I read in a previous post (I believe on MFK) from someone who worked in the water treatment field and they were suggesting that chlorine takes a bit to have full effect at "killing" things...
I would imagine it is similar to UV lighting... It starts "attacking" instantly, but not evertyhing "dies" so easily... some things take a bit...
So I would suggest that both Chlorine and Chloramine 'instantly' start destroying your bacteria... but may not 'instantly' kill it...
But either way, using a proper dechlorinator (I also use Prime) is very wise...