Concerns over the spread and virulence of the latest flu outbreak are increasing among health experts, following a doubling of cases in a week and the announcement today of 10 more deaths, taking the total this season to 27.

Influenza expert Professor John Oxford described the upward curve of the flu cases graph as "pretty horrible". According to the Royal College of GPs, confirmed cases have increased from 32.8 per 100,000 to 87.1 per 100,000 in the last week.

The true figure will be higher as some people will stay at home in bed without seeing a doctor.

The chief anxiety is that the predominant strain is H1N1 swine flu, which triggered last year's pandemic and is now coming around for a second season. Most over-65s are protected, because the strain is similar to one that circulated years ago, so it is a younger age group, including children and pregnant women, who are being hardest hit.

According to the Health Protection Agency's weekly statistical bulletin, of the 27 deaths, 24 were from swine flu and the rest from influenza B. Eighteen of those who died were adults and nine children.

About half of those who died had an underlying health condition, which would have made them especially vulnerable to flu and also makes them eligible for the vaccine on the NHS. The jab protects against three strains, including swine flu and influenza B. But the rest were apparently previously healthy.

The HPA acknowledges that a younger generation than usual are bearing the brunt of the outbreak. "A small proportion of flu cases are resulting in severe disease, particularly in people under the age of 65," it said in a statement. "This is due to the fact that H1N1 is more likely to infect young people and, unfortunately, a very small number of these may develop severe disease."

Professor John Watson, head of the respiratory diseases department at the HPA, which collects the data said: "The level of flu activity we are currently seeing is at levels often seen during the winter flu seasons, but due to the fact that H1N1 is one of the predominant strains circulating at the moment, we are seeing more severe illness in people under 65 than we would normally expect."

He urged people in at-risk groups for flu – generally the over-65s and those with health problems but also pregnant women in any stage of pregnancy – to go to their GPs for vaccination.

"Flu can be an extremely serious illness for people in 'at risk' groups, including pregnant women, the elderly and those with other underlying conditions such as heart problems, diabetes, lung, liver or renal diseases and those who have weakened immune systems," Watson said. The vaccine was safe and effective, he added.

The extent to which flu is affecting the under-65s became clear earlier this week, when the Department of Health released figures showing that more than 300 people were in critical care, only 23 of whom were over 65. The vast majority – 243 – were between 16 and 64, with 24 under the age of five and 12 aged between five and 15.

Vaccine uptake remains low (43%) in people considered high risk but aged under 65. In the older age group, 68.5% have been vaccinated.

Oxford said the vaccine is highly effective against swine flu, but suggested it would not be a good idea to urge everybody under 65 to get the jab. "It is best to prioritise, otherwise there will be a great rush," he said. "They will soak up the vaccine and maybe there will be some other people who are more vulnerable, who will not join the rush and may end up in hospital."

He thought the government would be well-advised to mount some sort of publicity drive to get the most at-risk vaccinated, and said he thought it was remiss not to have done so already.

"I think it shows that if you are into cost-cutting, you can get into endless problems," Oxford said.

H1N1 had shown itself to be a dangerous virus. "It is perfectly capable of killing a perfectly healthy young person," he said.

For most people, self-care at home, involving plenty of fluids, rest and painkillers, should be enough, the HPA said, but those showing severe symptoms should contact their GP or out-of-hours service.

The Department of Health has told GPs that antiviral drugs such as Tamiflu can be used in previously healthy people, as well as those in at-risk groups who show symptoms.

"It's hoped wider access to antiviral treatments will help reduce the number of severe cases we are seeing," said Watson.

Professor David Salisbury, director of immunisation at the Department of Health said: "Our influenza immunisation programme has been designed to protect those particularly at risk from serious disease, either from influenza itself or in whom influenza would make their underlying disease worse.

"This means that we use an age and risk group based approach in line with most other countries that have seasonal influenza vaccination programmes.

"We continue to monitor the current situation very carefully and JCVI keeps the policy for seasonal influenza vaccination under review as well."

NHS Direct has been under such pressure in recent days that people have been forced to wait for advice, but its head today denied this was connected to the flu outbreak. The problem appeared to be people's reluctance or inability to travel to see their GP in the extreme weather conditions. Some staff had also not been able to get to work.

"I think we're providing a good service. Not as good as we'd like," Nick Chapman told BBC Radio 4's Today programme, rejecting reports that NHS Direct was at "breaking point".

But he added: "Because we're not able to provide the normal service I think I should apologise ... What we have seen is a 50% increase in the overall demand for our service and we've found that is extremely difficult for us to cope with."