Influenza A (H3) viral aerosol shedding in nasally inoculated and naturally infected cases
Authors: Lai Jianyu , Mesquita P. Jacob Bueno de , Hong Filbert , Ma Tianzhou , Cowling Benjamin J. , Milton Donald K. 
Published: September 2024

Nasally inoculated influenza cases reported milder symptoms and shed lower viral RNA load in exhaled breath aerosols (EBA) than people with classic influenza-like illness including fever, in a previous study. Whether nasally inoculated influenza is representative of mild natural influenza infection, the majority of natural infections, is unknown. Here, we extend our previous analyses to include a broader range of community-acquired influenza cases. Previously, we reported on two groups: (A) volunteers intranasally inoculated with a dose of 5.5 log10TCID50of influenza A/Wisconsin/67/2005 (H3N2) and (B) cases with cough and sore throat plus fever or a positive rapid antigen test recruited on a college campus in the same year (2013). Here we added two additional groups from a later study: (C) cases from a 2017-2019 surveillance cohort of college dormitory residents and their contacts, and (D) cases recruited from a university health center in 2019. All cases had an influenza A(H3) infection. Using a Gesundheit-II sampler, we collected 30-minute EBA samples. Community-acquired cases from the surveillance cohort (C) shed more EBA viral RNA and were more symptomatic than the nasally inoculated cases (A) but shed less viral RNA than the natural cases that were selected for symptoms (B) in 2013, but not (D) recruited in 2019. Despite sharing a similar symptomatic profile with the 2013 selected natural cases (B), the 2019 community-acquired cases (D) recruited post-infection showed a lower fine aerosol viral RNA load. Nasal inoculation of influenza virus did not reproduce EBA viral RNA shedding or symptoms observed in mild natural infection. Circulating strains of influenza A(H3) may differ, year-to-year in the extent to which symptomatic cases shed virus into fine aerosols. New models, including possibly aerosol inoculation, are needed to study viral aerosol shedding from the human respiratory tract.

Author Summary

In this study, we compared influenza A (H3) viral aerosol shedding in the exhaled breath of four different groups of influenza cases: (A) volunteers given the influenza virus intranasally, naturally infected (B) college community members with classic influenza-like illness including fever recruited in 2013, (C) dormitory residents undergoing active surveillance, and (D) patients from a university health center recruited in 2019. We found that mild symptomatic cases among healthy college students (C) released more viral RNA in their exhaled breath than those nasally inoculated with influenza virus (A). We also observed that more symptomatic and medically attended cases from different flu seasons (B and D), although reporting similar symptom severity, shed different levels of viral RNA in their exhaled breath. Our findings indicate that influenza viral aerosol shedding varies from season to season. Most volunteers nasally inoculated at a high virus dose did not shed detectable viral RNA in their exhaled breath or show symptoms, suggesting that nasal inoculation may not accurately mimic natural infection. Our results highlight the need for improved models to study the spread of influenza virus in aerosol forms.