China must already have mass immunised and had the vaccine long ago for their country to be how it is now. But they just hid it from the world.
Please don't spread these dangerous, totally groundless conspiracy theories. It's genuinely damaging and based on zero facts or evidence.
China does not have some secret vaccine for a virus no one even knew existed two years ago. If you'd ever worked in virus research or knew even the slightest thing about how scientific research actually works, you'd know that what you're describing is physically impossible.
China is "back to normal" because they locked down immediately and locked down so tightly. (Meanwhile the British government encouraged people to eat in restaurants to help the economy, allowed a huge range of shops to stay open, had barely any enforcement of restrictions, and lots of British people flat out refused to do even basic tiny things like wear a mask.) Plus the Chinese government downplays the death rate and is probably not reporting figures accurately.
Other countries like Taiwan and South Korea have had success in containing the virus, are they supposedly in on the whole 'secret vaccine' thing too? Do the conspiracy theorists have any answer as to how a conspiracy that many millions of people would have to be in on was kept secret (the zillions of Westerners living in China prior to the pandemic just never noticed a mass government mandatory vaccine plot taking place under their very eyes? None of the Chinese journalists and citizen journalists and bloggers who have gone to prison for reporting this ever bothered to mention it?) considering how many conspiracies that only a handful of people knew about, have failed and been exposed?