More than 2,000 people in Japan have been diagnosed with rubella, or German measles, the worst outbreak in five years, prompting warnings to visitors – particularly pregnant women – to make sure their vaccinations are up to date.

The 2,454 cases that have been confirmed as of early December is 21 times the total number reported last year, with the outbreak centered on Tokyo and the surrounding prefectures of Kanagawa, Chiba and Saitama, according to the Atlanta-based Centers for Disease Control and Prevention.

There is also growing concern that similar outbreaks in the next two years – Japan hosts the Rugby World Cup in 2019 and the Olympic Games will be staged in Tokyo the following year – could expose tens of thousands of foreign visitors to the disease.

Rubella, which is also known as three-day measles, is spread through the coughing and sneezes of people who are already infected. The symptoms include a rash, sore throat, fatigue and a fever that typically lasts three days.

And while the illness is often relatively mild, complications can include bleeding problems, testicular swelling and inflammation of the nerves.