Can the hepatitis b vaccine alone (first shot for an unvaccinated person), given 48 hours post-exposure, prevent infection?

Not likely: If you have been definitively exposed to Hep B then will need HBIG/hep B immune globulin as well as the hep B vaccine. The immune globulin will act like a sponge and soak up any hep B virus present currently in your body. The hep B vaccine then stimulates your body to produce antibodies, which will then attach to and disable the viral particles. This process takes 2-4 weeks to make antibodies.