A team led by Professor Peter Openshaw of Imperial College, London, studied data from 631 patients admitted to UK hospitals with swine flu.
Their data showed that 28 per cent of children and 23 per cent of adults did not have a fever on admission, while just half had a fever over 38 degC.
'Such findings question the appropriateness of specifying fever of at least 38 degC as part of the clinical case definition in current diagnostic, treatment and infection control algorithms,' they said.
Professor Openshaw and his colleagues said their findings suggested that patients who had been admitted to hospital with illness compatible with influenza should have a chest X-ray.
They added that patients should also be actively monitored for altered level of consciousness, dyspnoea and low peripheral oxygen saturation.
'An abnormal chest X-ray or raised CRP level, especially in patients who are observed to be obese, have pulmonary conditions other than asthma or COPD or are pregnant, may suggest a potentially serious outcome,' they said.
But the researchers said their findings backed up the government's chosen vaccination scheme. '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 said.
They said their data also suggested that all patients with asthma might benefit from vaccination, rather than only those with severe disease.
The researchers also found that Asian and black people were three to four times more likely than others to be admitted to hospital with swine flu.
'The reasons for this excess are unclear, but could include language barriers affecting consultation behaviour or treatment access, overcrowding, household size and genetic susceptibility,' the researchers suggested.