 Project announced that Mitsui would not be able to perform in the 2008 spring tour due to her being in very poor health.Later in the week, it was discovered that she had suffered from acute appendicitis, and would be out of action for two more weeks.Between December 15, 2010 (the graduation of Linlin) and January 2, 2011 (the addition of Mizuki Fukumura, Erina Ikuta, Riho Sayashi and Kanon Suzuki), Mitsui was the only member of Morning Musume under the age of 20.