Figure 4. Airborne attack rate of Alpha and Delta SARS-CoV-2 variants.

Donor animals (N=7) were inoculated with either the Alpha or Delta variant with 10³ TCID₅₀ via the intranasal route and paired together randomly (1:1 ratio) in 7 attack rate scenarios (A–G). To each pair of donors, 1 day after inoculation, 4–5 sentinels were exposed for a duration of 4 hr (i.e. h 24–28 post inoculation) in an aerosol transmission set-up at 200 cm distance. (A) Schematic figure of the transmission set-up. (B). Day 1 sgRNA detected in oral swabs taken from each donor after exposure ended. Individuals are depicted. Wilcoxon test, N=7. Grey = Alpha, teal = Delta inoculated donors. (C) Respiratory shedding measured by viral load in oropharyngeal swabs; measured by sgRNA on days 2, 3, and 5 for each sentinel. Animals are grouped by scenario. Colors refer to legend below. 3.3=limit of detection of RNA (<10 copies/rxn). (D) Schematic representation of majority variant for each sentinel as assessed by percentage of Alpha and Delta detected in oropharyngeal swabs taken at day 2 and day 5 post exposure by deep sequencing. Grey = Alpha, teal = Delta, white = no transmission.

Figure 4—figure supplement 1. Airborne competitiveness of Alpha and Delta SARS-CoV-2 variants.

(A) Cumulative pathology score of sentinels on day 5 post exposure. Bar-chart depicting median, 96% CI, and individuals, Mann-Whitney test. (B) Lung histologic lesion scores 0, 1, and 2. Score 0, normal lung devoid of immunoreactivity. Score 1, a solitary focus of inflammation (circled) surrounded by normal lung. Bronchiole (*) and alveolar (>) immunoreactivity. Score 2, multiple foci of coalescing inflammation centered on airways. HE (top row) and anti-SARS-CoV-2 IHC (bottom row). (C, D) Viral load measured via infectious virus titer in swabs and lungs of sentinels on day 5. Bar-chart depicting median, 96% CI, and individuals, Mann-Whitney test. Grey = Alpha, teal = Delta, p-values indicated where significant. Abbreviations: TCID, Tissue Culture Infectious Dose.