Comparative Analysis of Titers of Antibody against Measles Virus in Sera of Vaccinated and Naturally Infected Japanese Individuals of Different Age Groups

The anti-measles virus (MV) antibody titers in the sera of vaccinees and naturally infected individuals of different age groups were measured to help assess the efficacy of the current MV vaccination in Japan. Neutralizing (NT) antibody titers induced by vaccination were 2(3.2) times lower than those induced by natural infection and declined significantly by age 20. The once-decreased NT antibody titers of the vaccinees increased 2(3.6) times during their twenties to titers comparable to those of naturally infected individuals of the same age, implying the possible occurrence of natural infection in vaccinees with decreased anti-MV immunity. Although the current field strains in Japan, types D3 and D5, were reported to differ antigenically from each other and from vaccine strains (type A) to some extent, as demonstrated by different reactivities to monoclonal antibodies, the sera of vaccinees neutralized the two types of field strains and the vaccine strain with the same efficiency. This result suggests that the current vaccine strain would be suitable to elicit protection against types D3 and D5, as long as viral antigenicity is concerned. However, when compared at given hemagglutination inhibition titers, NT antibody titers of vaccinees were 2(1.1) to 2(3.2) times lower than those of naturally infected individuals, suggesting a qualitative difference(s) of anti-MV antibodies between the two groups. It should be emphasized that protective immunity induced by the one-dose vaccination currently implemented in Japan may not be strong enough to ensure lifelong immunity. A two-dose vaccination program with higher vaccination coverage needs to be considered in order to effectively control measles in Japan.

为评估日本现行麻疹病毒（measles virus, MV）疫苗的免疫防护效果，本研究检测了不同年龄组疫苗接种者与自然感染者血清中的抗MV抗体滴度。研究发现，疫苗接种诱导的中和（Neutralizing, NT）抗体滴度较自然感染者低2(3.2)倍，并在年龄达到20岁时出现显著下降。疫苗接种者曾一度降低的中和抗体滴度在其20多岁阶段回升2(3.6)倍，达到与同年龄组自然感染者相当的水平，这提示抗MV免疫力下降的疫苗接种者可能曾发生过自然感染。尽管日本当前流行的MV野毒株分为D3和D5两种型别，据报道二者在抗原性上存在差异，且与疫苗株（A型）也存在一定程度的抗原差异，该差异可通过与单克隆抗体的不同反应性得以证实；但疫苗接种者的血清可同等有效地中和这两种野毒株与疫苗株。该结果表明，就病毒抗原性而言，现行疫苗株足以诱导针对D3和D5型野毒株的保护性免疫。然而，在相同血凝抑制（hemagglutination inhibition, HI）滴度水平下进行比较时，疫苗接种者的中和抗体滴度较自然感染者低2(1.1)至2(3.2)倍，这提示两组人群的抗MV抗体存在质性差异。需要强调的是，日本当前实施的单剂次疫苗接种方案所诱导的保护性免疫，可能不足以维持终身免疫。为在日本有效控制麻疹疫情，应考虑推行更高接种覆盖率的两剂次疫苗接种计划。