Why did the epidemic last for four years?

Why did the epidemic last for four years? The epidemic broke out from the end of February 20 19 to the end of February 20 19, which was intermittent for four years. Now, due to the variation of Okmiqiong, it is highly contagious and more concealed. The situation is all an umbrella step and it is still grim. It is difficult to determine when it will end, so the epidemic has been going on for four years.