The negative effects are compounded with each additional injection. Keep in mind the huge amount of variability in the injections themselves, from one manufacturer to another, and one batch to another within a single manufacturer.

This was intentional for "plausible deniability", among other things. If the negative effects of the injections were consistent and obvious, even the sheep would have been able to figure things out for themselves... which would have ruined the evil plans of the fascists.

Remember, the repeated periodic injections were supposed to be mandatory for EVERYONE - no exceptions, except for the fascist leadership themselves (look back to the groups that were quietly exempted from the mandatory injections).

The cumulative damage of the periodic boosters would have significantly shortened most people's lifespans, plus paved the way for digital injection ID's, central bank digital currency, and social credit score systems. High tech digital enslavement, exactly like current communist China, which is the model they wanted to follow for the whole world.   So, the fact that he only got the first injection reduced his own risk - plus he was smart enough not to get any of the "boosters". As mentioned in another comment, his one injection makes him more "relatable" to a huge number of people that also voluntarily got themselves injected, which is a good thing politically.