SEOUL - Falling temperatures, young asymptomatic spreaders and small social gatherings where people tend to take off their masks.

These are reasons cited by experts to explain South Korea's latest surge in coronavirus cases in what is viewed as the third major wave since February and possibly the biggest and toughest to curb.

South Korea, which has prided itself on using massive testing and aggressive contact tracing to fight the coronavirus outbreak without resorting to lockdowns, is now struggling to contain Covid-19 ahead of an all-important college entrance exam due on Dec 3.

What was once the worst-case scenario of 100 cases a day is now the new low benchmark in a five-tier scheme introduced earlier this month to reflect more realistic targets as the fight against Covid-19 stretches into the long haul.

Professor Choi Jae-wook of Korea University's College of Medicine pointed out that the government's response to community infections that started in the June-July period was ineffective to begin with, and that people got tired of social distancing.

"Now, it is impossible to keep the number of confirmed cases at 100 as in the past," he told The Straits Times. "Although inevitable, the quarantine policy should be re-established on the premise that there are 400-500 confirmed cases, and that the quarantine and treatment capabilities can be improved to enable control and management."

The country reported 382 cases on Wednesday (Nov 25) - the highest since August, when gatherings at a church and a public rally triggered the second wave of infections.

This brought the total tally to 31,735, with a death toll of 513.

Each patient currently infects 1.5 people, but experts fear this figure will grow as winter arrives and plunging temperatures confine people to cramped indoor spaces, where viruses thrive.

Instead of imposing pre-emptive measures such as a lockdown, South Korea has so far been successful in tracking down transmissions by tapping mobile phone data, credit card records and closed circuit television footage linked to a cluster.

But the new wave presents a challenge as there are many small clusters happening in vastly different locations, ranging from saunas to cafes, a high school and a music studio. Many of the infected are also asymptomatic, making them silent spreaders.

The wave appeared after social distancing rules tightened in August in response to the second wave were eased.

Since Tuesday, restrictions on social gatherings have been tightened in Seoul and greater Seoul (including Gyeonggi province and Incheon city).

Clubs and bars must close for two weeks, while restaurants have to switch to delivery and takeout after 9pm. Karaoke rooms and indoor sports facilities are only allowed to operate until 9pm.

The measures are aimed at curbing a growing number of infections from small gatherings in public places that contributed to the current spike, such as those that occurred at a pub in Incheon, a university in Seoul, and a children's cafe in Yongin city.

Experts said people tend to let their guard down and take off their masks at such gatherings, allowing the virus to spread more easily.

The measures also target young people who are silent spreaders and tend to socialise through the night.

Patients in their 20s account for about 20 per cent of all infections, up from 10 per cent two months ago and the highest among the various age groups. Their incidence rate of 88 per 100,000 cases is also the highest.

Dr Jung Eun-kyeong, chief of the Korea Disease Control and Prevention Agency, said young people pose a higher risk of spreading the virus in the community as they often show no or mild symptoms, and they continue to lead active social lives without first getting tested.

"We need to tighten anti-virus measures targeting those in their early 20s," she said at a briefing on Monday.

Easing social distancing measures before the virus situation stabilises is also a mistake, noted Dr Kim Woo-joo of Korea University Guro Hospital's infectious disease department.

A cluster in a club in May and one in a church in August both emerged after social distancing rules were eased too quickly due to concerns over the economic toll of a prolonged shutdown.

"In the end, the government will just be repeating the same mistakes and continuing a vicious circle," said Dr Kim.

"The lesson learnt is that only countries that took pre-emptive and bold measures suffered minimal damage."