Because an effective vaccine would be really good for the economy so a positive market reaction indicates that people researching this stuff think the likelihoo

Because an effective vaccine would be really good for the economy so a positive market reaction indicates that people researching this stuff think the likelihood of an effective vaccine being ready in the near future is higher. So it's not cheering the market reaction, it's cheering the potential for an effective vaccine and the market reaction is just a signal that the vaccine is legit.