Coronavirus has downgraded from a "tiger to a wild cat" and could die out on its own without a vaccine, an infectious diseases specialist has claimed.

Prof Matteo Bassetti, head of the infectious diseases clinic at the Policlinico San Martino hospital in Italy, told The Telegraph that Covid-19 has been losing its virulence in the last month and patients who would have previously died are now recovering.

The expert in critical care said the plummeting number of cases could mean a vaccine is no longer needed as the virus might never return.

His comments come after the Health Secretary announced on Thursday that a deal had been struck between pharmaceutical company AstraZeneca and Oxford University to begin the manufacture of a potential vaccine.

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"The clinical impression I have is that the virus is changing in severity," said Prof Bassetti.

"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. Also we are now more aware of the disease and able to manage it.

"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 in bed and they are breathing without help. The same patients would have died in two or three days before.

"I think the virus has mutated because our immune system reacts to the virus and we have a lower viral load now due to the lockdown, mask-wearing, social distancing. We still have to demonstrate why it's different now.

"Yes, probably it could go away completely without a vaccine. We have fewer and fewer people infected and it could end up with the virus dying out."

Prof Karol Sikora, an oncologist and chief medical officer at Rutherford Health, previously said it is likely the British public has more immunity than previously thought and Covid-19 could end up "petering out by itself".

However, Dr Bharat Pankhania, a senior clinical lecturer at the University of Exeter Medical School and a former Public Health England consultant, said the idea that Covid-19 would die out is "optimistic in the short term".

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"I don't expect it to die out that quickly," he told The Telegraph.

"It will if it has no one to infect. If we have a successful vaccine then we'll be able to do what we did with smallpox. But because it's so infectious and widespread, it won't go away for a very long time.

"My estimate is ranging from never to if we are really lucky and it sort of mutates and mutates, it may lose its virulence - we're talking years and years. I disagree with Prof Sikora that nirvana is around the corner."