Lack of human data on influenza virus aerosol shedding fuels debate over the importance of airborne transmission.
We show that sneezing is rare and not important for—and that coughing is not required for—influenza virus aerosolization.
We screened 355 symptomatic volunteers with acute respiratory illness and report 142 cases with confirmed influenza infection who provided 218 paired nasopharyngeal (NP) and 30-minute breath samples (coarse >5-μm and fine ≤5-μm fractions) on days 1–3 after symptom onset.
Fine- and coarse-aerosol viral RNA were positively associated with body mass index and number of coughs and negatively associated with increasing days since symptom onset in adjusted models. Fine-aerosol viral RNA was also positively associated with having influenza vaccination for both the current and prior season.
Sneezing was rare, and sneezing and coughing were not necessary for infectious aerosol generation.
Influenza virus RNA was detected in 76% of the fine-aerosol samples, 40% of the coarse-aerosol samples, and 97% of the NP swabs of enrolled volunteers.
The few sneezes observed were not associated with greater RNA copy numbers in either coarse or fine aerosols.
The day after symptom onset… was associated with a significant decline in viral RNA shed into fine aerosols…
[M]ales produced, on average, 3.2 times more virus than did females per cough. However, females also coughed significantly more frequently than males…
Self-reported vaccination for the current season was associated with a trend toward higher viral shedding in fine-aerosol samples; vaccination with both the current and previous year’s seasonal vaccines, however, was significantly associated with greater fine-aerosol shedding…
[W]e observed 6.3 times more aerosol shedding among cases with vaccination in the current and previous season compared with having no vaccination in those two seasons.
The association of vaccination and shedding was significant for influenza A but not for influenza B…
[A] significant fraction of influenza cases routinely shed infectious virus, not merely detectable RNA, into aerosol particles small enough to remain suspended in air and present a risk for airborne transmission.
The association of current and prior year vaccination with increased shedding of influenza A might lead one to speculate that certain types of prior immunity promote lung inflammation, airway closure, and aerosol generation.