Younger adults and children would be most vulnerable if a pandemic of swine flu were to break out next winter, the Government's chief medical officer warned yesterday. As two more cases were confirmed in Britain yesterday, in a child and a man in his thirties, bringing the total to 34, Sir Liam Donaldson said early analysis of the H1N1 swine flu virus suggested older people might have some immunity as they may have been exposed to the virus before.

"It is not seasonal flu and it is not Spanish flu [the cause of the 1918 pandemic]," he said. "It could have been around in the past, so older people might have some immunity. It may have come back and mixed with avian and pig viruses to make a new virus, but the human bit of it may not be new. We don't know that yet but it is what scientists are speculating."

If the speculation is correct, it could concentrate the virus among younger age groups and limit its spread. Evidence from Mexico suggests the virus is targeting adults aged 20 to 40.

Sir Liam said there were indications that the virus could infect 25 to 30 per cent of the population, which would be five to 10 times more than seasonal flu infects in an average year. The nature of the virus and its potential to cause a pandemic was still unclear but it was likely cases would increase over the next few weeks and that there would be a new wave of infection next winter, he said.

Flu is always worse in winter in all countries. It would be "very important" to look carefully at what was happening in Australia and New Zealand, which are just entering their winter, over the next few months, Sir Liam said.

"We don't know at this stage what the pattern of the disease will be. But it would be highly unusual for a new strain of flu to come along and not cause deaths," he said.

Responding to criticism from Sir David King, the Government's former chief scientist, that school closures were a mistake as early cases of swine flu appeared mild and it would be better for children to be infected and acquire some immunity against a potential pandemic, Sir Liam said that not enough was known yet about the virus and he believed it was better to be cautious.

Current advice that people were only likely to catch the virus if they spent an hour within a metre of someone infected may need to be reviewed, he added. That could mean advising people to avoid crowded places.

Alan Johnson, the health secretary, earlier told the Commons that 10 of the 34 cases confirmed in Britain had acquired the infection in this country and 13 were children. There was not yet evidence of "sustained" person-to-person transmission, but the situation "could rapidly escalate".

The Health Protection Agency said that it was investigating 464 suspected cases of swine flu in Britain yesterday.