Swine flu is killing more than a tenth of those it infects in California and up to 20% of older victims, a study has shown

Swine flu kills more than one in 10 of those it affects severely enough to put in hospital, a US study has shown.

Up to 20% of older patients taken to hospital with the virus ended up dead, researchers in California found.

Almost a third of cases required intensive care treatment, and deaths occurred among all age groups.

The findings, published in the Journal of the American Medical Association (Jama), suggest that H1N1 swine flu is potentially more dangerous than many people believe, according to the authors.

Dr Janice Louie, from the California Department of Public Health, led the study of 1,088 swine flu cases resulting in being taken to hospital or death in the state between April 23 and August 11 this year. Of these, 32% were children under the age of 18, with infants having the highest rates of hospitalisation. The midpoint age of patients was 27. In total, 340 patients (31%) were admitted to hospital intensive care units. Of the 297 intensive care cases for whom information was available, 65% required mechanical ventilation.

Chest X-rays of 833 patients revealed indications of pneumonia or acute respiratory distress syndrome. The researchers recorded 118 deaths, an overall mortality rate of 11%. Fatalities were most common among people aged 50 and over, for whom death rates rose to between 18% and 20%. Seven per cent of deaths were among children younger than 18.

Among fatal cases, it typically took just 12 days after the onset of symptoms for the patient to die. The most common causes of death were viral pneumonia and acute respiratory distress syndrome. The scientists wrote: "In contrast with the common perception that pandemic 2009 influenza A (H1N1) infection causes only mild disease, hospitalisation and death occurred at all ages, and up to 30% of hospitalised cases were severely ill."

Most patients admitted to hospital had underlying health problems, including high blood pressure and obesity, said the scientists. They suggested that obesity might be a newly identified risk factor for fatal swine flu infection.

The researchers added: "Clinicians should maintain a high level of suspicion for pandemic 2009 influenza A (H1N1) infection in patients presenting currently with influenza-like illness who are older than 50 years or have known risk factors for influenza complications, regardless of rapid test results. Hospitalised infected cases should be carefully monitored and treated promptly with antiviral agents."

A total of 78,000 new cases of swine flu were reported in England in the week ending October 29, according to latest figures from the Health Protection Agency. This was up from 53,000 the week before, but fell short of the 100,000 cases reported in July. A total of 751 patients with swine flu had been admitted to hospital in England, the largest number since July. They included 157 patients needing critical care. To date there have been 137 swine flu deaths in the UK. Nearly half have involved people with underlying health problems.