The infection is also causing the Serbian government to tighten entry regulations. From December 20th, only travelers with a negative PCR test will be allowed into the country. 

This must not be older than 48 hours. Serbian citizens and people with a Serbian residence permit can enter the country without a test, but have to go into a 10-day quarantine. You can test yourself from this if you undergo a corona test before the quarantine period expires and it turns out negative. This does not apply to drivers who work in international passenger and freight transport, as well as children up to the age of 12, provided the accompanying person has a valid test. Transit travelers and flight attendants are excluded from this, the transit is limited to a maximum of twelve hours.