American biotech firm Moderna has released early results from its phase III clinical trials, announcing in a press release on Monday (US time) that its COVID-19 vaccine has an efficacy of 94.5%, according to an “interim analysis” by an independent data and safety monitoring board.

Here’s how the percentage was calculated.

First, there are two groups in the study: the vaccinated group, who received the candidate vaccine, and the placebo group, who received a shot of salt water.

Among 30,000 trial participants, 95 people developed COVID-19 just in the one week after the final vaccination. But encouragingly, 90 of those positive cases occurred in the placebo group, and only five were in the vaccinated group.