The sudden deaths of a family doctor and a young girl who had contracted swine flu are being investigated by public health officials as research shows that the virus is more dangerous than seasonal flu.

Chloe Buckley, 6, from northwest London, and Michael Day, a GP from Bedfordshire, died after suffering from the virus which is sweeping Britain.

Both victims died quickly after being taken to hospital, according to reports. It is not yet known if either death was caused by the H1N1 virus, or if they were suffering any other health problems.

A post-mortem examination is to be carried out on Chloe, who died on Thursday at St Mary’s Hospital in Paddington a day after complaining of a sore throat. Dr Day died on Saturday in Luton and Dunstable Hospital, where tests revealed that he had swine flu. Seventeen Britons who had the virus have now died.

Research published in the journal Nature indicates that H1N1 virus is more severe than seasonal strains of the flu, suggesting that the pandemic should not be regarded as mild.

Scientists from America and Japan have discovered that the virus inflicts significantly more lung damage in animals than seasonal flu strains, in similar fashion to the highly virulent H5N1 bird flu virus.

The findings, from a team at the University of Wisconsin-Madison in the US, may explain why the swine flu pandemic has caused deaths among people with no underlying health problems. Though swine flu has often been described as mild, and most infected patients have had only minor symptoms, thousands of sufferers have been admitted to hospital and hundreds have died around the world.

Friends and colleagues paid tribute yesterday to Dr Day, a long-serving GP, while the head teacher at Chloe’s school said that it was “impossible to put into words the sorrow that the whole school feels”.

Anxious parents attended a meeting about Chloe’s death at St Catherine’s Primary School in West Drayton, West London. The school has been closed until further notice.

Chris Spencer, director of education at Hillingdon Borough Council, said that everyone was in shock at the death of a little girl “who until a few days ago in all our minds was a child that was perfectly healthy”.

Dr Paul Hassan, senior partner at Priory Gardens health centre in Dunstable, where Dr Day worked, said: “This news has come as such a shock to us all and we are completely devastated.”

Dr Day had lived with his wife for many years in the village of Stanbridge, near Leighton Buzzard. He was a member of the rugby club and neighbours said that he had shown no signs of any significant health problems. “He’s always been a very active man,” said one. “I’d have said he was fit and healthy.”

Professor Wendy Barclay, chair in influenza virology, Imperial College London, said that with so many people becoming infected more fatalities must be expected.

“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,” she said.

The first British patient without underlying health problems died on Friday after contracting swine flu. The patient, from Essex, died in Basildon.

Nearly 10,000 Britons have been confirmed with swine flu but hundreds of thousands more are thought to have the virus. The Government has ordered enough swine flu vaccine to cover the entire population, with the first doses arriving next month.

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