The situation is creating alarm as the transmission route is unknown for more than 13% of the cases — low compared with many other Asian countries but more than double the rate of about 6% in May, when there was an outbreak at Seoul nightclubs. Health officials said the latest infections, mostly linked to church gatherings, may pose a greater challenge than the previous flareup, which was quickly contained.

“We’re now in a critical situation as local infections are rising every day amid the vacation season,” said Kwon Jun-wook, deputy director of Korea Centers for Disease Control and Prevention. “We see signs that cases without symptoms or with mild symptoms, undetected for some time, may have been silently spreading.”

The jump in cases comes as some of the most successful countries in containing the pathogen are now wrestling with sudden flareups — a grim reminder that the coronavirus won’t be entirely stamped out without help from a vaccine. New Zealand and Vietnam have both reintroduced strict containment measures after seeing their first cases in months.

If the latest outbreak in South Korea worsens, it would be a setback in the global Covid-19 fight. Bolstered by its success with rapid testing and contact tracing, the country has been held up as a model for democracies looking to manage the virus without having to resort to restrictive lockdowns. Once the world’s second hardest hit, South Korea touts one of the lowest infection and fatality rates among developed nations.

Of the 103 new cases, 85 were locally infected while 18 were imported, according to the CDC. Most of the local cases are linked to flareups at churches, schools and traditional markets.

The spike follows a relaxation of social distancing rules, and ahead of a three-day holiday when many are expected to visit parks, shopping centers and restaurants. South Korea has in recent weeks allowed baseball and soccer fans in the stands as well as visitors to public parks and museums.

South Korea’s prime minister said Friday the government may re-impose stricter social distancing rules in Seoul and neighboring areas if the situation worsens, and urged the public to follow virus prevention guidelines, including wearing masks and washing hands.

If the alert is raised, high-risk facilities such as clubs, pubs and karaoke bars will be required to close temporarily. Churches and restaurants will need to require masks and to maintain lists of people entering and leaving the facilities.

South Korea’s virus-prevention measures are producing a positive side-effect. The CDC reported Thursday that the country is seeing a sharp drop in other infectious diseases, aided by social distancing, handwashing and mask wearing. Cases of measles fell 97% in the year through July compared with the year-earlier period, while both whooping cough and scarlet fever declined 62%.