South Korean authorities have sought to reassure people as the number who have died after being vaccinated against seasonal influenza rose to 13.
The doctor leading an inquiry into the deaths has said he does not believe the vaccine and fatalities are connected.
But trust in the vaccine programme has been dented, and the Korean Medical Association has called for it to be halted until its safety is assured.
So far around 13 million people in South Korea have received the flu jab.
The country launched a free flu vaccination programme for 19 million eligible people last month as the government tries to avoid a mass flu outbreak while it tackles Covid-19.
A 17-year-old boy who died two days after receiving a flu shot was the first to be noted by officials.
The other reported fatalities were all elderly. At least eight were in their 70s or 80s with underlying conditions.
Professor Kim Jun-kon, who is leading the damage investigation team, has tried to reassure South Koreans about the safety of the vaccination programme.
He said that the probe so far had concluded the deaths were not a result of the flu jab. Post-mortem examinations are under way and will also test for Covid-19.
This is the second time trust has been shaken in South Korea’s vaccination programme.
It was earlier suspended for three weeks after it was discovered that around five million doses, which need to be refrigerated, had been exposed to room temperature while being transported to a medical facility.
None of the people who have died received the recalled vaccine.
Professor Kim has recommended that the vaccination programme continue.