1. World Health Organization, COVID-19 Weekly Epidemiological Update 23, 19. Jan. 2021. https://www.who.int/emergencies/diseases/novel-coronavirus-2019/situation-reports (2021)
2. World Health Organization, COVID-19 Weekly Epidemiological Update 47, 6 July 2021. https://www.who.int/emergencies/diseases/novel-coronavirus-2019/situation-reports (2021)
3. World Health Organization, COVID-19 Weekly Epidemiological Update 59, 28 Sept 2021. https://www.who.int/emergencies/diseases/novel-coronavirus-2019/situation-reports (2021)
4. O. Diekmann, J.A.P. Heesterbeek, Mathematical Epidemiology of Infectious Diseases (Wiley, Chichester, 2000)
5. P. van den Driessche, J. Watmough, Math. Biosci. 180, 29 (2002)