The presence of a preexisting antibody modified the mean duration of illness of 4.4 days in participants with pre-hemagglutinin inhibition titers of <1/8 compared with 1.0 day in participants with pre-hemagglutinin inhibition titers of >1/8 after the inoculation of a wild-type A(H1N1) virus (Doyle, et al., 1994) .