Japan's National Institute of Infectious Diseases says 67 people in the country were confirmed to have the coronavirus during the three-month period through June despite being fully vaccinated.

The figure comes from the first survey of its kind in Japan. It was based on reports from municipal governments and medical institutions.

Full protection from the virus is believed to occur about 14 days after receiving the recommended doses of a vaccine. Infections after that are known as breakthrough cases.

Researchers at the institute say almost 80 percent were among people in their 20s to 40s, adding that none developed serious symptoms.

They say genetic analysis of 14 of the cases revealed that 12 were the Alpha variant first detected in Britain. The other two were the Delta variant first detected in India.

The researchers point out that although coronavirus vaccines are highly effective, it is still important to take anti-infection measures even after receiving all recommended doses.

Some other countries have also reported breakthrough cases.