All 1 million slots for vaccinations with Johnson & Johnson’s COVID-19 vaccine filled up Tuesday within about 16 hours after opening in South Korea.

Korea is set to get 1 million doses of Johnson & Johnson’s vaccine later this week to inoculate members of the military in their 30s or older, as well as members of the reserve and civil defense forces. As there are close to 4 million people who fit the bill, the vaccines were made available through an online system on a first-come, first-served basis.

The competition for appointments was tough, according to a Seoul man, 31, who agreed to give only his surname, Park.

“The first thing I did when I came into work this morning was look for available slots. I finally found one near my area after some waiting around so I snatched it,” he said.

“The demand seems to have outpaced the supply, clearly. I felt like I was competing with the entire cohort of Korean men in their 30s.”

Park said the biggest appeal for him was that the vaccine only required a single injection: “Just get one shot, and get it over with.” Plus, he said, he didn’t like the uncertainty of not knowing when he could expect a dose before the Johnson & Johnson vaccine became available.

A Gyeonggi Province resident surnamed Lee, 33, said signing up for the vaccine was “like getting a ticket to a hot band’s concert.”

“I went on the website exactly at Monday midnight when it opened and there were already about 30,000 people ahead of me. I waited for half an hour before I could land one for myself,” he said.

Health officials say the extra Johnson & Johnson supply could help Korea administer 14 million COVID-19 shots by the end of June, 1 million more than the earlier target.

Willingness to get vaccinated was rising among Koreans, the ministry said. Its May survey of 1,000 adults who weren’t yet eligible showed that 69 percent intended to get vaccinated, up nearly 8 percentage points from a month prior.

But the key is getting as many older adults to get the shot as possible, according to Dr. Paik Soon-young, a virologist at Catholic University of Korea. “Ideally the rate of vaccinations should be at least 80 percent among people in their 60s and older, who are more vulnerable to severe complications from COVID-19,” he said.

Only 40 percent of people aged 75 and up, who were offered the Pfizer vaccine beginning in early April, are fully vaccinated -- meaning they have received both shots -- despite more than 85 percent of them having made an appointment. For people in their 60s and early 70s, the AstraZeneca vaccine just became available Thursday.

As the vaccine rollout expands to people in younger age groups, infectious disease professor Dr. Kim Woo-joo of Korea University said educating people about the benefits as well as the risks of the vaccine they are getting was “essential” to overcome hesitancy in the long haul.

“Informing people about the signs and symptoms to watch out for, and where they can seek help, is also an important part of a safe vaccination campaign,” he said.

The age cutoff for the Johnson & Johnson vaccine in Korea is 30 due to a link to rare form of blood clots, but authorities believe the risk for men in their 30s and up is low.

Jeong Eun-kyeong, the Korea Disease Control and Prevention Agency’s commissioner, said in Monday’s press briefing that blood-clotting events after Johnson & Johnson vaccination were “mostly observed in those younger than 50, and primarily women,” citing the US data.

Three months into the vaccination campaign, Korea has now vaccinated just over 10 percent of its 51 million people with at least one dose of a COVID-19 vaccine. The number of people who are fully vaccinated is 2 million, or 4 percent of the population.

Dr. Jung Ki-suck, a respiratory disease specialist who was once the director of the Korea Centers for Disease Control and Prevention, said getting to the vaccination goal by the end of June seemed “plausible” based on the ministry’s calculations.

“The ministry says it’s prepared to vaccinate up to 1 million people a day,” he said. “The vaccine uptake appears strong so far. Korea’s forte is that anti-vaccine sentiment never really had a place here. The only issue is the supply.”

By Kim Arin (arin@heraldcorp.com)