LONDON (REUTERS) - A study of the lungs of people who have died from Covid-19 has found persistent and extensive lung damage in most cases and may help doctors understand what is behind a syndrome known as "long Covid", in which patients suffer ongoing symptoms for months.

Scientists leading the research said they also found some unique characteristics of Sars-CoV-2, the virus that causes Covid-19, which may explain why it is able to inflict such harm.

"The findings indicate that Covid-19 is not simply a disease caused by the death of virus-infected cells, but is likely the consequence of these abnormal cells persisting for long periods inside the lungs," said Professor Mauro Giacca at King's College London, who co-led the work.

The research team analysed samples of tissue from the lungs, heart, liver and kidneys of 41 patients who died of Covid-19 at Italy's University Hospital of Trieste between February and April this year.

In a telephone interview, Prof Giacca said that, while his research team found no overt signs of viral infection or prolonged inflammation in other organs, they discovered "really vast destruction of the architecture of the lungs", with healthy tissue "almost completely substituted by scar tissue".

He said: "It could very well be envisaged that one of the reasons why there are cases of long Covid is because there is vast destruction of lung (tissue).

"Even if someone recovers from Covid, the damage that is done could be massive."

Growing evidence from around the world suggests that a small proportion of people who have had Covid-19 and recovered from their initial infection can experience a range of ongoing symptoms, including fatigue, brain fog and shortness of breath.

The condition is often called "long Covid".

Prof Giacca said almost 90 per cent of the 41 patients had several characteristics unique to Covid-19 compared with other forms of pneumonia.

One was that patients had extensive blood clotting of the lung arteries and veins. Another was that some lung cells were abnormally large and had many nuclei - a result of the fusion of different cells into single large cells in a process known as syncytia.

The research, published in the journal Lancet eBioMedicine, also found that the virus itself was still present in many types of cells.

"The presence of these infected cells can cause the major structural changes observed in lungs, which can persist for several weeks or months and could eventually explain 'long covid'," Prof Giacca said.