Background: Due to the presence of maternal passive antibodies, the measles vaccine is ineffective if administered before age 12–15 months. The optimal timing for administering a live attenuated vaccine (LAV) after intravenous immunoglobulin therapy (IVIG) for Kawasaki disease (KD) has not been fully investigated. The recommended interval between vaccination and IVIG therapy for KD differs by country. The present study aimed to evaluate efficacy of LAV six months after IVIG therapy for KD in Japan. Methods: The present, single-arm, prospective, interventional study included patients aged 6 months or older with no medical history of measles, rubella, varicella or mumps or vaccinations against these diseases. The subjects received these vaccinations for the first time at six months after IVIG therapy. Virus-specific IgG levels for each virus measured by EIA was examined at nine months after IVIG therapy. If the results were negative, the subjects received a booster vaccination at 12 months after IVIG therapy. The primary outcome was the prevalence of positivity for antibodies after the initial and booster vaccinations. Results: The present study enrolled 32 subjects, 31% of whom were female, with an average age of 10.8 (standard deviation 2.8) months at IVIG therapy. At six months after IVIG therapy, 9% and 6% of the subjects were seropositive for measles and varicella titers, respectively, but were seronegative for the mumps and rubella titers. The seroconversion rate for measles, mumps, rubella, and varicella after the initial vaccination was 88%, 6%, 78%, and 16%, respectively. The seroconversion rate after a booster vaccination was 100% for measles and rubella, 97% for mumps, and 77% for varicella. Conclusions: The seroconversion rate was low for LAV at six months after a single dose of IVIG for KD, but seroconversion was achievable with a booster vaccination at 12 months. Clinical Trial Registration: UMIN-CTR, UMIN000007174, https://upload.umin.ac.jp/cgi-open-bin/ctr/ctr_view.cgi?recptno=R000008452.
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