BELGRADE, Serbia — A prominent Serbian epidemiologist has warned that soaring coronavirus infections could jeopardize the Balkan nation’s vaccination drive, which has been among the fastest in Europe.

Dr. Mijomir Pelemis told state RTS television on Wednesday that as more people get infected with the virus they cannot be vaccinated, which could slow down the process.

Serbia has introduced a lockdown until the end of the week, closing down all bars, restaurants and non-essential shops and businesses. The country of 7 million people has confirmed more than 5,000 new cases on Tuesday, which was the highest number in months and more than double than the daily cases reported in early March.

Health authorities in Croatia also reported reported a huge jump in new daily cases. They said Wednesday that 1,445 new cases were confirmed in 24 hours, compared to an average of several hundred in previous weeks. A 12-year-old girl with heart problems was among the dozen people who died, officials said.

Coronavirus infections have soared in the past days in neighboring Bosnia as well.

Serbia has vaccinated nearly 800,000 people with both doses and around 500,000 have received a dose.