The coronavirus (COVID-19), once an ‘aggressive tiger’ of a disease, has weakened and become more like a wild cat, according to a top Italian doctor.

Professor Matteo Bassetti said he is convinced the virus is ‘changing in severity’ and patients are now surviving infections that would have killed them before.

And if the virus’s weakening is true, COVID-19 could even disappear without the need for a vaccine, he claimed.

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He has said multiple times in recent months that patients with COVID-19 seem to be faring much better than they were at the start of the epidemic in Italy.

Professor Bassetti suggests this could be because of a genetic mutation in the virus making it less lethal, because of improved treatments, or because people are not getting infected with such large doses because of social distancing.

But other scientists have hit back at the claims in the past and said there is no scientific evidence that the virus has changed at all.

Professor Bassetti, the chief of infectious diseases at San Martino General Hospital in Genoa, Italy, told The Sunday Telegraph the virus could wither away on its own.

He said: ‘It was like an aggressive tiger in March and April but now it’s like a wild cat. Even elderly patients, aged 80 or 90, are now sitting up n bed and they are breathing without help. The same patients would have died in two or three days before.’

Italy was one of the worst-hit countries in the world during the pandemic’s early stages and has now recorded more than 238,000 positive cases and 34,000 deaths.

Scientists have said the elderly population there, the virus spreading in rural areas and the suddenness of the outbreak contributed to the country’s high death toll.

Professor Bassetti suggests that one of the reasons the virus might be causing less serious illness is a genetic mutation which has made it less damaging to people’s lungs.

Or, he said, people may simply be receiving smaller amounts when they get infected, because of social distancing and lockdown rules, making them less sick.

This theory depends on the severity of someone’s illness being affected by their ‘viral load’ – the amount of virus that gets into someone’s body when they’re first struck by it.

Professor Bassetti said: ‘The clinical impression I have is that the virus is changing in severity.

‘In March and early April, the patterns were completely different. People were coming to the emergency department with a very difficult to manage illness and they needed oxygen and ventilation, some developed pneumonia.

‘Now, in the past four weeks, the picture has completely changed in terms of patterns. 

‘There could be a lower viral load in the respiratory tract, probably due to a genetic mutation in the virus which has not yet been demonstrated scientifically.’ 

The infectious disease doctor has made similar claims in the past but sparked criticism for being over-optimistic.

He said at the beginning of June: ‘The strength the virus had two months ago is not the same strength it has today.‘

But other scientists did not welcome the idea and said there was no evidence to back up Professor Bassetti’s claims.

Dr Gideon Meyerowitz-Katz, from the University of Wollongong in Australia, told MailOnline that the idea the virus has disappeared ‘seems dubious’.

The epidemiologist warned Italy – which was the centre of Europe’s coronavirus crisis in March – was still recording new Covid-19 cases and deaths, showing the virus was still a danger.

At the start of June, in response to Professor Bassetti’s claim, Dr. Angela Rasmussen, from Columbia University, tweeted: ‘There is no evidence that the virus is losing potency anywhere.’ 

She added less transmission means fewer hospitalizations and deaths – but warned: ‘That doesn’t mean less virulence.’

The virulence of a virus is how dangerous the illness is but may not directly relate to how contagious it is.

Dr Seema Yasmin, an epidemiologist from Stanford University, said the idea was ‘bulls***’.

Dr Oscar MacLean, of the University of Glasgow, added: ‘These claims are not supported by anything in the scientific literature, and also seem fairly implausible on genetic grounds.

‘The vast majority of SARS-CoV-2 mutations are extremely rare, and so whilst some infections may be attenuated by certain mutations, they are highly unlikely to be common enough to alter the nature of the virus at a national or global level...

‘Making these claims on the basis of anecdotal observations from swab tests is dangerous. 

‘Whilst weakening of the virus through mutations is theoretically possible, it is not something we should expect, and any claims of this nature would need to be verified in a more systematic way. 

‘Without significantly stronger evidence, no one should unnecessarily downplay the danger this highly virulent virus poses, and risk the ongoing society-wide response.‘