Before Syd puts the virus into Edward Porris's lip, we see the plunger of the syringe has been pulled back a bit, as it would be had Syd filled it with the virus. However, in the close up of the needle piercing the skin, you can see the plunger is all the way down, so when it was stuck into Edward's lip there was nothing in it.
When Syd injects himself, he taps on his arm, like one would do when they are trying to make a vein pop out. He doesn't inject into a vein, so tapping on his arm is pointless and does absolutely nothing.