1. From the Division of Blood Transfusion and Transplantation Immunology, Neonatal Intensive Care Unit, and, Department of Obstetrics and Gynecology, Fukushima Medical University School of Medicine, Fukushima City, Fukushima;
2. Mitsubishi Kagaku Bio‐Clinical Laboratories, Itabashi‐ku, Tokyo;
3. Immunology Division and Division of Molecular Virology, Jichi Medical School, Tochigi, Japan.