As nations race to develop treatments and vaccines for Covid-19, Japan has become a victim of its own success as slowing new infections has led to a shortage of patients to enroll in clinical trials, Reuters reports.

Clinical trials are underway for more than a dozen potential vaccines, including at least six in China, but Japan's first human trials are expected to start next month.

In development of treatments, Russia and India approved Fujifilm Holdings Corp's Avigan as a Covid-19 therapy, but Japan, whose Prime Minister Shinzo Abe has touted the drug's potential and hoped to approve it in May, won't see a decision until at least July.

"Due to the decreasing number of coronavirus infections, we believe it will take some time before clinical research is completed," said Tetsuya Nakamura, who is running a trial of Avigan at Gunma University Hospital in central Japan.

"It's a pity that Avigan has been approved overseas but not in Japan."

Japan has fared better than most developed nations in tackling the disease that has killed more than 470,000 worldwide. While the epidemic drove Japan's medical system to the brink of collapse in recent months, serious cases now number about 60 nationwide.