Figure 2. Proportion of participants with seroprotected (titers ≥ 1:40) at Day 21 by type and by the count of seropositive historical influenza strains among those seronegative at Day 0. The number of historical viruses for which each specimen had a titer ≥ 1:40 was called the “historical count”—for example, if a given serum specimen were tested against 6 historical viruses and had a titer ≥ 1:40 for 4 of them, the historical count was 4.