By Nick Triggle
Health reporter, BBC News
Children have been partly hard hit by the swine flu virus
Up to a third of children in some areas may have had swine flu, but many will not have been ill, analysis shows.
The Health Protection Agency has reviewed blood tests which showed higher levels of infection among children than originally thought.
In hotspot areas, such as London and the West Midlands, a third of school-aged children may have had the virus, but only one in 10 or less got ill.
Across the UK, the figure is probably about a fifth, the HPA said.
The findings reinforce the fact the pandemic is a mild strain of flu.
SWINE FLU IN CHILDREN
The most common symptoms of swine flu are a cough, sore throat, fever and muscle aches
A minority of patients have also suffered vomiting and diarrhoea
But as health officials are pointing out there are a very large number who have had the virus without having any symptoms
If children do fall ill, parents can contact their GPs or the flu service to get drugs to lessen the symptoms and shorten the illness
Children with health problems and all under-fives are among the priority groups being offered vaccination
Flu, whether it is seasonal or swine, always affects more people than actually get ill.
The ratio is normally about 50-50 and pandemic flu is probably not much different, the HPA said.
The agency has been carrying out blood tests on sample groups as part of its on-going surveillance programme to look for signs that individuals had come into contact with swine flu.
These have revealed high levels of infections without symptoms among children - between three to fives times as many have had the virus than have fallen ill.
The revelation is interesting because it means the 1m figure for the overall number of cases seen so far is likely to be an underestimate by some way.
The findings also suggest there may be a significantly higher degree of immunity in children than thought - once someone has had swine flu they cannot get it again unless the virus mutates.
Coupled with this, between 30% to 40% of over 50s are thought to have some immunity from previous strains they have come into contact with.
Swine flu is from the same family of the virus as the normal seasonal flu and previous pandemics.
Impossible to predict
The HPA admitted this could explain why the virus had not really taken off this autumn - the number of new cases is currently half of what they were in the summer.
However, Justin McCracken, chief executive of the HPA, said it was still impossible to predict what would happen next.
He added the latest analysis just gave an idea of what had been going on so far.
"Those that meet the case definition are just the bit of the iceberg above the water. We are giving an idea of how big the iceberg is under the water."
HPA officials also tried to calm fears about the emergence of the drug-resistant strain of flu seen in Cardiff last week.
Maria Zambon, director of the HPA's Centre for Infections, said the revelation was "not unexpected", but she added many mutated strains struggle to replace the original strain as the more dominant one.
"We will have to see if it can outcompete the non-resistant one."