Ignoring statistical significance for the purposes of this discussion, the observed rate suggests that the vaccine is 90% effective at preventing severe infecti

Ignoring statistical significance for the purposes of this discussion, the observed rate suggests that the vaccine is 90% effective at preventing severe infections as well, not just mild infections. In a worse world it could have been the case that e.g. the vaccine was 90% effective at preventing infections but still had the same number of severe infections because it did nothing to help the most vulnerable patients.