Why was Japan's vaccine rollout slow compared with other countries? Experts have pointed out the stringent regulations of Japan's approval process for new drugs, which require conducting clinical trials at home even in times of emergency — like the COVID-19 pandemic.

But during a radio program on Tuesday, vaccine rollout minister Taro Kono revealed that U.S. drugmaker Pfizer Inc. had included more than 100 Japanese residents in the United States in its clinical trials last year, since Japan was not among the countries where it conducted trials. The pharmaceutical company conducted trials in the U.S., Germany, Turkey, South Africa, Brazil and Argentina on more than 40,000 participants.

"When Pfizer said in July (2020) that it was going to conduct an international clinical trial, Japan was excluded because the number of patients (at that time) were considerably small compared with Western countries," Kono said. "They thought that it would be meaningless to do it in Japan because it would only take time."