What TCRanch said... if your animals have been previously inoculated with Covexin-8, Cavalry (not Calvary... that's where Christ was crucified)-9 will serve just fine as the booster dose; no second dose of Cavalry-9 needed.
I've used both in the past, but felt like I got fewer or smaller 'knots' with Cavalry-9 than with Covexin-8.
Not picking on the Cavalry/Calvary thing... it's just a peeve... almost like fingernails grating on a chalkboard (dating myself there). I had to quit listening to Bill O'Reilly's audiobook of 'Killing Lincoln', about halfway through, because he kept talking about 'Stuart's Calvary', 'Custer's Calvary', etc. I just couldn't take it. I think there were enough complaints registered that he went back and re-recorded those to be 'Cavalry'
Cavalry - think 'cheval'- 'horse' in French, or 'caballo', 'horse in Spanish.