Professor Sir Liam Donaldson, the Government's chief medical officer, has said it is "premature" to assume that the swine flu virus is a mild infection, simply because none of the British victims has so far died.
His warning comes after Sophie De Salis, 12, a pupil from Alleyn's School in south London who contracted swine flu said it was just like having a cold.
"I just felt like I had a cough and was under the weather," she said.
Sir Liam told BBC Radio 4's Today programme that there was still a need for caution.
"There have been deaths in Mexico and the Mexican health system is probably not as well supplied as we are with drugs like anti-virals.
"So at this stage I think it would be premature to conclude that this is a mild infection.
"We need to be very cautious when we are dealing with children and that is why in each of the school outbreaks so far we have had local public health teams tracing contacts and getting children and others on antiviral drugs on a precautionary basis."
He said there was no need for people worried about the virus to stock up on Tamiflu and other anti-virals: "We have a large stockpile of anti-virals – the largest in the world. These will be made available through the NHS at the right moment when numbers increase."
He confirmed that a second, larger wave of infections could be expected during the usual flu season in the autumn and winter, and said officials were closely monitoring the progress of swine flu in southern hemisphere countries like Australia and New Zealand, which are currently experiencing their winter.
Sir Liam said that the last public health information campaign on this scale dealt with Aids in the 1980s.
Leaflets being distributed to homes around the country will "reinforce the basic hygiene principles that can play a very important part in halting the spread and reducing the risk to individuals", he said.
These involve covering the mouth when sneezing or coughing, disposing of tissues after sneezing and washing hands.
"These things are common sense, they are time-honoured, but they are also underpinned by science which shows that they help a great deal," he said. "Getting into these habits early is very important."