Flu Can Infect Many Without Causing Symptoms: Study

[tetano]

SUNDAY, March 16, 2014 (HealthDay News) -- Think you know who has the flu? Think again: a new study finds that three-quarters of people infected with seasonal flu and swine flu in recent years showed no symptoms.

Researchers analyzed data gathered in England during the winter flu seasons between 2006 and 2011, including the 2009 H1N1 "swine flu" pandemic.

Overall, about 18 percent of unvaccinated people became infected with an influenza virus, but only 23 percent of them went on to develop flu symptoms, the researchers reported March 16 in The Lancet Respiratory Medicine.

What's more, only about 17 percent of infected people became ill enough to see a doctor, the British study found. And compared with some of the seasonal flu strains, the 2009 swine flu strain actually caused much milder symptoms.

The findings suggest that relying on data about flu-related visits to primary care doctors underestimates the extent of flu infections and illnesses, the researchers said.

Overall, the infection rate for the winter flu seasons as calculated in the study were an average of 22 times higher than the rates recorded by standard methods.

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[tetano]

Re: Flu Can Infect Many Without Causing Symptoms: Study

The original article

[Gert van der Hoek]

Three quarters of people with seasonal and pandemic flu have no symptoms

This finding seems consistent with the "rule of thumb" regarding infectious diseases: the real number of patients is 5 to 10 times the number of reported patients.

Three quarters of people with seasonal and pandemic flu have no symptoms

17 March 2014

Around 1 in 5 of the population were infected in both recent outbreaks of seasonal flu and the 2009 H1N1 influenza pandemic, but just 23% of these infections caused symptoms, and only 17% of people were ill enough to consult their doctor, according to new UCL-led research.

These findings come from a major new community-based study comparing the burden and severity of seasonal and pandemic influenza in England over 5 years, published in The Lancet Respiratory Medicine journal.

“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”, explains lead author Dr Andrew Hayward (UCL Infection & Population Health).

“Most people don’t 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 community 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 Watch study tracked five successive cohorts of households across England over six influenza seasons between 2006 and 2011. The researchers calculated nationally representative estimates of the incidence of influenza infection, the proportion of infections that were symptomatic, and the proportion of symptomatic infections that led to medical attention.

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“Surveillance of medically attended illnesses provides a partial and biased picture, and is vulnerable to changes in consulting, testing, or reporting practices. As such, it is clear that reliable estimates of the infection and clinical attack rates during the early stages of an influenza epidemic requires the collection of standardised data across the whole range of disease severity, from the community, primary care, and secondary care.”

More: UCL News

Research paper in The Lancet Respiratory Medicine

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