Stats: 77% of People Who Have the Flu Don't Show Symptoms

Researchers from the United Kingdom have discovered that a majority of individuals who have seasonal and pandemic influenza do not exhibit symptoms, and most patients who do develop symptoms do not seek medical attention.

“The study is important because it tells us flu infection is very common, but usually our immune system tackles it without it causing illness,” said lead author Andrew C. Hayward, MD, UCL Research Department of Infection and Population Health, University College London, London, UK. “If it does cause illness, this is usually mild.”
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He continued by explaining that these findings were consistent with the 2009 H1N1 pandemic “when there was little or no antibody-based immunity. This suggests that other aspects of our immune response can be highly protective.  If we can understand this better, we should be able to develop better vaccines.”

For this analysis, Hayward and colleagues compared the community burden and severity of seasonal influenza and the 2009 H1N1 pandemic, and aimed to ascertain how much traditional surveillance underestimates this burden.

Traditional surveillance only measures cases in which the patient goes to a doctor, and many cases may be missed and not reported if the patient does not have the typical symptoms. “Traditional surveillance therefore misses many mild cases of influenza,” noted Hayward. “This can lead us to overestimate what proportion of cases end up in hospital or die.” Traditional surveillance also cannot account for asymptomatic infection.

“We wanted to find out how much flu there is in the population and how severe it usually is,” he told Consultant360. “We knew that just studying people who seek medical attention would underestimate this, but we didn’t know by how much.”

Researchers analyzed data from the Flu Watch study. Seasonal and pandemic influenza were tracked in 5 cohorts in England during 6 flu seasons from 2006 to 2011. Blood samples were collected from the study participants before and after each influenza season. In addition, they were asked about the presence of cold, cough, sore throat, or flu-like illness each week. If those symptoms were present, patients provided a nasal swab on the second day of illness.

“By looking at participants’ antibody responses to influenza before and after the flu season, we could see if they developed antibodies to fight off infection,” said Hayward. “By contacting participants every week throughout the flu season, we could find out if they had been ill and how severe the illness was.”

Influenza infected an average of 18% of participants who were not vaccinated each winter; of those who were infected, only 23% developed symptoms. Further, only 17% of individuals with symptomatic influenza visited their physician.

The findings were similar for both seasonal and pandemic influenza, although the pandemic strain caused milder symptoms than seasonal influenza, which was 1 of several unexpected findings according to Hayward.

The results also demonstrate that traditional surveillance has significantly underestimated the extent of influenza infections and illness.

Hayward noted that the study has implications for controlling the spread of influenza. “An infection that causes minimal symptoms may spread easily as those affected are still able to be in contact with lots of other people,” he said.

Despite the fact that influenza is very common but often mild does not mean that it isn’t worth preventing, noted Hayward.

“Those at higher risk of serious complications, namely the elderly and those with chronic illness, should still be priority groups for vaccination,” he said. “It is also important for people working in health services to get vaccinated to stop them spreading influenza to vulnerable patients. As it is hard to spot the symptoms of influenza and these symptoms are often mild, unvaccinated health care workers could be infecting people without even suspecting they might have flu.”

In terms of future research, Hayward explained that they are using the specimens and data they collected to understand the influence of T cell–based immunity on the development of symptoms and viral shedding in individuals with influenza.

“This type of immunity is potentially important as it could provide cross protection between different strains of influenza,” he said. “We would also like to do new research to find out if those who are infected but have no symptoms are infectious.”

This study was funded by Medical Research Council and the Wellcome Trust, and was published in The Lancet Respiratory Medicine.

-Meredith Edwards White

Reference

Hayward AC, Fragaszy EB, Bermingham A, et al; on behalf of the Flu Watch Group. Comparative community burden and severity of seasonal and pandemic influenza: results of the Flu Watch cohort study. The Lancet Respiratory Medicine. Early online publication, March 17, 2014. doi:10.1016/S2213-2600(14)70034-7.