Exhaled aerosol transmission requires a donor threshold viral load.

<p>Two pairs (designated A and B) of donor ferrets (designated donor1 and donor2 of pair A, donor3 and donor4 of pair B) were infected i.n. and i.t. with a total dose of 10⁶ FFU of Cal/04. Three recipients (R1, R2, R3 exposed to pair A; R4, R5, R6 exposed to pair B) were exposed individually to one of the donor pairs for 3 h approximately 24 h after donor infection. Exposures to donor ferrets Donor1 and Donor2 with low viral loads in nasal wash (green and red, left graph) did not result in transmission indicated by negative cultures (yellow squares) in each of the 3 recipients, whereas exposure to ferrets with higher titers of nasal wash virus (red bar) resulted in transmission and positive nasal wash cultures (yellow bars, right graph). Viral RNA (‘PCR’) is expressed as genome equivalents/mL nasal wash; Viral culture (‘virus’) is expressed as FFU/mL nasal wash.