Daily COVID-19 cases fell below 500 for the first time in a week on Sunday, largely as fewer people were tested over the weekend. 

The Korea Disease Control and Prevention Agency(KDCA) said on Monday that 488 people were newly infected the previous day, raising the accumulated total to 123-thousand-728. 

About 57 percent of the 465 domestic cases were from the capital area, including 123 in Seoul, 129 in Gyeonggi Province and 14 in Incheon. 

One more death was reported, bringing the death toll to one-thousand-834. The fatality rate stands at one-point-48 percent. The number of critically ill patients fell by six to 164.

Meanwhile, concerns are rising over a possible vaccine shortage after more than three million people received their first dose of COVID-19 vaccines in a little over a month. 

The Central Disaster and Safety Countermeasures Headquarters said one-thousand-561 people received their first COVID-19 vaccine shots on Sunday, lifting the cumulative number of those who have begun their two-dose inoculation to around three-point-39 million, or six-point-six percent of the population. 

According to vaccine authorities on Monday, however, a considerable number of vaccine centers around the nation have limited new appointments or stopped them altogether for first-dose Pfizer inoculations. 

This comes after authorities informed local governments to refrain from scheduling appointments for first doses as there could be a supply shortage for second shots.

With the efficacy of COVID-19 vaccines also remaining a concern, an infectious disease expert says reaching herd immunity may be difficult.

Seoul National University professor Oh Myoung-don, who heads the country's central clinical committee on new infectious diseases, said at a press conference on Monday that COVID-19 is likely here to stay.

While herd immunity is believed to be attained when 70 percent of the population is vaccinated, Oh said that is not the case, noting there isn't a vaccine yet that is over 95 percent effective in preventing secondary transmissions.

The professor said the Pfizer vaccine's 95 percent efficacy refers to the prevention of contracting the virus and not the prevention of the spread.