You will still catch it, you'll just have the antibodies to fight it off before you spread it. The old vaccines would harvest bits of the virus, inject its remains into your body, and then let your immune system work out how to kill it. That takes a lot of the dodgy science out of the equation, since you're just introducing one bit of biology into another bit of biology, and letting nature take care of the science.
These new vaccines don't use the actual virus (which by pure chance makes them far cheaper and easier to roll off a production line, nothing to do with profit, just pure chance that that's the way it is). So they instead inject bits of its genetic code into your cells, your cells then start making the bits of the virus which correspond to that bit of code, and again your body figures out how to kill it. This requires more difficult science, an understanding of genetics, and protein synthesis, and trying to predict what will happen from the most basic building blocks possible. If the bit of genetic code they've picked out for this strain is (1) actually critical to fighting it off, and (2) common amongst all strains, then you've solved the problem. Otherwise you've done nothing but figure out a way to make lots of money for about the next year or so. I would bet on the money.
Also lots of people don't like the idea of having genetic code inserted into their cells, and having their body synthesise proteins selected by a virologists.