At the start of the Covid-19 pandemic, the narrative was simple. Either you were one of the lucky 80% who suffered from ‘mild’ Covid-19, or you were one of the 20% who required hospitalisation. The ‘mild’ cases – who ranged from totally asymptomatic to ‘very poorly but can recover at home’ – had nothing to worry about over the longer term.

Unfortunately, we have since gained an insight into another category of patient – those who struggle with symptoms on an ongoing basis. Some of these patients were hospitalised, but many weren’t. Some were older, with pre-existing conditions, but a surprising proportion were young and in good health.

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These patients have become known as ‘long-haulers’, and suffer with Covid-19 far beyond the expected two weeks. In a Facebook group for coronavirus survivors, long-haulers list 200 different symptoms ranging from fatigue (the most common) to joint pain, phantom smells, night sweats and irritability.

The British Medical Journal defines ‘long Covid’ as “illness in people who have either recovered from Covid-19 but are still reporting lasting effects of the infection or have had the usual symptoms for far longer than would be expected”. While we don’t know the true prevalence of this condition, data from the UK Covid-19 Symptom Study app suggests that one in ten people with Covid-19 are sick for three weeks or more.

What are the potential long-term health impacts of Covid-19, and what kinds of treatments might be required in the years ahead?

Lung damage

One of the scariest long-term Covid-19 complications is a condition called post-Covid fibrosis or post-ARDS fibrosis. This is irreversible lung damage, occurring when fluid builds up in the alveoli (air sacs in the lungs) and causes portions of the lung to die. Symptoms include shortness of breath, coughing and fatigue.

While we know the extent of lung damage is related to the severity of illness, it’s hard to determine the cause. It might be the viral infection itself, but it might also be the cytokine storm that can follow infection, and it might even be related to ventilation.

In June, doctors performed the first known double lung transplant on a Covid-19 patient in the US. The patient, aged just 28 and otherwise healthy, had been in intensive care for six weeks. While the infection eventually left her body, the lung damage was so severe that a transplant was her only chance of survival.

Lung transplants, however, will remain a last resort, and they certainly aren’t suitable for every patient. The patient needs to be strong enough to recover from the surgery, but sick enough that they would otherwise die.

“It’s such a paradigm change,” the patient’s surgeon, Dr Ankit Bharat, told the New York Times. “Lung transplant has not been considered a treatment option for an infectious disease, so people need to get a little bit more of a comfort level with it.”

In the UK, patients who have been admitted to hospital with Covid-19 receive chest radiography three months after discharge. Speaking to the BBC, one doctor estimated that 20%-30% of the patients he was seeing showed some form of early lung scarring. These patients will need antifibrotic drugs to slow the progression of the disease, and there is likely to be a growing need for new therapies.

“The interstitial lung disease community should pull together to investigate the long-term consequences of Covid-19 and develop evidence-based strategies to deal with this emerging problem,” concluded a review in The Lancet.

Neurological symptoms

While estimates vary, it seems that around half of all Covid-19 patients experience neurological symptoms during the acute phase of their illness. These can range from mild symptoms such as anosmia (loss of smell) to bouts of delirium, confusion and seizures.

Unfortunately, recovery can be protracted. Previous research has shown that one in five survivors of acute respiratory distress syndrome (ARDS) deal with ongoing cognitive impairment.

At this stage, we don’t know how this will manifest in relation to Covid-19, but many patients with ‘long Covid’ report problems with attention and concentration; dysfunction in the peripheral nerves; and mental health problems like anxiety, depression and PTSD. A recent Lancet study, which looked at 60 Covid-19 patients, found that 55% of them were still experiencing neurological issues three months down the line.

Professor David Nutt of Imperial College London is planning a study on Covid-19 patients who have developed depression to scan their brains for inflammation or other abnormalities. “People who are discharged from the ICU with Covid-19 need to be monitored systematically long term for any evidence of neurological damage – and then given interventionist treatments if necessary,” he told BBC Future.

Blood clotting, heart damage and other issues

A number of Covid-19 patients have dealt with blood clots, leading to complications such as strokes, pulmonary embolisms and deep vein thrombosis. This appears to be caused by inflammation, and while hospitalised patients are the most susceptible, women taking the contraceptive pill or hormone replacement therapy are also at elevated risk.

“During this pandemic, we need additional research to determine if women who become infected with the coronavirus during pregnancy should receive anticoagulation therapy or if women taking birth control pills or hormone replacement therapy should discontinue them,” said Dr Daniel Spratt, the author of a paper on the subject.

Worryingly, a number of patients suffered strokes or clotting abnormalities even after they had recovered, or while showing no other Covid symptoms. Doctors have called on patients to be vigilant for the signs, and have advised them to keep taking anticoagulants even after leaving hospital.

In some cases, the virus also inflames the heart muscle. In July, two studies from Germany suggested that Covid-19 could presage cardiovascular problems. The first looked at cardiac MRIs of Covid-19 survivors, and found they were more likely to have evidence of heart damage than controls. The other looked at autopsy results of patients who’d died of Covid. In around two thirds of cases, they had high levels of the virus in their hearts.

We don’t know yet whether this damage is sustained in the long term or whether it’s a temporary reaction to the virus.

The future

Since Covid-19 is a new illness, with the earliest patients having fallen sick late last year, we can’t say much for certain about how long its effects might endure. What we need are ongoing studies, such as the recently launched UK government-funded study that will follow 10,000 Covid-19 survivors.

“It is vitally important that we rapidly gather evidence on the longer term consequences of contracting severe Covid-19 so we can develop and test new treatment strategies for them and other people affected by future waves of the disease,” said Professor Chris Brightling, chief investigator of the study.

What we know for the time being is that the virus can attack a bewildering array of tissues in the body, and that ‘long Covid’ sufferers face difficulties accessing appropriate treatment. Even once we have a vaccine, the disease is likely to impact people’s lives, and weigh on health systems, for some time to come.