Majority of rhinovirus infections are asymptomatic

By Laura Cowen

Asymptomatic human rhinovirus infections outnumber symptomatic infections by a factor of four to one, Canadian researchers report.

The virus was detected in an estimated 60% of asymptomatic university students during an 8-week study period, reported Andrea Granados (McMaster University, Hamilton, Ontario) and colleagues at the 112th General Meeting of the American Society for Microbiology.

A total of 545 undergraduate students participated in the study, during September/October, when rhinovirus activity usually peaks. Half were enrolled during 2010 and half during 2011.

The students were asked to collect weekly nasal swabs for 8 weeks and provide details of symptoms of upper respiratory tract infections.

A total of 167 cold-like illnesses were reported over the 2 years, and rhinovirus was detected in 54.1% of these symptomatic cases.

To assess the level of asymptomatic infection, the researchers screened a random sample of 25 swabs from asymptomatic participants for rhinovirus RNA each week (n=400 in total).

They detected 35 infections in these samples, giving an incidence rate of 8.8% per week. This compares with a weekly incidence of 2.1% for symptomatic infections. Using these figures the researchers estimated that 60.5% of the asymptomatic student population was infected with rhinovirus over the 8-week study period.

Of note, the mean viral load was significantly lower in individuals with asymptomatic infections than in those with symptomatic infections, at 5.30 versus 6.43 log₁₀ copies/mL.

"Decreased amounts of the virus may be responsible for the lack of symptoms; however, larger studies are necessary to confirm this finding," Granados remarked in an associated press statement.

The researchers also found that 11 (31.4%) of the 35 asymptomatically infected individuals were shedding the virus for at least 8 days, while three (8.6%) shed virus for at least 15 days.

"A high occurrence of asymptomatic infections indicates that university students can spread infections to classmates, or individuals in the community without knowing they are infected," said Granados.

She concluded: "These findings in students can be extended to include other populations and is a convenient setting for evaluating novel antivirals."

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