Fig. 1
From: COVID-19 vaccine development: milestones, lessons and prospects
The milestones of COVID-19 vaccine development. With the maturity of vaccine platforms, more and more COVID-19 vaccines have entered clinical trials and been approved for emergency use in many countries. However, the appearance of VOCs has brought great challenges to existing COVID-19 vaccines. By changing the administration route, the protection provided by vaccines can be enhanced, and more vaccination strategies are applied to cope with VOCs. In addition, more vaccine development methods are applied, such as developing polyvalent vaccines and improving adjuvant and delivery systems. These enormous changes form a milestone in the COVID-19 vaccine progress compared with post-years
