Evaluating Modes of Influenza Transmission (EMIT-2): Challenges of a Controlled Human Influenza Virus Infection Transmission Trial (CHIVITT).

Background: The relative importance of influenza transmission via inhalation (airborne), direct deposition (spray), and contact (touch) remains poorly understood. We implemented a controlled human influenza virus infection transmission trial to study transmission pathways.

Methods: We established a quarantine hotel with individual rooms for participants and recruited healthy Recipients for two-week cohort stays. We recruited Donors with community-acquired influenza virus infection, confirmed by qRT-PCR. Donors and Recipients interacted in a room with limited ventilation and controlled humidity. We collected ambient and personal bioaerosol, Donor exhaled breath, and fomite surface swab samples. We analyzed aerosol and surface swab samples using dPCR and florescent focus assays, and sera by hemagglutination inhibition (HAI) assay and enzyme-linked immunosorbent assay.

Results: We enrolled four Recipient cohorts (one Feb 2023 and three Jan-Feb 2024) and recruited Donors for two 2024 cohorts. We exposed 11 Recipients (mean age: 36; 55% female) to 5 Donors (mean age: 21; 80% female; two H1N1, three H3N2) for 82 Donor-hours. Among 16 breath samples from Donors, 7 G-II fine (<5µm), one G-II coarse (≥5µm), and 2 of 24 NIOSH ambient air (one ≥4µm and one 1-4µm) tested PCR positive. We cultured influenza virus from 1 G-II fine aerosol and 1 of 23 surface swab samples. No Recipient developed influenza-like illness or PCR-positive swabs or saliva. Admission sera for 8 of 11 Recipients had HAI titers ≤10, and 9 of 11 had stronger binding antibody responses than respective Donors, against vaccine strains corresponding to Donor viruses.

Conclusions: We successfully recruited community-acquired influenza Donors and exposed healthy Recipients under controlled conditions. However, the unpredictable timing of influenza seasons, a small eligible Recipient pool, pre-existing immunity, and limited virus shedding by Donors posed significant challenges. Addressing these factors will be critical for optimizing future study designs and improving our understanding of influenza transmission dynamics.

Author Summary: We designed a controlled intervention trial to identify whether influenza virus spreads between people mainly by breathing contaminated air (airborne/inhalation transmission), spray (direct deposition) of large semi-ballistic droplets at close range, or touch (contact). Understanding how influenza spreads can help improve strategies for prevention, vaccine development, and public health policies. We recruited four cohorts of healthy volunteer Recipients to stay in a quarantine hotel for two weeks. We asked them to interact in a poorly ventilated room with Donors who had recently become infected with influenza virus during their normal activities. We successfully recruited a total of five Donors to interact with 11 Recipients over the course of two quarantine cohorts. We detected small quantities influenza virus in Donor exhaled breath, in room air, and on surfaces, and none of the Recipients became infected. Unpredictability of the influenza season, a small pool of eligible Recipients, Recipient immunity, and Donors more than 24-hour post symptom onset with low rates of shedding presented significant challenges. These results suggest that new designs capable of bringing together larger numbers of Donors, early in the course of infection, with susceptible Recipients will be required to increase the odds of observing transmission.