Anti-monkeypox virus-neutralizing activities of human immunoglobulin manufactured between 1999 and 2021 and derived from donors in the United States and Japan

In May 2022, the United Kingdom reported the first case of chained transmission of the monkeypox (mpox) virus without any known epidemiological links to west or central Africa. The monthly number of mpox patients currently has passed a peak and is declining globally, and infected patients include both non-vaccinated and vaccinated individuals. Herein, the virus-neutralizing (VN) activity against vaccinia viruses, which are considered to cross-react with the mpox virus, in the intravenous immunoglobulin (IVIG) lots derived from donors, including vaccinated Japanese populations, was evaluated to clarify the status of the Japanese blood donor population. VN titres against vaccinia and human mpox viruses in IVIG lots derived from donors in Japan and the United States manufactured between 1999 and 2021 and 1995 and 2001, respectively, were evaluated by neutralization testing. VN titres of IVIG derived from donors in Japan and the United States against vaccinia and mpox viruses showed a slowly decreasing trend between 1999 and 2021. VN titres are expected to decrease in the future since the percentage of vaccinated donors in the donor population seems to have decreased. Therefore, continuous monitoring of VN titres is required.