Influenza infection is very common – about one in five of us are infected each year. But, surprisingly, the majority of infections don’t cause any illness. In a study published in The Lancet Respiratory Medicine, we found that in two recent outbreaks of seasonal flu and the H1N1 influenza pandemic in 2009, about three quarters of people who were infected had no symptoms. And only 17% of people who were unwell enough to visit their doctor.

Given that surveillance usually measures flu cases that are picked up when people go to their doctors (and people are generally discouraged from visiting surgeries if they are otherwise fit and healthy) it is likely that we are not only underestimating the number of overall cases in the community but overestimating the proportion of cases who end up in hospital or die.

When we think of flu, we usually think of a severe illness that will put you out of action for several days. While flu can be severe and very occasionally fatal, most influenza illnesses are not like this. Often they appear more like a common cold than what we typically think of as flu. This is true not only for the usual strains of flu that circulate each year but also during the pandemic.

We wanted to get as accurate a picture as we could and with the support of the Medical Research Council and Wellcome Trust we set up Flu Watch, one of the biggest and most detailed studies of community influenza to date. We recruited people of all ages from across England, tracking five successive cohorts of households over six flu seasons from 2006 to 2011. Before and after each flu season we took blood samples from participants to see if they developed antibodies to fight influenza infection.

We also contacted every household each week over the winter to see if anyone had been ill – noting any cases of cold, coughs, sore throats or “flu-like illnesses”. And those who were ill were asked to keep detailed symptom diaries and take a swab from their nose. These were then sent to the lab so we could see whether they were shedding influenza virus from their nose.

From these tests we found that an average of 18% of people who were not vaccinated against flu were infected each winter season, which was about the same amount during the 2009 pandemic. And 77% of the time, influenza infection did not cause symptoms. Our findings suggest that the rate of influenza across all winter seasons was on average 22 times higher than that reported by the Royal College of GPs. It suggests that reported cases of influenza have until now represented just the tip of the iceberg.

From an evolutionary perspective, a successful infection is one that spreads easily. If it makes us so ill that we can’t get out of bed we are less likely to pass it on to others than if we are out and about feeling generally well and perhaps coughing and sneezing on our friends and colleagues. Our immune system has also evolved to be very sophisticated so that it can usually fight flu very successfully. Along with rates of infection, this study will help us understand the immune response much better so that we can work out why some people get ill and some do not.

Although the 2009 pandemic caused much international concern and some deaths in otherwise healthy adults, it was relatively mild. We were lucky that the strain was not more severe. Flu pandemics are very unpredictable and we need to be prepared for the worst. The 1918 strain, for example, killed more people than World War I.

We need more research to find out why most people have mild illness while for some it can be catastrophic. And there is huge value in the samples and information provided by our participants that we will be able to use in the future, for example to study the immune response to infection.