There is a constant learning curve with COVID. But the human immune system is something that has been very well-studied. Vaccines all work in a similar way. They induce an immune response so the body develops antibodies to a pathogen, and those antibodies provide future immunity or protection when that pathogen is encountered again. It’s not like the relevant aspects of human cellular biology and the human immune system are these big, unstudied mysteries. Scientists know how it works down to the molecular level. They know a lot about viruses and how viruses work. So, in the development of the COVID vaccine, it’s not like they were working with total blank slates of ignorance. They were trying to find the best way to deal with a specific virus and develop a specific vaccine, but neither viruses, nor vaccines, nor human bodies are these vast unknown realms. We have been vaccinating and studying the effects of vaccines on people for over two centuries now.
The specific COVID vaccines were extensively studied and tested in carefully conducted human trials. The side effects are known. The vaccines have been certified as safe and effective by the agencies whose job it is to require rigorous documentation of the testing and quality control. Where scientists are still learning things is when it comes to questions about how effective the vaccine is against new variants, how long its immunity lasts, how well it curbs spread in populations under certain parameters.
But none of what they learn in those areas is going to change the basic idea that the COVID vaccine protects you against getting infected and in the chance that you get infected anyway, reduces the severity of your symptoms. It’s not going to change the fact that billions of people have been safely vaccinated and dangerous side effects are extremely rare and completely treatable. That evidence is already well-documented and this idea that “scientists just don’t know” is fear-mongering, not fact.