The assumption that individuals infected with influenza viruses automatically transmit the infection to others by sharing the same indoor space appears to be flawed. A new US study showed that influenza is not inevitably transmitted, even during close contact between infected and uninfected individuals.

Instead, viral release through coughing, air movement, and individual immune responses plays a decisive role in transmission. These findings have recently been published in PLOS Pathogens.

Real-World Experiment

Researchers at the University of Maryland, College Park, Maryland, have conducted unusual real-world trials. This study was conducted in two periods, between January and February 2024.

Healthy individuals from the four 2-week hotel quarantine cohorts and naturally infected individuals were identified as influenza virus donors for the two cohorts using quantitative reverse transcription polymerase chain reaction. Five donors (mean age, 21 years; 80% female; two H1N1, three H3N2, one for cohort 24b and four for 24c) who were exposed (mean age, 36 years; 54% female; eight in cohort 24b and three in 24c) were recruited from a hotel room with limited ventilation and a high air recirculation rate.

The participants lived together under household-like conditions. They ate meals together and engaged in activities such as yoga, conversation, and sharing objects. The fresh air supply was deliberately restricted, whereas fans and space heaters ensured constant air circulation.

During their 2-week stay, participants underwent daily assessments. Researchers collected exhaled breath, ambient and personal bioaerosols, fomite swabs, and sera and analyzed samples using digital polymerase chain reaction, hemagglutination inhibition assays, enzyme-linked immunosorbent assay, and fluorescence-based culture assays.

Despite prolonged close interactions, none of the healthy participants were infected. Infected donors exhibited high viral loads in the upper respiratory tract but coughed infrequently.

Compared with previously studied community-acquired influenza cases, we detected viral RNA (44%) and culturable virus (6%) less frequently and measured fewer viral RNA copies (79-8.9 × 10³ copies per 30 minutes) in donors’ exhaled fine aerosols. One of the 23 surface swab samples tested positive in the culture assay. Constant air movement is an additional protective factor against respiratory infections. Fans and heaters disperse and dilute viral particles within an enclosed space, thereby reducing exposure intensity.

Age-related immune differences may have also contributed to these findings.

At admission, 8 of the 11 recipients had hemagglutination inhibition titers ≤ 10, but 9 of the 11 had stronger binding antibody responses than the donors against vaccine strains corresponding to the donor viruses. Potential explanations and insights regarding the lack of transmission include the importance of cough and seasonal variation in viral aerosol shedding by donors, potential cross-reactive immunity in middle-aged recipients with decades of exposure, and exposure to concentrated exhaled breath plumes limited by rapid air mixing from environmental controls that evenly distribute aerosols.

The authors concluded that the greatest transmission risk occurred during direct person-to-person contact in poorly ventilated environments. Portable air cleaners may help reduce this risk; however, when coughing occurs at a close range, wearing an N95 respirator is the most effective protective measure.

These findings also shed new light on the current influenza season, which is characterized by high case numbers and the emergence of a new subtype, the subclade K. These results suggest that influenza transmission is more complex than previously assumed and cannot be explained by proximity alone.

Further trials over multiple seasons, in donors who cough, in younger recipients, and in environments that preserve normal exhaled breath plumes are required to observe transmission from naturally infected donors under controlled conditions and generate new insights into influenza transmission dynamics.

This story was translated from Univadis Germany, part of the Medscape Professional Network.