Infectious virus in exhaled breath of symptomatic seasonal influenza cases from a college community

Abstract

Little is known about the amount and infectiousness of influenza virus shed into exhaled breath. This contributes to uncertainty about the importance of airborne influenza transmission. 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. We assessed viral RNA copy number for all samples and cultured NP swabs and fine aerosols. We recovered infectious virus from 52 (39%) of the fine aerosols and 150 (89%) of the NP swabs with valid cultures. The geometric mean RNA copy numbers were 3.8 × 10⁴/30-minutes fine-, 1.2 × 10⁴/30-minutes coarse-aerosol sample, and 8.2 × 10⁸ per NP swab. 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. NP swab viral RNA was positively associated with upper respiratory symptoms and negatively associated with age but was not significantly associated with fine- or coarse-aerosol viral RNA or their predictors. Sneezing was rare, and sneezing and coughing were not necessary for infectious aerosol generation. Our observations suggest that influenza infection in the upper and lower airways are compartmentalized and independent.

Keywords: aerosol; airborne infection; influenza virus; vaccination effects; viral shedding.

Fig. 1.

Histograms of symptom scores. (A) Upper respiratory symptoms (runny nose, stuffy nose, sneezing, sore throat, and earache, score range 0–15). (B) Lower respiratory symptoms (chest tightness, shortness of breath, and cough, score range 0–9). (C) Systemic symptoms (malaise, headache, muscle/joint ache, fever/sweats/chills, and swollen lymph nodes, score range 0–15).

Fig. 2.

Viral shedding: (A) infectious influenza virus (fluorescent focus counts) in NP swabs and fine aerosols and (B) RNA copies in NP swabs, coarse, and fine aerosols. (C and D) Scatter plots and Spearman correlation coefficients of infectious virus plotted against RNA copies for (C) NP swabs and for (D) fine-aerosol samples. (E) The effect of day after symptom onset on RNA copies observed in NP swabs, coarse, and fine aerosols plotted as GM adjusted for missing data using Tobit analysis with error bars denoting 95% CIs. (F–H) The effect of cough frequency on RNA copies observed in (F) NP swabs, (G) coarse aerosols, and (H) in fine aerosols. Coarse: aerosol droplets >5 µm; Fine: aerosol droplets ≤5 µm in aerodynamic diameter.