Mean virus titers in nasal swabs of naïve sentinel pigs at 1 to 5, 7, 9, and 11 days post indirect contact (dpc) with pigs with maternally-derived antibodies (MDA) challenged with homologous or heterologous virus. Treatments in Study 1 (A) were: pigs with no MDA and with MDA from vaccination of naïve sows with H1N2-δ1(1:7) WIV, challenged with homologous H1N2-δ1(1:7) or heterologous H1N1pdm09(2:6). Treatments in Study 2 (B) were: pigs with no MDA, and with MDA from H1N1pdm09 WIV vaccination of seropositive sows, from experimental infection of seropositive sows with H1N1pdm09 (EXP-MDA), or from a non-boosted H1N1pdm09 naturally exposed sow (NAT-MDA) challenged with homologous H1N1pdm09(2:6) or heterologous H1N2-δ1(1:7). The dotted line indicates the assay's limit of detection.