Immunity

Immunity plays a huge role in the Young Dracula sequal The Gathering.

"So the girl..She's immune right? She's the cure and the key to end all vampires from existing" 

- Braig Tanaka questioning Maria's immunity to the vampires.

It''s exactly unknown what causes immunity to a human, but Maria Clarke is the only known person in this case. According to Vladimir Dracula, the one who infected her, believes she is a special case.''

Once a human is bitten by a vampire, they become a half-fang. The transformation can be immeaditly, hours or even days after infection. However, some have taken months to turn. But most are likely to die after turning. Maria was bitten almost a year and still didn't turn.

When Lisa Simpson did a test on her, the infection had disintergrated from her body, like it had completly vanished. So a chance of her turning is impossible. Apparently she is also immune to zombie bites.