South Korea restricted the use of AstraZeneca’s COVID-19 vaccine in people younger than 30 years old over the risk of unusual blood clots, but some experts say a higher age limit is advisable.

Korea University’s infectious disease professor Dr. Kim Woo-joo said the move is “questionable” when the latest research found that blood-clotting conditions were reported in the vaccine’s recipients ranging in age from their early 20s to mid-50s.

For people in their mid to late 50s, benefits of getting the vaccine so far seemed greater than the risks of side effects. But the same could not be said for people aged between 30 and 55, he said.

He said clotting reactions of a milder grade were probably not being picked up by the vaccine safety monitoring in the country. A slight drop in platelet count -- not so much as thrombocytopenia, or abnormally low levels of platelets -- was observed in some younger recipients of the vaccine in their 20s to 30s at the hospital where he is located.

“What Korea is doing now is passive surveillance. But to be able to survey the full spectrum of the side effects,” he said, “recipients -- at least those who are younger -- should be monitored for up to four weeks after inoculation.”

The specific condition now being called vaccine-induced thrombotic thrombocytopenia was probably on the extreme end, he added.

He said asking young and middle-aged people to accept the risks of the vaccination, however minuscule, for the goal of immunizing the herd appeared “ethically complicated.”

According to the analysis by the government’s advisory committee on immunization practices, the difference in the evaluated risks for those in their 20s and 30s were slim. The estimated respective death tolls from the clotting disorder in a scenario where the entire Korean population is given the AstraZeneca vaccine was 3.99 for those in their 20s and 4.02 for those in their 30s.

Kim said the immunization advisory committee failed to take into account the risks of getting seriously sick from the clotting conditions. “Even if they don’t die from it they may suffer serious complications and sustain lasting injuries,” he said.

Dr. Jean-Louis Excler, a member of the vaccine safety group within COVAX, said recommendations in Korea should have “some alignment with other countries.”

With the exception of the UK, and now Korea, the age limit for AstraZeneca vaccinations elsewhere falls somewhere between 50 years of age and up. In Canada it’s 55, and in Germany, 60.

“France, for example, said three days ago that the AstraZeneca vaccine should not be given to anyone under 55, and those who have received a fist injection would now receive another vaccine for the second. Those under 55 who aren’t yet vaccinated would receive Pfizer or Moderna vaccines,” he said.

Despite excluding people under 30 from the vaccinations, Korea will direct those who already have received an AstraZeneca inoculation to receive a second dose of the same vaccine.

Epidemiologist and vaccine scholar Dr. Chun Byung-chul of the Korean Vaccine Society said an upper age limit would have been “a better precaution,” but that the cutoff at 30 was an “understandable compromise,” considering more than 60 percent of the country’s current supply consists of AstraZeneca doses.

“There are other considerations like delivery timeline that must also factor into the equation. For Korea, major changes to vaccination schedule will be inevitable if we had a higher age restriction,” he said.

“Still, safety is paramount,” he added. “I don’t think it’s right to include younger people, especially women, in AstraZeneca vaccinations when the rare but fatal conditions are mostly observed in them.” He pointed out that health authorities have repeatedly stressed in response to criticism that Korea was falling behind in vaccinations that they would make “safety, not speed” the top priority.

He said as the vaccination program targets almost all Korean adults, even reports of suspected adverse reactions that aren’t listed in official guidelines deserved attention.

“When the first blood clot report emerged in the country, there was a tendency to downplay the event and disregard it. But as I’ve said before, we need to be humble before what we don’t know,” Chun said.

Dr. Song Joon-young, who is on the immunization advisory committee, said on capping the age for vaccinations as 30, “For now, the understanding is that the benefits of AstraZeneca vaccinations are nearly nil for people in their 20s, given that they are significantly less likely to die or fall badly sick from COVID-19 compared to their older counterparts.”

He said for young people in their 20s, they could wait for “safer alternatives,” as they weren’t equally at mercy of vaccination to fare through the pandemic.

The dangers of catching COVID-19 are “slightly greater” for 30-somethings, he said. The death count from the infection here so far stands at three for those in their 20s, and seven for those in their 30s.

He admitted that opinions varied on how high the age limit should be for the AstraZeneca vaccine, and that the current advice was “not completely set in stone.”

Jeong Eun-kyeong, the Korea Disease Control and Prevention Agency’s commissioner, said in announcing the AstraZeneca vaccine will only be used for people 30 or older on Sunday that the decision was “subject to change.”

“The age limit could be raised or lowered depending on the evolving pandemic situation and vaccine availability, as well as emerging data on the clotting symptoms following vaccination,” she said. She cited guidelines from the UK, which also set the bar for the jab at 30, as one of the references for deciding to curb the vaccine’s use in the age group.

So far, Korea has seen two cases of potentially vaccine-linked blood clots in people in their 20s. One was diagnosed with a clotting condition in the brain called cerebral venous sinus thrombosis, and the other one with a blockage in the lung arteries, or pulmonary embolism.

By Kim Arin (arin@heraldcorp.com)