The number of rubella cases reported so far this year has surpassed 5,000, more than twice the number for all of 2012, the National Institute of Infectious Diseases said Wednesday.

The rise has prompted the Health, Labor and Welfare Ministry to encourage vaccination, particularly among people in their 20s to 40s who tend to lack immunity to the disease.

The number of reported cases totaled 5,442 as of April 28, including 526 in the week of April 22 to 28, the institute said. There were 2,392 cases last year.

Since April, the weekly tally of new cases has been more than 500.

By prefecture, Osaka had the most new cases during from April 22 to 28 at 135, followed by 124 in Tokyo, 61 in Kanagawa and 43 in Hyogo, the institute said.

Experts have been calling for women in the early stages of pregnancy to be vigilant because the disease can lead to the development of congenital rubella syndrome in fetuses that can result in hearing difficulties, eye abnormalities and heart disease.

The health ministry said around 15 percent of men and women in their 20s to 40s are believed to have no immunity due to low vaccination coverage, among other reasons.

The ministry mandated reporting of all rubella cases in 2008.