Less than a quarter of patients infected during recent flu outbreaks showed any symptoms, researchers have claimed.
Experts said that almost one in five people were infected in recent outbreaks of seasonal flu and the 2009 H1N1 influenza pandemic. But only 23 per cent of these infections caused symptoms, they said.
It is believed those remaining flu victims who did not show symptoms continued to mix within their community, thereby spreading the infection.
The Flu Watch study published in the Lancet Respiratory Medicine Journal today states that reported cases of flu are “the tip of the iceberg”. Experts say the findings show a more accurate picture of infection rates and could help health services plan the best way to combat outbreaks of the virus.
Lead author Dr Andrew Hayward, of University College London, said: “Reported cases of influenza represent the tip of a large clinical and subclinical iceberg that is mainly invisible to national surveillance systems that only record cases seeking medical attention.
“Most people do not go to the doctor when they have flu. Even when they do consult they are often not recognised as having influenza. Surveillance based on patients who consult greatly underestimates the number of cases, which in turn can lead to overestimates of the proportion of cases who end up in hospital or die. Information on the community burden is therefore critical to inform future control and prevention programmes.”
The flu virus is spread in droplets of fluid coughed or sneezed into the air by an infected person and then breathed in by anyone within range. It can also be spread by touch.
Elderly people and those with certain medical conditions are more likely to develop a serious case of flu. Around 600 people in the UK die from complications stemming from seasonal flu every year, with the figure rising to around 13,000 during an epidemic. Dr Hayward said: “Despite its mild nature, the 2009 pandemic caused enormous international concern, expense, and disruption. We need to prepare for how to respond to both mild and severe pandemics.
“To do this we need more refined assessments of severity, including community studies to guide control measures early in the course of a pandemic and inform a proportionate response.”
The number of flu cases being reported by health services across the country greatly underestimates the extent of infection and illness. The rate of influenza across all winter seasons was on average 22 times higher than rates of disease recorded by the Royal College of General Practitioners Sentinel Influenza-Like Illness Surveillance Scheme.
Dr Peter William Horby, of the Oxford University Clinical Research Unit based in Vietnam, said: “A large number of well individuals mixing widely in the community might, even if only mildly infectious, make a substantial contribution to onward transmission. Surveillance of medically attended illnesses provides a partial and biased picture, and is vulnerable to changes in consulting, testing or reporting practices.”