Those (spike protein) are the tests that are readily available, although they do have others that target other relatively stable portions of the virus. In fact

Those (spike protein) are the tests that are readily available, although they do have others that target other relatively stable portions of the virus. In fact the way that the "English'-B.1.1.7 and "South African" B.1.351 were originally discovered (before sequencing) was that they didn't replicate one of the PCR primers, but they are still targeting the functional element, which is required for replication/infection. There are lots of things your body might respond to and even effectively fight off a COVID infection, it's just that not all of them are particularly stable. If you go to Nextstrain you can actually see which protein the different mutations are in (S is for spike). They're all over the place, but many change so much that you could have multiple strains within a single infection. RNA viruses are not very stable. Only the required functionality of the spike provides a relatively stable attack point. spike (S), envelope (E), membrane (M), nucleocapsid (N) https://nextstrain.org/ncov/global https://nextstrain.org/ncov/global