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Three-quarters of people with flu have no symptoms

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“‘77% of flu infections’ have no symptoms, say experts,” reports ITV News.

The news is based on a large community-based study carried out in England, which found that most people with influenza (“flu”) don’t have symptoms, and even if they do, only a small proportion go to a doctor.

The study was part of Flu Watch – a larger, ongoing study to assess the impact of flu on public health in England – and analysed five groups of people over six periods of influenza transmission, between 2006 and 2011.

Participants provided blood samples before and after the influenza season, so that the amount of antibodies in the blood could be measured. They were then contacted every week so that cough, cold, sore throat, or any “flu-like illness” could be noted down. If any of these were experienced, participants were asked to complete a symptom diary and to take a nasal swab to test for the influenza virus.

Approximately 20% of people had an increase in antibodies against influenza in their blood after an influenza “season”. However, around three-quarters of infections were symptom-free, or so mild that they weren’t identified through weekly questioning.

This is very much a “good news, bad news” story. It is good news in that so many people with a flu infection are spared the burden of a nasty infection. However, limiting the spread of a future pandemic could be challenging, as it would be unclear who is infected.

This reinforces the importance of practising essential hygiene habits to stop flu spreading, such as frequent hand washing and cleaning surfaces so that they are free of germs – especially if there is an ongoing flu pandemic.

Where did the story come from?

The study was carried out by researchers from the Flu Watch Group from University College London, London School of Hygiene and Tropical Medicine, Public Health England, University of Oxford, University of Nottingham and Imperial College London. It was funded by the Medical Research Council and the Wellcome Trust.

The study was published in the peer-reviewed medical journal The Lancet Respiratory Medicine.

Coverage by ITV News was accurate, if a little brief.

What kind of research was this?

This was a cohort study that aimed to measure the:

  • Proportion of the UK population infected with influenza each season.
  • Proportion of those infected who developed symptoms attributable to influenza.
  • Proportion of those with symptomatic disease who had detectable nasal shedding of
    the influenza virus.
  • Proportion of people who went to see a doctor.

Researchers also looked at the symptoms of people with confirmed influenza.

During the study, there was an influenza pandemic: the 2009 “swine flu” pandemic (influenza A H1N1). The researchers also measured the development of immunity to this pandemic strain.

This was the ideal study to address this issue.

What did the research involve?

The researchers analysed five groups of people, aged five years or older, across England between 2006 and 2011. Each year, a random sample of people registered at general practices across England was selected, and their households were recruited.

Participants were followed over the 2006-2007, 2007-2008 and 2008-2009 periods of seasonal influenza circulation, and the first (spring/summer 2009), second (autumn/ winter 2009) and third (winter 2010-2011) waves of the pandemic.

There were around 300 households and their members recruited for the November-March flu season each year.

However, during the 2009 swine flu pandemic, slightly more were recruited, with around 300 at the start of the pandemic (May-September 2009), and then almost 1500 households covering the normal flu season period of that year (October 2009-February 2010).

Participants provided blood samples before and after the influenza season so that the levels of antibodies in the blood could be measured. They were contacted every week to see if they reported symptoms such as cough, cold, sore throat, or a “flu-like illness”.

If any of these were experienced, participants were asked to complete a symptom diary and to take a nasal swab to test for influenza virus.

The researchers also reviewed GP’s records, to see if people went to the doctor.

What were the basic results?

The overall follow-up time was 5448 person-seasons (e.g. just over 1000 people, followed up in 5 flu seasons). On average, based on levels of antibodies in the blood, influenza infected 18% of unvaccinated people each winter.

Approximately three-quarters of infections were either symptom-free, or so mild they were not identified through the weekly surveillance of illness:

  • Of those infected (based on the levels of antibodies in the blood), the rate of respiratory illness attributable to influenza was 23 respiratory illnesses, including 18 influenza-like illnesses per 100 person-seasons (e.g. in one flu season, 18 influenza-like illnesses per 100 people).
  • A quarter of people with flu antibodies in their blood also had influenza confirmed from nasal swabs.

Most people with influenza confirmed from nasal swabs did not consult a doctor. Among those who did, influenza or influenza-like illnesses were rarely recorded in medical notes:

  • Only 17% of those with confirmed influenza and 21% of those with influenza-like illnesses consulted their family doctor.
  • Only 8% of people consulting their doctor had influenza or influenza-like illnesses recorded in their medical records.

How did the researchers interpret the results?

The researchers concluded that “seasonal influenza and the 2009 pandemic strain were characterised by similar high rates of mainly asymptomatic infection, with most symptomatic cases self-managing, without medical consultation”.

Conclusion

This large, community-based study found that most people with influenza in England don’t have symptoms, and even if they do, only a small proportion go to a doctor.

Approximately 20% of people had an increase in antibodies against influenza in their blood after an influenza “season”. However, about three-quarters of infections were symptom-free, or so mild they weren’t identified through weekly questioning about whether participants had a cough, cold, sore throat, or a “flu-like illness”.

People who reported being ill were asked to take a nasal swab to test for the influenza virus. Among those with illnesses and with confirmed influenza, only 17% went to see their doctor; among those that did, influenza or influenza-like illnesses were rarely recorded in medical notes.

This information is important, as it indicates that current surveillance systems that rely on people visiting their doctor underestimate the extent of infection and illness in the community. This, somewhat counterintuitively, can lead to overestimates of the severity of the disease (only people with the most severe symptoms are identified as being infected).

There is also the worry that people unaware they are infected may pass it on to people more vulnerable to infection, such as those with weakened immune systems.

Find out if you are in a “vulnerable group”. If you are, get the annual flu jab.

Analysis by Bazian. Edited by NHS Choices.  Follow Behind the Headlines on Twitter.  Join the Healthy Evidence forum.

 

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