Ratten will sit out Saturday’s contest against Gold Coast at Marvel Stadium with Saints senior assistant Brendon Lade to fill the void.
Michael Voss (Carlton), Justin Longmuir (Fremantle) and Ben Rutten (Essendon) have all been forced to isolate for games during the first four rounds.
All three of those teams won when their coaches were watching from home in what the Saints will take as a promising sign of still being able to defeat the improving Suns.
“Brett undertook a rapid antigen test (on Monday afternoon) after developing minor symptoms at home that morning and it returned a positive result,” St Kilda’s football boss David Rath said.
“Pleasingly, he is feeling well and early signs are pointing to him returning to our program once his isolation ends.
“In the meantime, we’re well placed in that we have a very knowledgeable and experienced coaching team who will be able to physically step in while Brett is away from the club.
“Brett will still be a big part of our preparation for the game and we’re looking at ways we can involve him on Saturday, but Brendon is more than capable to lead the charge.”
After losing their opening-round encounter against Collingwood, St Kilda have won their past three games to rise up to fourth on the ladder.