The panel could not find any trace of flu in the third child, Or Ho-yeung, 27 months, although his elder sister had influenza B.

Panel chairman Yuen Kwok-yung, head of the University of Hong Kong's department of microbiology, said the circulating flu strains had changed a little, making the very young vulnerable. But he said the flu situation this year was 'not worse than' in previous years.

Professor Yuen also issued a stern warning against doctors using multiple drugs with similar functions, which he called 'polypharmacy'.

The panel found that Po-yi was given similar medicines for flu that increased the risk of her abnormal heartbeats.

The deaths of the three children sparked public alarm that prompted the early closure of primary schools and preschools ahead of the Easter holidays to prevent the spread of flu. It was the first time since the Sars outbreak in 2003 that schools were closed in response to an outbreak.

Professor Yuen noted that in the case of Ho-ming, the boy's worried parents had sought medical treatment for him seven times in 10 days.

Professor Yuen said doctors should pay attention to such 'soft medical history' when handling flu patients.

Ho-ming, who died of severe swelling of the brain, had asthma and had been treated with steroids and other drugs.

The panel found no medical negligence involved in the three deaths.

It called for the government to look at widening the recommended age group for childhood flu vaccinations. The government now advises that children aged six months to 23 months be vaccinated but only families on welfare or children with certain chronic illnesses can get them free.

Law Yuen, father of Ho-ming, said he reluctantly accepted the results.

'They say they will make some improvements, say, to the medical staff's attitude,' Mr Law said.

The father of Ho-yeung said: 'The finding is not convincing ... So many things remained unknown.'

Flu expert Malik Peiris, of HKU's department of microbiology, said the polypharmacy problem was linked to the bad practice of doctor-shopping.

'When a doctor sees a patient for the first time, the doctor gives them some treatment and if the patient continues to have problems and get worse, you expect the patient to go back to the same doctor,' he said. 'But if the patient or their parents bring them to another doctor, that doctor will start again from scratch.'

Chinese University professor and head of respiratory medicine David Hui Shu-cheong said the combination of 'impatient patients' and general practitioners being easily accessible in Hong Kong was deadly.

How they died

February 26 Or Ho-yeung, 27 months old, dies at Prince of Wales Hospital

Cause: inflammation of heart

March 1 Ho Po-yi, three, dies at Tuen Mun Hospital

Cause: irregular heartbeat possibly triggered by flu

March 11 Law Ho-ming, seven, dies at Tuen Mun Hospital

Cause: brain lesions caused by flu