A shortage of doctors and nurses in Japan’s aging population contributed to the country’s sluggish Covid vaccine rollout, says Keio University professor Sayuri Shirai.

As of June 21, only 18.3% of Japan’s population has received at least one Covid vaccine dose, according to Our World in Data. In comparison, more than 50% of people in the U.S. have received one dose of the vaccine while that figure is even higher in the U.K. at 63.6%.

“In an aging society it’s very difficult to find doctors and nurses. And local government, they don’t have enough people to administer [the vaccines]. So that delayed this vaccination,” Shirai told CNBC’s “Street Signs Asia” on Wednesday.

Countries with aging populations and declining birth rates also risk a critical shortage of workers in the future. A United Nations report from 2019 showed Japan had the world’s highest old-age dependency ratio, with the figure expecting to rise even further by 2050.

Even before the coronavirus pandemic, a 2017 World Health Organization report noted Japan was facing a “critical shortage of doctors.” It also noted that, despite efforts to boost the workforce in this sector, “the declining birth rate in particular is expected to impact on the future population of health care professionals.”

Japan’s race to vaccinate its population ahead of the Olympics faced another hurdle. An article published in medical journal The Lancet earlier this month noted the country only allows nurses and medical doctors to vaccinate citizens and listed this as one of the reasons behind the sluggish rollout.

Still, that situation may be improving. Local media report the Japanese government will relax medical rules to allow more workers to administer Covid vaccinations. Meanwhile, Bloomberg reported Wednesday that Japan has crossed 1 million doses of Covid vaccinations given on a single day.

“The vaccine rollout finally started to accelerate from mid-May,” Naohiko Baba, chief Japan economist at Goldman Sachs, told CNBC on Wednesday. “We now expect half of the population will be vaccinated by end of summer.”

Baba said that development was “good news” for the Tokyo Olympics, which is set to begin in about a month despite public fears over its potential to become a super-spreader event. A recent poll by local news agency Kyodo News showed about 86% of people in Japan are “concerned about a rebound” in Covid cases if the Tokyo Olympics and Paralympics are held this summer.

As of Thursday, the Japanese prefecture of Okinawa was still under a state of emergency while multiple areas including Tokyo were under “priority preventative measures,” according to information from the country’s Ministry of Health, Labour and Welfare. Japan has recorded more than 789,000 Covid infections and at least 14,506 deaths, according to data compiled by Johns Hopkins University.

Disclosure: CNBC parent NBCUniversal owns NBC Sports and NBC Olympics. NBC Olympics is the U.S. broadcast rights holder to all Summer and Winter Games through 2032.