As others have mentioned it depends. For instance the initial trials with pembrolizumab weren't the silver bullet the media makes them out to be today, until th

As others have mentioned it depends. For instance the initial trials with pembrolizumab weren't the silver bullet the media makes them out to be today, until they identified what features might common among patients that did see a response, e.g. mismatch repair deficient cancers, because the tumor needs to be spitting out enough of these tumor-specific neoantigens to be targted by the immune system. That generally happens with highly mutated tumors such as those found in mismatch repair deficient tumors. The reason why the immune system was not targeting the tumors like it should have given the neoantigens until you take the drug is because the tumor cells presents the correct receptors that the immune system expects from healthy cells, which prevents the immune response. When you block that receptor as these drugs do, and also have many of these tumor specific neoantigens being expressed, then the immune system is able to target the tumor cells for death and the drug works well.