The coronavirus may trigger depression, even when the infection is mild, research suggests.
Early in the pandemic, experts warned the outbreak could have a "profound" and "pervasive" impact on people's mental health for some time.
Initially considered an airway infection, the coronavirus is now known to affect many parts of the body. With some enduring fatigue, headaches and a loss of taste or smell, the virus is thought to affect the brain, either directly or indirectly.
A team from University College London (UCL) has now reported just under one quarter (23%) of people who have the coronavirus endure depression, regardless of whether they become seriously ill with the infection itself.
Off the back of the results – published in the Journal of Neurology, Neurosurgery and Psychiatry – one scientist has warned the coronavirus' negative affect on mental health "is the norm, rather than the exception".
"We had expected neurological and psychiatric symptoms would be more common in severe COVID-19 [the disease caused by the coronavirus] cases, but instead we found some symptoms appeared to be more common in mild cases," said lead author Dr Jonathan Rogers.
"It appears COVID-19 affecting mental health and the brain is the norm, rather than the exception."
The scientists analysed 215 studies, carried out in 30 countries up to July 2020. These were made up of more than 105,000 coronavirus patients between them, with the participants experiencing varying levels of ill health.
A loss of smell was the most common symptom overall, affecting more than two in five (43%) of the patients. This was followed by weakness (40%), fatigue (38%) and a loss of taste (37%).
One quarter (25%) of the patients endured muscle pain, while 23% had depression, 21% headaches and 16% anxiety.
Early research suggests the coronavirus is mild in four out of five cases, however, it can trigger severe complications.
Within the UCL study, 1.9% of the patients endured an ischaemic stroke – when a clot blocks oxygen-rich blood from reaching the brain – while infected with the coronavirus.
A haemorrhagic stroke – caused by a blood vessel rupturing or abnormal ones forming in the brain – affected 0.4% of the participants, while 0.06% experienced a seizure.
The scientists have stressed their study was disproportionately made up of severe coronavirus patients, with many of the 215 trials taking place in hospitals. Even community-based studies do not generally include people with very mild or no symptoms, they added.
Among the patients who were not hospitalised, more than half (55%) endured fatigue, while just slightly fewer (52%) lost their sense of smell.
Muscle pain, a loss of taste and headaches were reported by 47%, 45% and 44% of the non-hospitalised participants, respectively.
The scientists did not investigate how the coronavirus causes psychiatric or neurological complications.
It has been suggested the infection may enter the brain or trigger an inflammatory immune response that indirectly affects the vital organ.
Some coronavirus patients may have also struggled with the social restrictions and economic uncertainty that came from the UK's various lockdowns.
This may explain why mental health disorders appear to be more common with the coronavirus than seasonal flu, the latter of which has long had a successful vaccination programme, preventing the need for extreme restrictions.
"Many factors could contribute to neurological and psychiatric symptoms in the early stages of infection with COVID-19, including inflammation, impaired oxygen delivery to the brain, and psychological factors," said Dr Rogers.
"More studies are needed to understand these links better."
The majority of the 215 studies focused on fatigue and muscle pain, rather than depression, anxiety or post-traumatic stress disorder.
While further research is carried out, the scientists have stressed medics should be prepared to treat a surge in mental health disorders as the pandemic continues to unfold.
"Neurological and psychiatric symptoms are very common in people with COVID-19," said co-author Dr Alasdair Rooney, from the University of Edinburgh.
"With millions of people infected globally, even the rarer symptoms could affect substantially more people than in usual times.
"Mental health services and neurological rehabilitation services should be resourced for an increase in referrals."
Watch: How coronavirus can affect the brain in severe cases
The circulating coronavirus is one of seven strains of a virus class that are known to infect humans.
It is genetically similar to fellow-strain severe acute respiratory syndrome (Sars), which killed 774 people during its 2002/3 outbreak.
Sars has been described as "clinically neurotoxic, causing mental health and neurological disorders".
Its 2002/3 outbreak was linked to a 30% increase in suicide in those aged 65 or over, while half (50%) of the recovered patients "remained anxious".
The long COVID phenomenon, when former a coronavirus patient continues to endure complications after testing negative for the infection, "may be a continuation of some of those [symptoms] experienced in the acute phase of the illness", wrote the UCL scientists.
Long COVID patients have reported a range of symptoms, including persistent brain fog, fatigue and depression.
Experts have stressed, however, a depression epidemic does not have to be an inevitable outcome of the coronavirus pandemic, with lessons being learnt from the Sars outbreak.
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Watch: Depression doubled among UK adults amid pandemic