Over half of UK swine flu hospital admissions and inpatient deaths occurred in people with no underlying health problems or obvious risk factors, reveals research published in Thorax today.
The data back up the government's policy of prioritising pregnant women, the under 5s, and those with long term respiratory problems for vaccination against swine flu.
But they also suggest that everyone with asthma might benefit from vaccination, not just those with severe disease, and they question whether a high fever helps to decide who has swine flu infection.
The findings are based on an analysis of clinical data from 55 hospitals in 20 urban areas during the first wave of the swine flu pandemic during May to September 2009. The data were collected as part of the government's Influenza Clinical Information Network (FLU CIN) surveillance programme.
Between April 27 and September 30 2009, data were collected on 631 people with swine flu - 405 of them adults - admitted to the 55 hospitals.
Their ages ranged from 3 months to 90 years. One in three (36%) were under 16 and one in 20 (5%) were aged 65 and older. Those from black and minority ethnic backgrounds made up over 60% of admissions.
Twenty seven patients were pregnant, representing 4% of admissions and 18% of women aged between 16 and 44. This indicates that pregnant women are around three times as likely to require hospital admission - once infected with swine flu - as women who were not pregnant, say the authors.
On average, two days elapsed between the start of symptoms and hospital admission. The most common presenting symptoms were fever and cough in both adults and children.
But around one in four adults and children did not have a fever on admission, and over half did not have a high fever, prompting the authors to question the wisdom of using a high fever as a key symptom of swine flu infection.
Around one in eight (13%) were admitted to intensive care or a high dependency unit. One in 20 (5%) died.
Risk factors for death included an abnormal chest x ray or raised levels of a protein indicative of inflammation (CRP), especially in those who were obese or with any underlying serious respiratory condition.
Just under half of the patients had underlying conditions - mostly asthma. But almost half of those with asthma were not routinely using steroid inhalers or taking oral steroids, which suggests they had mild, rather than severe, asthma.
And over half of all admissions (55%) and inpatient deaths (59%) occurred in people with no previous health problems.