Sforno on Numbers 17:13/ספורנו על במדבר י״ז:י״ג

1ויעמוד בין המתים ובין החיים, after he had already reached that spot he waited in order to help cure the ones who had taken sick but had not died yet. Instead of distancing himself from the congregation of sinners (verse 21) Aaron did the opposite in order to save whom he could save.

2ותעצר המגפה. Not a single Israelite was struck by the pestilence from that moment on.