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Eurwangni beach in Incheon is pictured on Monday. (Yonhap News)

The South Korean authorities believe that the Delta variant of the coronavirus, which currently makes up the majority of locally transmitted cases of COVID-19, will soon account for nearly all cases of the disease. This means that Delta has become the dominant strain of the virus faster than the authorities’ original prediction of August.

“We believe that Delta, which already represents at least half of coronavirus cases, is driving the country’s current wave of COVID-19,” said Kwon Jun-wook, second deputy director of Korea’s Central Disease Control Headquarters.

“That was the situation even before last week,” said Kwon, who pointed out that Delta’s 48.6% rate of detection in local cases applied to samples that underwent genomic analysis on July 14-25.

“In this situation, we can assume that Delta will soon account for the entire outbreak. It’s safe to say that Delta has already become the dominant strain of the coronavirus,” he added.

It’s only been about two and a half months since the World Health Organization described Delta as a “variant of concern” on May 11. South Korea’s disease control authorities first categorized Delta as a “major variant” on May 18.

As recently as the fourth week of last month, Delta was only detected in 3.3% of domestic cases. But over the past month, that rate has soared to 48.6%, making it the dominant variant.

Lee Sang-won, director of the CDCH’s epidemiological analysis team, mentioned “the possibility that Delta will gain dominance sometime in August” on July 13. But that variant surpassed the authorities’ prediction in just two weeks.

Delta is believed to be 140% more infectious than the original coronavirus and 60% more infectious than Alpha. The risk of hospitalization for Delta is also 126% higher than for Alpha.

For now, Korea’s disease control authorities don’t think it will be necessary to change the current disease control system and basic vaccination campaign even if Delta gains dominance.

“We’ll respond to Delta or any other variant in the same way, with social distancing and more vaccinations. Our top priority is a stricter implementation of social distancing,” Kwon said.

“Research shows that vaccines are 10-20% less effective at preventing [Delta]. But vaccines definitely reduce its hospitalization rate, case fatality rate, and transmissibility, so there’s no reason to doubt their effectiveness,” he added.

That said, the public health authorities are paying close attention to the argument that the proliferation of Delta may mean that the vaccination rate needed for herd immunity may be higher than 70% of the entire population.

“Some think we need experts to reconsider the scope and scale of herd immunity since Delta has raised COVID-19’s basic reproduction number a little. We in the government are also familiar with that opinion, and we intend to discuss that with local experts based on the example of other countries,” Kwon said.

“For the moment, we can’t raise social distancing any higher, and we can see from other countries that it’s not easy to raise the vaccination rate above 70%. Our only option is to comply with the current level of distancing while continuing vaccinations and to ensure that nothing goes wrong with our medical response system,” said Lee Jae-gap, a professor of infectious disease at Hallym University Kangnam Sacred Heart Hospital.

“Because we’ve vaccinated the high-risk group, Delta isn’t causing the number of serious cases to rise as we saw before. We need a flexible response that takes the economy and education into account while making the reduction of severe cases and deaths the top priority of disease control,” said Kim Nam-jung, a professor of infectious disease at Seoul National University.

In a related story, an infectious disease institute that belongs to the Korea National Institute of Health, which is affiliated with the Korea Disease Control and Prevention Agency, has completed a domestic study on the effectiveness of vaccine mixing, in which different manufacturers make people’s first and second doses. This study found that people who’d taken different vaccines had a higher level of virus-neutralizing antibody formation than those who’d taken two doses of the same vaccine.

The findings suggest that vaccine mixing may be as effective as, or more effective than, administering two doses of the same vaccine.

However, experts noted that these findings shouldn’t be taken as conclusive given the small size of the clinical study. No serious adverse events were reported among either group (mixed vaccination or identical vaccination).

Researchers carried out the study on 499 medical workers at 10 hospitals in the greater Seoul area. The subjects consisted of 100 people inoculated with AstraZeneca first and Pfizer second, 199 people inoculated with two doses of AstraZeneca, and 200 people inoculated with two doses of Pfizer. The researchers observed the formation of neutralizing antibodies, changes in the antibodies’ ability to neutralize variants, and adverse events.

Subjects’ ability to neutralize the virus was found to decrease by the same amount in regard to the Delta variant in both the mixed vaccination and identical vaccination groups.

“These research findings provide grounds for continuing mixed vaccination. That said, we don’t plan to administer the Pfizer vaccine to those who are currently signed up for a second dose of AstraZeneca,” said Hong Jeong-ik, head of the vaccine management team in the government’s vaccination campaign, in the daily briefing on Monday.

“It’s significant that this research has found that mixed vaccination raises the level of neutralizing antibodies since we already have no choice but to mix vaccines because of supply issues. But since this study only covered hundreds of people, we shouldn’t conclude that vaccinating people with the same vaccine — which has been tested in clinical studies of tens of thousands of people — is less effective than mixed vaccination,” said Bang Ji-hwan, head of the central infectious disease hospital operations center at the National Medical Center.

By Kim Ji-hoon, staff reporter

Please direct comments or questions to [english@hani.co.kr]

Caption 2-1: Eurwangni beach in Incheon is pictured on Monday. (Yonhap News)