From 8 February 2021, persons who are allowed to enter Switzerland from Serbia must announce their arrival at the following website https://swissplf.admin.ch/home, only if they travel by airplane, bus, train or ship. All air travellers flying to Switzerland from Serbia must present a negative PCR test from the above mentioned date.

All travellers coming from a State or area with a high risk of infection (https://www.bag.admin.ch/bag/de/home/krankheiten/ausbrueche-epidemien-pandemien/aktuelle-ausbrueche-epidemien/novel-cov/empfehlungen-fuer-reisende/quarantaene-einreisende.html#-2060676916) any time during the 10 days prior to their entry into Switzerland are required to announce their arrival to the cantonal authorities at https://swissplf.admin.ch/home. They must present a negative PCR test carried out within 72 hours prior to entry into Switzerland and are required to stay in quarantine for a period of 10 days. The person under quarantine may, at the earliest on the 7th day of quarantine, be re-tested and the quarantine can be terminated.
At the moment, Republic of Serbia is not on the list of State or area with a high risk of infection.