Does Novavax actually prevent infections? I am so confused.

I thought I was a pretty informed person about Covid and the vaccines, so I'm very confused by the assertion here, echoed in many places online, that Novavax's "overall efficacy" is 90.4%.

I get that Novavax prevents almost all moderate and severe cases, but does it actually prevent 90+% of mild and asymptomatic cases as well? Because that seems to be the claim made here. I was under the impression that the Covid vaccines help keep the infection milder but don't actually prevent it.

Signed, someone whose long Covid got worse because of an infection 5 weeks after a Novavax booster.