It means it didn't help people who already had severe symptoms, but might help people if it is given to them earlier.

This is actually quite well known when treating the flu with anti-virals. My doctor has said in previous years, if I wake up with full body ache and feel like it's the flu, come in immediately and he'll prescribe anti-virals. He said if I wait, there's no point, though.

Makes sense the anti-virals might only be enough to stop viral replication if given early on before it's all over the place.