South Korean authorities are investigating the deaths of two people, both with pre-existing conditions, who died within days of receiving AstraZeneca’s COVID-19 vaccine.

AstraZeneca said it was aware of the investigation by the Korea Disease Control and Prevention Agency (KDCA), but the safety of the vaccine has been extensively studied in clinical trials with data confirming it is generally well tolerated.

“KDCA is conducting epidemiological surveys with relevant local authorities...to confirm any link with inoculation,” KDCA Director Jeong Eun-kyeong told a briefing.

But it did not yet confirm any causal relationship to the vaccine.

A 63-year-old nursing home patient with cerebrovascular disease, developed symptoms including high fever, after being given the vaccine four days ago, Jeong said.

The man was moved to a larger hospital on Tuesday, but died after showing symptoms of blood poisoning and pneumonia, Yonhap news agency reported.

Another nursing home patient in his 50s with a cardiac disorder and diabetes died on Wednesday after suffering multiple heart attacks, having received the vaccine a day earlier, the agency said.

Jeong said there were no cases of fatalities from receiving COVID-19 vaccines developed by AstraZeneca or Pfizer/BioNTech. However, did urge people to take the shot when they are feeling in good health.

The KDCA said that out of the people who had received the coronavirus vaccines, 207 had adverse reactions, including three cases of severe allergic reactions, known as anaphylaxis.

South Korea began vaccinating its population last week. By Tuesday midnight, 85,904 people had received the first doses of the AstraZeneca vaccine and 1,524 had been given Pfizer shots, KDCA said in a statement.

South Korea reported 444 new confirmed COVID-19 cases on Tuesday, up from 344 on Monday, raising the country’s tally to 90,816 infections, with 1,612 deaths.

The agency earlier said it would provide compensation of over 430 million won ($383,500) for deaths from the COVID-19 vaccine.