For the second straight week, a Japanese medical expert questioned whether the Tokyo Olympics can move forward next summer without a vaccine for the novel coronavirus.

“In my view, it would be difficult to hold the Olympics unless effective vaccines are developed,” Yoshitake Yokokura, a surgeon who heads the Japan Medical Association, told reporters in a video news conference Tuesday.

The Tokyo Games were postponed by a year amid the pandemic, which has caused 389 deaths in Japan, the country’s health ministry said Tuesday. In announcing the postponement last month, Prime Minister Shinzo Abe said the Olympics would be held in full in 2021 ″as a testament to victory over the infection.”

But Tuesday, the president of Japan’s organizing committee told the Nikkan Sports newspaper that the Games would not be postponed again if the pandemic is not under control by next summer.

“No, in that situation, it will be canceled,” Yoshiro Mori said, per the AP. “In the past, when there were such problems, like wartime, it has been canceled. This time, we are fighting an invisible enemy.”

Earlier this month, the chief executive of the Tokyo Olympic organizing committee, Toshiro Muto, told reporters that he could not guarantee that the Olympics will go on as planned next summer, a sentiment Mori echoed Tuesday.

“This is a gamble for mankind,” he said. “If the world triumphs over the virus and we can hold the Olympics, then our Games will be so many times more valuable than any past Olympics.”

A vaccine is seen as key to the Olympics moving forward, and last week a professor of infectious disease at Kobe University expressed his doubts that one will be developed in time.

“I am very pessimistic about holding the Olympic Games next summer unless you hold the Olympic Games in a totally different structure such as no audience, or a very limited participation,” Kentaro Iwata said.

Scientists around the globe are working to develop a coronavirus vaccine, with one laboratory at Oxford University hoping to have a few million doses available by September if its vaccine proves effective. Meanwhile, the Coalition for Epidemic Preparedness Innovations, which is funding nine vaccine projects, said Monday that a vaccine could be ready to administer to health-care workers and other vulnerable groups this year.

Abe declared a state of emergency in seven of Japan’s 47 prefectures April 7 and expanded it nationwide April 16 to battle the pandemic, though the measures are far less stringent than in other countries. The government has prohibited large gatherings but has not banned residents from leaving their houses, and bars and restaurants are allowed to remain open until 8 p.m. And while the daily count of new coronavirus infections has stabilized, it has not fallen sharply, casting doubt on Abe’s plan to end the state of emergency May 6.