Korea on Tuesday reported its first two cases of delta plus, a new variant of COVID-19, both of which were “breakthrough infections” affecting vaccinated people.

Lee Sang-won, the head of the Korea Disease Control and Prevention Agency’s epidemiological investigations team, told a briefing that both delta plus patients had been fully vaccinated with AstraZeneca’s COVID-19 vaccine.

One of them is a man in his 40s who had no recent history of overseas travel, but sought testing after experiencing respiratory symptoms. He was positive in test results that came on out July 26. His son was also positive, but it was unclear whether he was also infected with delta plus.

The other patient returned from the United States on July 23 and had tested positive during an airport screening.

Lee said delta plus “appears to differ only slightly from delta in terms of its transmissibility or virulence.” “Since breakthrough infections are possible, social distancing, masking and practicing personal hygiene remain important even for people who have been vaccinated,” he said.

Delta plus is a sub-lineage of delta with an extra mutation in the spike protein called K417N -- the same mutation found in beta and gamma, variants of concern first spotted in South Africa and Brazil, respectively. South Africa stopped using AstraZeneca’s vaccine in February after a study showed it did not work well against the beta variant.

When a mutation occurs in the spike protein on the surface of SARS-CoV-2, the virus causing COVID-19, it likely increases the ability of the virus to bind to receptors on the human cells and infect the host, according to Dr. Paik Soon-young, an emeritus professor of virology at Catholic University of Korea.

“The location of the mutation boosts the binding affinity of the virus to the human ACE2 receptors,” he said. “But we don’t know yet if that leads to increased replication after entering the human cell, which is significant in understanding its transmissibility.”

So far, there is no evidence to suggest delta plus renders the vaccines less effective, according to infectious disease epidemiologist and the Korean Vaccine Society’s vice president Dr. Chun Byung-chul.

“Beta, which carries the same mutation, somewhat weakened AstraZeneca’s vaccine. The mRNA-type vaccines fared better in comparison,” he said. “Korea needs to keep a close eye on the new variant, delta plus, for sure, but a separate analysis is necessary to know if it is capable of evading vaccines.”

Chun added he was worried the recent breakthrough cases might be used as a means to discredit the vaccines. “To be clear, breakthrough infections can happen with any vaccine, not just COVID-19 vaccines.”

Delta is as transmissible as chickenpox, and patients with breakthrough infections of the variant may be able to spread it as readily as unvaccinated patients, according to internal report by the US Centers for Disease Control and Prevention.

Korea’s national disease control agency says delta is already the dominant strain in Korea. Delta showed up in 61.5 percent of patient samples analyzed last week, according to the agency’s Tuesday report.

There are 1,132 known cases of breakthrough infections in Korea, which translates to 17.8 cases for every 100,000 fully vaccinated people. Breakthrough infections were most common among recipients of Johnson & Johnson’s vaccine, at 51.4 cases for every 100,000 people, followed by recipients of AstraZeneca’s, at 24.3 per 100,000, and Pfizer’s, at 7.8 per 100,000.

Korea posted 1,202 additional cases on Tuesday, bringing the cumulative tally to 202,203. In the last seven days, the number of new cases found each day has averaged 1,526.

More than five months into rollout, the country has vaccinated 20 million people, or 39 percent of its population, with at least one dose of a vaccine as of Tuesday, while 14 percent have been fully vaccinated.

By Kim Arin (arin@heraldcorp.com)