It’s been a very bad year for the flu. For the week ending February 3, 2018, the Centers for Disease Control and Prevention estimates that almost 8% of people saw their healthcare providers for an influenza-like illness (ILI), the highest rate of ILI since the 2009 pandemic. Many more can be expected to be felled by the flu until the bug burns itself out around April. Bit not all ILI is caused by the influenza virus. Depending on the population, 60%-80% of ILI is caused by influenza, and the rest by other acute respiratory viruses masquerading as the flu. Does the distinction matter? Medscape turned to influenza expert Andrew T. Pavia, MD, professor of pediatrics at the University of Utah, to clarify the subtleties between influenza and its imitators.
Medscape: What is meant by the term “influenza-like illness”?
Dr Pavia: ILI is a way of tracking the burden of influenza. The term began to be used back in the days when relatively little influenza testing was done. ILI defined a patient with a documented fever and either a sore throat or a cough. These symptoms increase the likelihood that a respiratory illness is due to influenza and not some other virus, although this depends a lot on the time of year and how much influenza is circulating. So ILI is a surrogate—a way of looking for influenza mixed in with other things.
Medscape: So the likelihood of a patient having an ILI instead of influenza depends on the season?
Dr Pavia: Yes; the proportion of patients who come in with ILI depends on the time of year and what’s circulating. Right now, there is so much influenza around that if you meet the definition of ILI, you probably have the flu. However, a fair amount of ILI may be caused by other viruses, like
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