Oh, I misunderstood. My apologies.

Hepatitis B takes 60-90 days to incubate. Because of the slow incubation, the vaccine works best if started within 24 hours of exposure, and can still work if administered up to 7 days after exposure. Newborns are typically exposed during birth, not while growing in the womb.

Compare to, say, the COVID-19, where the incubation is 2-14 days. The vaccine takes about 10-14 days to set in, so if you take it post-exposure in most cases you’ll be done with the actual infection before the vaccine even starts working.