The reaction depends on the cat.
I have chosen never to give more than one shot per visit. I'd rather drag them back in to get the other shot.

Now, kittens can have a problem with their sugar level if they don't eat.
Nutrical is good.
I would try putting a small finger full of Karo syrup (light) on the roof of their mouths. (please check with your vet before taking any of my advice)
The fosters in my group do that and it peps them up and gives them appetite, if the sugar level had gotten too low from not eating.

My Snowy had a reaction and vet said give (some portion of) St. Joseph's baby aspirin. Snowy was trembling. Maybe he was feverish?

Mr. Jones goes into a deep funk for two days.

Bella had no reaction.