Measles seroepidemiology and decay rate of vaccine-induced measles IgG titers in Taiwan, 1995-1997.

Community-based seroepidemiologic studies were conducted to monitor the effectiveness of measles immunization programmes and to estimate the decay rate of vaccine-induced measles IgG titres. Sera collected from a mountain (792 sera), rural (875 sera) and urban (894 sera) populations in 1995-1997 were available. Measles IgG was quantified using a commercial EIA kit. Measles IgG seroprevalence and geometric mean titre (GMT) were calculated by setting the cut-off titre at 50 mIU/ml. The decay rate of measles IgG titres was estimated by assuming that the measles IgG titres, without exposing to wild measles virus, decay exponentially and constantly after 1 year post vaccination. The half-life of measles IgG titres was calculated from the corresponding decay rate. Measles IgG seroprevalences in these three populations have reached >95% in school children (7-18 years old) and >98% in young adults (19-25 years old) but varied from 87 to 96% in pre-school children (4-6 years old). Two-dose vaccinees, comparing with 1-dose vaccinees, had a significantly higher seroprevalence (98 versus 92%, P<0.01) and a slightly longer half-life of measles IgG titres (61 versus 27 months, P=0.08) but the measles IgG GMT in the two groups did not differ significantly (675 versus 618 mIU/ml, P=0.78).