As new Covid-19 cases have returned to an upward trend, Thursday’s daily tally nearly doubled that of the previous week.

The Korea Disease Control and Prevention Agency (KDCA) reported 18,511 new virus cases, including 194 cases from overseas, pushing the cumulative caseload to 18,451,862. The daily tally compared with 9,522 on Thursday last week.

The nation also reported 10 more Covid-19 deaths, raising the death toll to 24,593. The fatality rate stood at 0.13 percent. The number of critically ill patients came to 56, down five from the previous day, the KDCA said.

The KDCA said the pace of the virus' reproduction has recently gone up amid the nation's increasing mobility in the summer vacation season under eased social distancing rules. In addition, as the number of inbound travelers increases after the quarantine exemption for inbound travelers and the expansion of international flights, the number of inbound cases has risen.

While the number of imported cases from overseas was 194 on Thursday, 30 fewer than the previous day’s 224, imported cases have recorded a triple-digit figure for 14 consecutive days since June 24.

Health authorities warned against another wave of Covid-19 resurgence, as subvariants of the virus are spreading and more people are traveling for the summer holidays.

A health expert predicted that Korea might see 12 million reinfection cases, about two-thirds of the cumulative number of confirmed cases in Korea, if BA.5, a subvariant of Omicron, becomes a dominant strain.

"If BA.5 leads to a resurge, I estimate that reinfection will occur in two-thirds of the 18.45 million cumulative confirmed cases in Korea," Professor Eom Joong-sik, an infectious disease expert at Gachon University Gil Medical Center, said. "According to studies in countries where BA.5 has become the dominant strain, the ratios of initial and second infections are half and half."

Therefore, the longer the epidemic persists, the more likely the number of reinfection cases will increase, Eom said, explaining that this is because the neutralizing antibody and antibody binding capacity decreases with time.

Professor Kim Woo-joo of the Infectious Disease Department at Korea University Guro Hospital agreed.

"We are going through the same situation that the U.S. and Europe experienced a few months ago," Kim said. "The sixth viral wave has already begun."

As BA.5, which has a stronger transmission power than the existing Omicron, evades immunity caused by vaccines and spreads rapidly, the risk of breakthrough infection or reinfection is also greatly increased, Kim added.

Kim stressed that as pathogens with high immunity evasion and hosts with weakened immunity are bad factors, the outcome of this epidemic will vary depending on the government's initial response and quarantine policy.

"The number of tests should significantly increase to lower the serious morbidity rate through early diagnosis and treatment," Kim said.

The government reiterated the importance of vaccination to prepare for a possible spike in summer. It urged high-risk groups, including those 60 and older, immunocompromised patients, and residents at nursing hospitals and facilities, to complete vaccination as soon as possible.

Health officials also stressed that they are discussing with experts about providing a second booster shot to all Koreans.

However, experts did not fully support the idea of vaccinating all Koreans with the second booster shot as the vaccine currently in use targets the original Covid-19 virus, so it is more effective in preventing the progression of an aggravation than preventing variant infection.

"It is also difficult to secure sufficient evidence that shows that a second booster shot will be effective in preventing the recent outbreak," Professor Eom said. "Also, we have to bear various manpower and costs if we want to conduct a large-scale inoculation, and it is difficult to expect the effect of such costs."

As of Thursday, 44.63 million, or 87 percent of the population, had completed the full two-dose vaccinations, and 33.38 million, or 65 percent, had received their first booster shots.

About 4.47 million people, or 8.6 percent of the population, had gotten their second booster shots, according to the KDCA.