More than half of people who died from swine flu or were admitted to hospital with it during the first wave of the H1N1 pandemic in Britain were previously healthy people with no underlying risks, a study has found.

The research findings support many health authorities' policies to prioritise pregnant women, children under the age of 5 and those with long term respiratory problems such as asthma for vaccination against the H1N1 virus known as swine flu.

But they also suggest that everyone with asthma might benefit from vaccination, not just those with severe disease, researchers from the University of Nottingham said.

"Our findings support the use of H1N1 pandemic vaccine in pregnant women, children aged under five years and those with chronic lung disease as a priority, including patients with asthma, regardless of severity," they wrote.

The findings, published on Tuesday in the journal Thorax, a British Medical Journal title, were based on an analysis of clinical data from 55 hospitals in 20 towns and cities in the first wave of the H1N1 pandemic during May to September 2009.

Between late April and late September 2009, data were collected on 631 people with swine flu -- 405 of them adults.

Their ages ranged from 3 months to 90 years. Around 36 percent were under 16 and one in 20 (5 percent) were aged 65 and older and 27 patients, or 4 percent, were pregnant.

This indicates that pregnant women were around three times as likely to require hospital admission once infected with swine flu as women who were not pregnant, the researchers said.

The World Health Organisation (WHO) said last month that the H1N1 pandemic is not yet over, although its most intense activity has passed in many parts of the world.

Britain was one of the first European countries hit by H1N1, which emerged in Mexico in March 2009 and was declared a pandemic by the WHO in June 2009.

Drug companies such as GlaxoSmithKline, Novartis, Sanofi-Aventis and Baxter, among others, developed H1N1 vaccines and governments ordered millions of doses for immunisation campaigns to slow the virus' spread.

As in other countries, health authorities set priorities for pregnant women, the sick and the young to be first in line for vaccines, particularly those with lung conditions such as asthma.

The Thorax study found that 55 percent of all hospital admissions and 59 percent of H1N1 deaths in hospital occurred in people with no previous health problems. Just under half of patients had underlying conditions, mostly asthma.

WHO experts have said the H1N1 virus, which has so far claimed at least 18,000 lives worldwide, remains a threat to some vulnerable people, notably pregnant women, young children and those with respiratory problems, and such groups would continue to need vaccinations.