They said their findings raised the possibility that current influenza infection control measures may not be enough to protect healthcare workers and patients during routine care in hospitals.
“A significant proportion of individuals appear to shed influenza virus without harbouring any symptoms”
The researchers tracked almost 700 staff and inpatients over two flu seasons at the University Hospital in Zurich, uncovering several transmission clusters that were undetected by routine surveillance.
They noted that their results were consistent with previous research suggesting that flu may be spread to others by just breathing, and that coughing or sneezing were not required for transmission.
Knowing whether people are infectious in the absence of symptoms was a major concern for infection control in hospitals, warned the study authors.
While hospital acquired infection from asymptomatic individuals may occur, no previous prospective studies have investigated the transmission of flu in the absence of symptoms in acute care.
To provide more evidence, the researchers studied flu virus transmission trajectories in 542 patients on medical wards and 152 acute care staff on the same wards during the 2015-16 and 2016-17 flu seasons.
The team tracked flu infection through nasal swabs collected daily and performed diagnostic multiplex real-time PCR and RNA sequencing on specimens.
Contacts between participants were traced, and participants were asked to completed daily diaries of any illnesses.
“Influenza vaccination is not perfect but remains the best tool we have”
During the study, 11% healthcare workers and 4% inpatients tested were diagnosed with an influenza infection.
Most of these participants experienced influenza symptoms, particularly respiratory symptoms, when their tests were positive.
However, several of them remained asymptomatic despite testing positive for influenza infection – 13% of healthcare workers and 11% of inpatients.
Importantly, the researchers said 17% of influenza-positive swabs from healthcare workers and 8% from patients were collected on days that they did not report flu symptoms.
In addition, among symptomatic individuals, 14% of healthcare workers had a positive flu test before symptoms developed.
Further analysis revealed at least seven clusters of potential transmission events among healthcare workers, among inpatients, or between healthcare workers and inpatients.
Lead author Dr Stefan Kuster said: “Our findings suggest that influenza infection in acute care is common and a significant proportion of individuals appear to shed influenza virus without harbouring any symptoms, making the spread of flu very difficult to control even with self-diagnoses and current infection control practices.”
He added: “Influenza vaccination is not perfect but remains the best tool we have to protect healthcare workers and their patients from severe illness.”
But the authors highlighted that more research was needed before it could be firmly established whether those with no symptoms may be contributing to the spread of the virus without realising.
The findings were presented at this year’s European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) in Amsterdam in the Netherlands, which is on during 13-16 April.
- See attached PDFs below for study abstract and poster