A report by a team of 25 experts from 11 countries estimates that around 5 per cent of the adult population around the world in any year is living with depression.

In high-income countries, about half of people suffering from depression are not diagnosed or treated, and this rises to 80-90% in low and middle-income countries.

The pandemic, the study says, has created additional challenges, with "social isolation, bereavement, uncertainty, hardship, and limited access to healthcare taking a serious toll on the mental health of millions".

Global response needed

The report's authors - the Lancet-World Psychiatric Association Commission - are now calling for a "whole-of-society response to reducing the global burden of depression".

At its worst, depression can lead to suicide. Studies indicate 70-80% of people who die by suicide in high-income countries - and around half of those in low and middle-income countries - suffer from mental illness, of which depression is the most common cause.

The report further points out it also has an "enormous, under-recognised social and economic toll on individuals, families, communities, and countries".

It goes on: "Even before the COVID-19 pandemic, the loss in economic productivity linked to depression cost the global economy an estimated US$1 trillion [£738 billion] a year."

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Commission chair Professor Helen Herrman, of the University of Melbourne in Australia, said: "Depression is a global health crisis that demands responses at multiple levels."

She said the Commission's findings offered "an important opportunity for united action to transform approaches to mental health care and prevention globally".

The study stresses the current system of classifying people with symptoms of depression into just two categories - either that they have clinical depression or not - is too simplistic.

It argues depression is a more complex condition with many differing signs and symptoms, severity levels and duration.

This, it says, should be treated in a more personalised, staged approach than the current broad brush.

'Avoidable suffering'

Christian Kieling, from the Universidade Federal do Rio Grande do Sul in Brazil and the commission's co-chair, said: "There is arguably no other health condition which is as common, as burdensome, as universal, or as treatable as depression, yet it receives little policy attention and resources."

The report, entitled Time for United Action on Depression, calls for "concerted and collaborative efforts" by governments, healthcare providers, researchers, people living with depression, and their families to "improve care and prevention, fill knowledge gaps, and increase awareness of the condition".

• For help with depression, visit the Suicide Prevention Lifeline organisation or the Samaritans - or call 116 123.