Previous exposure with seasonal LAIV or H1N1 <i>ca</i> virus induced a faster H5N1-specific immune response.

<p>(A) Ferrets were intranasally inoculated with either medium or trivalent LAIV on day 0. Six weeks later, ferrets were intranasally inoculated with the H5N1 HK03 <i>ca</i> vaccine and five days later, lymphocytes isolated from TLN were examined for ASC against H2N2 MDV-A, H5N1 HK03 <i>ca</i> viruses and rH5 HA antigens by B cell ELISpot analysis. Ferrets were intranasally inoculated with medium or the monovalent H1N1 NC99, H3N2 CA04, H5N1 HK03 <i>ca</i> vaccine viruses on day 0 and intranasally inoculated with the H5N1 HK03 <i>ca</i> vaccine six weeks later. The B cell ELISpot analysis was performed with lymphocytes isolated from TLN using the indicated <i>ca</i> vaccine virus (B) or rHA (C) as antigens. The IgG antibody secreting B cells are presented as the number of ASC per 10⁶ lymphocytes.