Japan will stop short of offering blanket coronavirus tests, and instead target the vulnerable and those most at risk in seeking to prevent a second wave of infections, the minister in charge of policies to combat the health crisis said.

Japan is currently well behind other major economies in the number of completed coronavirus tests, drawing criticism from some experts that it is not doing enough to trace the virus and prevent clusters.

Yasutoshi Nishimura, Japan's economy minister who also oversees coronavirus policies, defended Tokyo's approach, saying that conducting effective polymerase chain reaction (PCR) tests on the entire population would be difficult.

Even if someone initially tests negative, the person could return a positive test days later depending on his or her behaviour during that period. Ideally, Japan would test the entire population all at once and isolate those who are positive, though that would be "impossible realistically," he said.

"It's therefore important to ensure that those with higher risks (of being infected) or deemed by doctors as in immediate need can get tested," Nishimura told Reuters in an exclusive interview conducted on Saturday.

"I don't side with the view that everyone should take PCR tests regardless of their conditions. The key is to what extent we conduct these tests, which we're debating now."

Japan has a daily maximum testing capacity of more than 27,000 cases. But actual tests being conducted is currently about 10,000 cases at the most, which Nishimura said was due to a sharp decline in the number of new infections.

After peaking at 720, the number of infections has gradually fallen and now hovers around 40 per day.

Japan so far has avoided the spike in infections seen in countries like the United States and Italy. It now has about 17,200 infections with around 900 deaths, far lower than the 1.9 million cases and more than 110,000 deaths in the United States. 

Reuters