Complement-mediated neutralization test for determination of mumps vaccine-induced antibody.

When assessed by a conventional plaque neutralization (NT) test, about one third of recipients of live mumps vaccine were found to have failed to seroconvert, although the majority of them became seropositive by the complement-mediated neutralization (CNT) test. The discrepancy between the conventional NT and CNT was found to result from two factors. First, the predominant production of antibodies to the fusion (F) protein during the early phase of antibody response, and second, a low efficiency of NT of the early antibodies in the absence of complement. These features of antibody response were also seen in natural mumps infection and in experimental infection of monkeys, but were particularly prominent in vaccinees because of the limited extent of antibody response in the latter. The discrepancy between conventional NT and CNT diminished with time after vaccination, after natural infection, and also after experimental infection of monkeys. The CNT test was therefore considered better suited than the conventional NT test for assessment of the outcome of vaccination, at least until 6 to 9 weeks after vaccination. The protective role of the antibody in vivo, which requires the addition of complement for neutralization, was inferred from the observation that the antibody incorporated in the agar overlay significantly reduced the size of plaques formed by mumps virus.