“…Moreover, a great effort has been put forward to develop vaccine candidates using various approaches, including classical (inactivated, live-attenuated, recombinant vaccines) and more innovative (mRNA, DNA, adenoviral vector vaccines) ones. The COVID-19 vaccines were made available with unseen speed due to years of basic and applied research, technological advances and platforms that enable the rapid development of candidates (e.g., mRNA), significant funding, running multiple trials in parallel, and regulatory agencies working at an extraordinary pace [ 2 ]. Until the end of 2021, four COVID-19 vaccines were approved in the European Union: two mRNA vaccines, BNT162b2 (BioNTech/Pfizer, Germany, Mainz/New York, NY, USA) and mRNA-1273 (Moderna, Cambridge, MA, USA), given as two doses 21 and 28 days apart, respectively, as well as two adenoviral vector vaccines: AZD1222 (Oxford/AstraZeneca, UK/Sweden), administered as two doses 4–12 weeks apart, and Ad26.COV2.S Janssen/Johnson & Johnson, Leiden, Netherlands/New Brunswick, NJ, USA), given as a single dose.…”