First, a disclaimer: I know nothing about this specific trial, so what follows is general.
There are a couple of mechanisms for closing trials early.
An independent datamonitoring committee normally oversees this.
They will see safety reports coming in and act to stop a trial if safety data shows unexpectedly poor results. That can be permanent or temporary whilst investigated. You may recall this happening for a couple of the vaccine trials.
Secondly, an interim readout may be taken. This has to be specified in advance. The trial will be designed to take, say 200 patients, and an assumption on how effective the drug is dictates that size. For our 200 patient study, let's say that's designed to reliably show a 60:40 ratio of outcome in favour of the drug over the placebo. So we get 60 of 100 pts improve on drug, 40 of 100 on placebo; that shows the drug is effective, and the numbers are large enough we can be sure is not just a random effect.
But what if the drug is far more effective than placebo? Maybe 90:10 rather than 60:40? If the investigators think that's likely they will specify an interim analysis, which means results will be available much earlier. Lets' do that for just 40 pts. We'd see roughly 18 of 20 improve on the drug, but just 2 of 20 on placebo. Those numbers are obviously meaningful and the drug is clearly effective.
Now, it's not just unnecessary to continue the trial, it's unethical, as we're giving patients a useless placebo when we know we can save their lives with the drug.
Hope that makes sense.