Swine flu has been blamed for two more deaths – of a family doctor and a six-year-old girl – as the first full analysis of the virus shows it is five times more virulent than ordinary seasonal flu.

Dr Michael Day died at Luton and Dunstable hospital on Saturday, taking the UK total of confirmed and suspected deaths from the virus to 17. A swab test showed he was infected with swine flu but in a statement NHS East of England said the exact cause of death was unknown and his case had been reported to the coroner.

Chloe Buckley, six, from north west London, died on Thursday at St Mary's Hospital, Paddington. Dr Simon Tanner of NHS London said a post mortem would be carried out to determine if she had any underlying medical condition but teachers said she was "perfectly healthy". On Friday, a patient from Essex became the first to die from swine flu in Britian without underlying health problems.

Chris Spencer, director of education at Hillingdon Borough Council, where Chloe attended St Catherine's School, confirmed that the school would now be closed.

Speaking outside St Catherine's he said: "It's an exceptional set of circumstances. This is a little girl who until a few days ago in all our minds was a child that was perfectly healthy so everybody here is in a deep state of shock and with just a few days to go until the end of term, we've decided it's in the best interests of all involved."

Experts said the deaths were "very sad", although not unexpected given the tens of thousands infected, but warned that any cluster of deaths could indicate the virus had mutated and become nastier.

Wendy Barclay, professor of influenza virology at Imperial College, London, said: "With so many people becoming infected we must expect that some fatalities will occur. It is very important to keep a careful look for clusters of severe cases that might indicate that a mutated virus has arisen that can be more virulent than the swine flu that has circulated until now. Each severe or fatal case should be carefully characterised using our best science to understand the reasons why this has happened."

The first full analysis of the H1N1 virus, published in Nature, shows it causes more lung damage in animals than seasonal flu. For two strains of virus tested, five times less was needed to cause the same damage as seasonal flu. Damage to the lungs increases the risk of pneumonia which is the commonest cause of complications, severe illness and death in flu epidemics.

Professor Ian Jones, director of research at the University of Reading, said: "This complete analysis of the current H1N1 is what we've been waiting for. For a number of measures it shows that the new virus is more serious than seasonal H1N1 but that, nonetheless, the major outcome to infection is recovery. For the few cases of severe infection the data should help in the clinical management of hospitalised patients."

The British Medical Association said it was saddened by the death of GP Dr Day, and the other victims, but urged the public not to panic. Dr Day, who qualified as a doctor in 1970, had recently retired from the Priory Gardens Health Centre in Dunstable but still worked there part time as as a locum.

Dr Paul Hassan, senior partner at the health centre, said: "This news has come as such a shock to us all and we are completely devastated. Dr Day was a work colleague and also a personal friend to everyone at the practice.The news will also come as a great shock to our patients, many of whom have known him for many years."

Dr Laurence Buckman, chairman of the BMA's GPs committee, said: "Doctors have always accepted that there are risks associated with their job. Obviously these are smaller than they used to be with the advent of modern medicine, but they can never be eliminated altogether.

"It is understandable that people will be worried when they hear that a GP has died but we urge them to follow the recommended advice and contact their family doctor, rather than physically going to the surgery if they have symptoms. The vast majority of people will recover quickly by taking paracetamol or ibuprofen, and drinking plenty of fluids. Anyone who is not recovering quickly should get extra advice as a small number will need more intensive treatment.

"We must remember that every year there are deaths from complications of seasonal flu; this is unfortunately inevitable with any strain of influenza."

Liberal Democrat health spokesman Norman Lamb said: "Dealing with the flu crisis is a mammoth task and NHS staff need access to support and advice given the anxiety that many of them may face.

"The Government needs to ensure that all frontline NHS staff are given access to flu vaccines as a matter of urgency.