Twenty-seven people have died from flu since October, including nine children, according to the latest UK figures.
Data from the Health Protection Agency (HPA) reveals that of those who died, 24 had swine flu and three were suffering from another strain, flu type B. Almost half of those who died were in an "at risk" group, such as those suffering from diabetes, heart disease or asthma.
The HPA would not confirm if any of the deaths were among pregnant women because of worries over identification, but confirmed that just one of the people who died (and whose vaccination status was known) had received this year's flu vaccine.
A spokeswoman for the HPA said the flu jab is only 70% to 80% effective, meaning somebody could still potentially die from flu if they are vaccinated but have an underlying serious illness. "It is not a vaccine failure; it means the person's illness is so serious that they are very weak," she said.
Figures published on Wednesday showed rates of flu infection have more than doubled in the last week. Cases of flu have risen to 87.1 per 100,000 people, from 32.8 in the previous week, according to England and Wales data from the Royal College of GPs. Rates of flu are highest in youngsters aged five to 14, followed by those under four, then people aged 15 to 44.
Professor Dame Sally Davies, interim chief medical officer for England, said rates of flu are within the expected range for the time of year.
She told the BBC: "Clearly, any death is sad for the family and the patient and we don't like it. But 27 deaths at this stage of seasonal flu is not a large number. What we are seeing with the data at the moment is a rise in consultations with GPs around flu which are within the normal range for winter flu."
But she said the difference is that the dominant strain, swine flu, affects younger people, adding: "They are young, some are pregnant, and it is not only people in at-risk groups."
Overall, the numbers in at-risk groups getting vaccinated "continues to remain low", the HPA said. Some 43% of at-risk groups under the age of 65 in England have had the jab, compared with 68.5% of over-65s.
The HPA data also showed 19 patients are receiving specialist intensive care treatment, known as extra corporeal membrane oxygenation (ECMO), while 302 people are currently in intensive care with flu.