There are two ways to do clinical trials on vaccine effectiveness. One is to vaccinate someone and then deliberately infect them; this gives you good data quickly, but due to the deadliness of COVID-19, this is generally regarded to not be an option. Another is to vaccinate a very large number of people and wait to see if they get sick with the virus from natural exposure. This is ongoing right now on a very large scale; millions of people have been vaccinated, and sooner or later people will come in contact with these new variants. However, it takes time to collect enough data with this approach to provide useful results, and in this case it can be confounded by the lack of a control group (you don't know how many exposure events happened that did not lead to infections).

Regardless, the current vaccination campaign is highly effective against the original - still very widespread and deadly - virus, so it's worth getting it, and as people get vaccinated with it, it will eventually be apparent whether it works against various variants as well. If it doesn't, it's likely that a booster vaccination can be developed for these variants (and indeed, Moderna has begun work, as an insurance policy, on a booster for the south african variant).