How Common is Flu Without Fever?

Fever is thought of as the most common presenting symptom of flu in hospital emergency departments, although it’s known that adults may have atypical presentations. But according to a study by researchers from University of New South Wales in Australia, The Beijing Centre for Disease Prevention and Control in China, The National Institute of Hygiene and Epidemiology in Vietnam, and Arizona State University, > 75% of adults with a viral respiratory infection (including influenza) do not have fever.

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The researchers analyzed data from 158 health care workers who had laboratory-confirmed viral infections. The viruses included rhinovirus, respiratory syncytial virus, parainfluenza virus, influenza virus, human metapneumovirus, coronavirus, and adenovirus. In 15 cases, > 1 virus was isolated, including 9 cases with influenza co-infection.

Twenty-five percent of 75 patients with rhinovirus had fever. Of 10 patients with influenza A, 3 had fever. Co-infection with > 1 virus was the strongest predictor of fever. When cases of influenza and a co-infection were included, 8 of 22 patients had fever (36%).

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The accepted definition of “influenza-like illness” includes fever as a symptom, the researchers note. But when it comes to treating and reducing the risk of transmission, it helps to be aware that a diagnosis of viral respiratory infection is possible in the absence of fever. That awareness could be especially important for infections that are transmissible when patients are asymptomatic or presymptomatic. The main clinical implication, the researchers say, is that the case definition for influenza-like illness in adults may be “highly insensitive.”