The two subvariants of the Omicron variant -- BA.4 and BA.5 -- can easily cause infections even in recovered Covid-19 patients and fully vaccinated people, a U.S. study showed.

Researchers at Harvard Medical School and Beth Israel Deaconess Medical Center in Boston found that neutralizing antibodies to BA.4 and BA.5 in cured Covid-19 patients and vaccinated individuals were significantly lower than when exposed to other variants, according to the study.

Among the 27 subjects participating in the study, those who received vaccination only produced 1/21 of the neutralizing antibodies against BA.4 and BA.5 compared to the novel coronavirus. Also, the level of neutralizing antibody formation against BA.4 and BA.5 of those fully cured after infection was low at 1/8.

"Our data suggest that these new Omicron subvariants will likely be able to lead to surges of infections in populations with high levels of vaccine immunity as well as natural BA1 and BA2 immunity," Dr. Dan Barouch, an author of the paper and director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center in Boston, was quoted as saying by CNN.

However, vaccine immunity will still provide substantial protection against severe diseases with BA4 and BA5, Barouch added.

BA.4 and BA.5 infections have recently spread in the U.S., where nearly 95 percent of the population has either received the vaccination or been cured of the infection.

According to the U.S. Centers for Disease Control and Prevention, BA.4 and BA.5 infections last week accounted for 35 percent of all Covid-19 cases, up from 29 percent the previous week.

However, the BA.4 and BA.5 subvariant have yet to become a dominant strain in Korea as the BA.2.12.1, also known as stealth Omicron, accounted for most of the Omicron subvariant cases here until recently, according to officials and experts.

As of June 14, the cumulative number of Omicron subvariant reached 176 -- 127 BA.2.12.1, 14 BA.4, and 35 BA.5 subvariant cases.

The Korea Disease Control and Prevention Agency (KDCA) also released a report which affirmed that Paxlovid, Pfizer's oral Covid-19 pill, was effective in treating Covid-19 patients 60 years and older.

The headquarters compared and analyzed the risk of aggravation and death of 35,287 older adults who took or did not take Paxlovid from Jan. 14 to Feb. 28.

Among the subjects, 7,063 people took Paxlovid, and 28,224 people did not.

As a result of the analysis, the government confirmed that the aggravation and mortality rate was 58 and 46 percent lower in the Paxlovid group compared to those that did not take the drug.

On Thursday, Korea's new virus cases stayed below 10,000 for 14 consecutive days.

Korea added 7,497 Covid-19 infections, including 92 cases from overseas, bringing the total caseload to 18,305,738, the KDCA said.

The numbers have declined steadily after hitting more than 620,000 in one day in March at the height of the Omicron spread.

It reported 14 more Covid-19 deaths, raising the death toll to 24,488. The fatality rate stood at 0.13 percent. The number of critically ill patients came to 58, down from 64 the previous day, the KDCA said.

Health authorities predicted the slowing pace in the Omicron spread has reached its limit, and the virus curve will maintain the current level for some time.

However, health authorities also raised concerns that relaxed virus curbs could prompt virus resurgence in the summer.

As of Thursday, 44.62 million, or 87 percent of the population, had completed the full two-dose vaccinations, and 33.35 million, or 65 percent, had received their first booster shots.

About 4.3 million people, or 8.4 percent of the population, had gotten their second booster shots, according to the KDCA.

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