I have little understanding of this but someone who actually works on vaccines told me this theory: Your body upon coming in contact with the vaccine starts to product antibodies (the first shot). This is generally enough for a normal immune system to fight the disease if in the future the body was to come in contact with the virus. The second shot is for hyper immunity after 4 weeks. What it does is that it basically tells your body make a lot more antibodies now and the second shot is actually a booster shot. So every time you take a shot afterwards (like in the middle of a pandemic) your body produces an abundance of antibodies and will likely fend off the virus much more easily than if there was no booster dose of any kind and just a vaccine shot. This is important because simply protecting your body from the virus is not enough since you could be carrying the virus and spreading it with mild infection which will not be a problem for you but will not protect others and so the hyperimmunity is important for preventing spread during a wave. I won’t be surprised if this one is a yearly dose as you need to be hyperimmune to not show symptoms and spread.