NIAID-Funded Scientists Tried to Make Influenza Spread—They Couldn’t Despite 80+ Hours of Close Human Exposure: Journal 'PLOS Pathogens'
Researchers engineered real-world transmission conditions using infected volunteers and sealed indoor exposure sessions, but no secondary infections were detected.
A new peer-reviewed study reports that researchers were unable to produce a single confirmed case of influenza transmission in a controlled human exposure experiment, despite prolonged close contact between infected individuals and healthy volunteers.
The study, published last month in PLOS Pathogens, tested whether naturally infected people could transmit influenza to others under tightly controlled indoor conditions designed to facilitate spread.
Transmission did not occur.
“[N]o transmission was observed in this study,” the authors confirm.
Researchers recruited individuals with confirmed influenza infection and placed them in repeated exposure sessions with healthy participants inside a sealed hotel-based quarantine environment.
Volunteers interacted face-to-face, shared objects, and spent hours together in a room with intentionally low ventilation—conditions chosen to support transmission.
Even under these circumstances, no recipient developed influenza-like illness, tested PCR-positive, or showed serological evidence of infection.
The study was funded primarily by an NIAID U19 cooperative agreement, along with support from the University of Maryland Baltimore Institute for Clinical & Translational Research, the MPowering the State partnership, and private gifts from The Flu Lab and the Balvi Philanthropic Fund.
The authors were affiliated with institutions including the University of Maryland (School of Public Health, School of Medicine, and engineering programs), Icahn School of Medicine at Mount Sinai, University of Hong Kong, University of Michigan, Medical University of Vienna, and related research centers.
Follow us on Instagram @realjonfleetwood & Twitter/X @JonMFleetwood.
If you value this reporting, consider upgrading to a paid subscription.
For advertising & sponsorship opportunities, contact us by clicking below.
Controlled Exposure—Zero Secondary Infections
The trial included:
5 individuals with influenza A infection, said to be confirmed through PCR testing
11 healthy recipients
23 exposure events
more than 80 cumulative hours of close contact
shared surfaces and prolonged conversation
low ventilation conditions
Despite these factors, investigators reported no secondary infections among exposed participants.
The outcome surprised researchers because the experiment was specifically designed to observe transmission from infected individuals rather than laboratory-adapted viruses used in earlier studies.
The authors’ summary reads:
“Human-to-human influenza virus transmission under controlled conditions could provide insights leading to better control of epidemics and pandemics. However, a previous study using laboratory adapted viruses produced minimal transmission. Therefore, we aimed to study transmission from people naturally infected with circulating viruses. We recruited four cohorts of healthy volunteer Recipients to stay in a quarantine hotel for two weeks. We could not recruit Donors for the first two cohorts. In the last two cohorts, one Donor exposed eight Recipients in the first and four Donors exposed three Recipients in the second. The Donors coughed infrequently and shed less virus into the air than we had observed during previous influenza seasons. No Recipients became infected. Possible explanations include that people infected during mild influenza seasons or who cough very little may be minimally contagious.”
Per the abstract:
“A previous controlled human influenza transmission trial produced minimal transmission using nasal inoculation of an egg adapted virus. Therefore, we implemented a new trial with naturally infected Donors. We recruited healthy Recipients for four, two-week hotel quarantine cohorts and naturally infected, qRT-PCR confirmed Donors for two cohorts. Five Donors (mean age: 21; 80% female; two H1N1, three H3N2, one for cohort 24b and 4 for 24c, Jan-Feb 2024) exposed Recipients (mean age: 36; 54% female, eight in cohort 24b and 3 in 24c) in a hotel room with limited ventilation but a high air recirculation rate. We collected exhaled breath, ambient and personal bioaerosols, fomite swabs, and sera, and analyzed samples using dPCR and fluorescent focus assays, hemagglutination inhibition (HAI) assay, and enzyme-linked immunosorbent assay (ELISA). 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/30-min) in Donors’ exhaled fine aerosols. One of 23 surface swab samples was culture positive. At admission, 8 of 11 Recipients had HAI titers ≤10 but 9 of 11 had stronger binding antibody responses than Donors against vaccine strains corresponding to Donor viruses. No Recipient developed influenza-like illness, PCR-positive respiratory samples, or serological evidence of infection.
Authors Theorize Why No One Got Infected
The researchers offer reasons they believe the purported virus may not have spread.
Their “potential explanations” include: infected people weren’t coughing much, many participants likely already had some immune protection, and airflow in the room quickly diluted the breath they exhaled, reducing exposure.
The study reads:
Potential explanations and insights regarding lack of transmission include importance of cough and seasonal variation in viral aerosol shedding by Donors, of potential cross-reactive immunity in middle-aged Recipients with decades of exposure, and of exposure to concentrated exhaled breath plumes limited by rapid air mixing from environmental controls that distributed aerosols evenly. Future trials over multiple seasons, Donors that cough, younger recipients, and environments that preserve normal exhaled breath plumes will be required to observe transmission from naturally infected Donors under controlled conditions and generate new insights into influenza transmission dynamics.”
Not the First Time Scientists Have Struggled to Produce Infection
The findings echo challenges reported in controlled COVID-19 human challenge studies.
In several SARS-CoV-2 reinfection trials recorded in a May 2024 Nature publication, researchers deliberately exposed volunteers—including some given escalating viral doses—yet failed to reliably produce sustained infections in participants with prior exposure or immunity.
Together, the influenza and COVID experiments highlight a recurring experimental reality: exposure alone does not consistently produce infection, even under prolonged controlled conditions.
The new findings also come amid growing evidence that pre-existing immune responses to purported respiratory viruses may be more widespread than assumed—including a recent Immunity study reporting detectable H5-reactive antibodies in most individuals with no known exposure to H5N1.
Bottom Line
In a controlled human exposure experiment specifically designed to observe influenza transmission, scientists were unable to produce a single confirmed infection among exposed volunteers.
The findings demonstrate that transmission in controlled environments can be far more difficult to produce than often assumed—even for influenza—and that host immunity, symptom severity, and environmental conditions may be decisive factors.
Follow us on Instagram @realjonfleetwood & Twitter/X @JonMFleetwood.
If you value this reporting, consider upgrading to a paid subscription.
For advertising & sponsorship opportunities, contact us by clicking below.
That PCR testing was used nullifies it for me. Exactly how did the original five "infected" people come to be infected? This whole malarkey experiment just assumes there are viruses and there is transmission from the get go. Grade F---
What a waste of taxpayer funds, once again. Now we can assume that conditions have to be "just right" for transmission to occur? That means the whole shebang is a bunch of crap.
The inmates are running the asylum!