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Some women in Japan have contracted rubella during the early stages of pregnancies, resulting in birth defects in 31 cases, even though they had received vaccinations against the virus during childhood, researchers said Monday.

The findings suggest the protection provided by the vaccination wore off with age and that people should be immunized twice, once in early childhood and again in early adulthood, according to Shigetaka Kato, a member of the research group who also works for the U.S. Centers for Disease Control and Prevention.

The study was conducted jointly by Kato and the state-run Zentsuuji Hospital in Kagawa Prefecture.

In one of the cases, a woman in her 40s, who was immunized at 14, tested positive for rubella antibodies for her first two pregnancies, which progressed normally.

But she contracted the virus when one of her relatives had the disease in the early stage of her third pregnancy at the age of 34.

Although she did not develop any symptoms, such as a rash, a blood test showed the antibodies had drastically increased and she had become infected. As a result, her third child was born with cataracts in both eyes and also carried the antibodies.

According to Kato, people who had been vaccinated normally do not develop symptoms of rubella even if they are infected, making it difficult to know whether they had contracted the disease.

Japan ordered junior high school girls to receive rubella vaccinations in 1977. Since 1995, following the revision of the Preventive Vaccination Law, the government has recommended that both males and females from 1 to 7 1/2 years old receive them.

But some experts say an increased number of vaccinations may increase the likelihood of side effects, suggesting it is not necessary to be vaccinated twice.