Attenuation of antibody titer of measles and rubella virus among university students of department of healthcare providers during 2015-2018 in Japan.

Background: In Japan, measles elimination was confirmed in March 2015. Nevertheless, some outbreaks with cases imported from abroad were reported even after certification. A large rubella outbreak has been occurring since 2017. This study examines measurement of the speed of attenuation of antibody titer for a measles virus comparison with rubella virus.

Method: Student subjects born from April 2, 1996 through April 1, 2000 were selected at Ibaraki Prefectural University of Health Sciences for this study: 177 for measles and 114 for rubella. They had available dates of additional immunization and antibodies in the following period and were judged as requiring additional immunization. We used enzyme immunoassay for IgG antibody testing. We regressed post-antibody titers of measles or rubella on pre-antibody titers and functions of duration between inoculation to post-evaluation. Functions of duration were selected according to the adjusted coefficient of determination.

Results: For measles, only a linear term of duration or log of duration was found to be significant without the quadratic terms. For rubella, we selected a five-order linear model which indicated that titer after vaccination would converge to 19.2.

Discussion: Results demonstrate that measles antibody decreased monotonically. If the pre-antibody titer was 15, vaccination raised titer quickly to 26; then it attenuated by 0.014 per day. Antibody titer is expected to be less than 16, which is the protection level of titer, after 704 days. For rubella, however, when pre-vaccination titer was evaluated at its average, the lower limit was 19.2. Therefore, protection can be maintained for a long time. This difference might reflect some circumstances of outbreaks of the respective diseases.

Conclusion: This report describes the speed of attenuation and the epidemiological situation. The speed of attenuation can be expected to rise. Therefore, additional vaccination every several years might be necessary to maintain a protection level if a disease is almost eliminated.