Ministry of Foreign Affairs of the Republic of Bulgaria:
https://www.mfa.bg/en/customnews/main/24737
EASA:
https://www.easa.europa.eu/the-agency/coronavirus-covid-19
European Commission, DG "Mobility and Transport":
ICAO:
https://www.icao.int/covid/Pages/default.aspx
EUROCONTROL:
https://www.eurocontrol.int/covid19
https://www.iata.org/en/youandiata/travelers/health/
World health organisation (WHO)
https://www.who.int/emergencies/diseases/novel-coronavirus-2019