and honorary associate professor and deputy lead of clinical and professional practice at UCL Medical School, London.
Mariam Molokhia is a clinical reader in epidemiology and primary care at King’s College London and a GP in NW London.
Competing interests: CR is in the shielding group for covid-19 due to underlying conditions. AUL is on the Board of Directors of the International MotherBaby Childbirth organisation. She is a company director of a small publishing company called Docamali Ltd. MM none declared.
Acknowledgements: MM is supported by the National Institute for Health Research Biomedical Research Center at Guy’s and St Thomas’ National Health Service Foundation Trust and King’s College London. We are grateful to Mark Ashworth and Patrick White for their comments on an earlier draft of this article.