The number of cases of swine flu in England almost doubled this week to 53,000, the chief medical officer said today.

Sir Liam Donaldson also said the number of deaths of people in the UK suffering from swine flu had risen from 106 last week to 122 – 93 in England, 15 in Scotland, eight in Northern Ireland and six in Wales.

The number of people with the infection in England last week was about 27,000.

Donaldson said he was concerned by the high proportion of deaths in younger age groups, describing it as a "very unusual pattern for flu".

More than half (54%) of deaths have been in the under-45 age group while 77% have been in the under-65 group.

With seasonal flu, the majority of deaths would be expected in the over-65s.

The announcement, during Donaldson's regular briefing at the Department of Health, came as the Scottish health secretary, Nicola Sturgeon, revealed that four people had died in Scotland in the past 24 hours, including a teenage boy. She said all had underlying health problems.

The deaths bring the total in Scotland to 21 since the outbreak began in spring."It is with sadness and regret that I have to report an additional four deaths from swine flu in the last 24 hours," said Sturgeon.

The four people with H1N1 virus who died in Scotland in the past 24 hours were a 16-year-old boy, a 77-year-old man, a woman, 54, from Glasgow, and a 47-year-old woman from Fife.

The UK's mass vaccination programme against swine flu was launched yesterday, targeting 11 million priority patients and frontline health workers in its initial phase.

Amid fears that there would be a low take-up among NHS staff, Gordon Brown, told the Commons that the country was "ahead of the world" in its fight against the potentially fatal infection.