A new study says that mild H1N1 infection may go undetected using standard diagnostic criteria and concludes that coughing or other respiratory symptoms are more accurate in determining influenza infection than presence of a fever. 

A team led by Sang Won Park, MD, professor at the Seoul National University, investigated confirmed cases of H1N1 who were hospitalized and quarantined during the early stages of the pandemic in 2009. The study's results showed only 45.5 percent of the case subjects had fever. Individuals with mild infection and no fever have the potential to evade detection at airports or medical triage units, thus continuing the chain of infection.

Currently, public health officials rely on body temperature (detecting fever) to screen individuals for potential infection with H1N1. For example, during a pandemic, standard screening at airports relies on body temperature scanners to detect the presence of fever. However, the new study's authors found that coughing, not fever, is a more reliable indicator of infection because nearly half of the individuals with mild infection may not have fever.

"Our study found that fever is not reliable for case definition, even though it has been regarded as a key factor in determining influenza infection," said Dr. Park. "We are aware of other studies that show fever present in as few as 31 percent of confirmed cases of influenza. We found that the most sensitive indicator was cough."

Dr. Park adds that that "screening should take any kind of respiratory manifestation into account."

To be published in the August issue of the American Journal of Infection Control.