Figure 1.

Experimental plan. A, Ferrets were infected intranasally with the primary virus and then challenged intranasally with a second virus. Virus shedding was assessed daily in nasal washings (Δ) as a measure of upper respiratory tract infection. B, Six intervals between primary infection and challenge were assessed. Primary infections were staggered to allow challenge of all animals on the same day, enabling direct comparison of virus shedding after challenge. C, Examples of virus shedding patterns after primary infection and challenge. Each graph represents data from an individual ferret. Shedding patterns were compared to those for controls that were infected with the challenge virus only on day 14 (bottom row).