KITAIBARAKI, Ibaraki -- A group of junior high school pupils who fell ill earlier this year have been confirmed as Japan's first group outbreak of a potentially deadly measles virus, health officials said Friday.

The 109 pupils from Kitaibaraki who fell ill from February to April were afflicted with a H-1 measles virus of a type that has struck widely in China and South Korea.

Over 70 percent of the afflicted pupils had been immunized against measles, yet still fell ill.

"We need to look into what caused their immunity to weaken," a spokesman from the Ibaraki Prefectural Government's preventative medicine section said.

Kitaibaraki Municipal Government officials said the prefectural government will examine why so many of the 109 pupils fell ill even though they had received shots to prevent them from picking up measles.

Usually when measles breaks out in Japan, it is carried by D3 or D5 viruses. The differences between these and the H1 virus are not great, but enough for the H1 virus to counter some defense barriers formed by immunization. (Mainichi Shimbun, Nov. 29, 2002)