How low is a child’s chance of chronic HBV without vaccination?

For a child whose mother is not a hepatitis B carrier, the chance of developing chronic HBV without vaccination is very low.

The risk depends entirely on whether the mother carries the virus. In a low-endemicity country like the US, about 0.5-1% of pregnant women are HBsAg-positive [1]. For a child of an HBsAg-negative mother, the chance of ever being exposed to HBV in childhood is small, and even if exposed, the risk of becoming a chronic carrier depends on age at infection. Infected as a newborn, about 90% become chronic carriers. Infected as a toddler, about 30% do. Infected as an older child or adult, about 5% do [3].

But the absolute risk for a child of a negative mother is the product of two small numbers: the chance of encountering the virus multiplied by the chance of chronicity if exposed. In a country with universal infant vaccination, circulation is low, so exposure is rare. The retrieved evidence does not give a single number for this combined risk, but it is clearly very small.

The retrieved evidence does not include a modern prospective study of unvaccinated children of negative mothers in a low-endemicity country. That study would settle the question. What exists are older serosurveys from high-endemicity settings where 60% of infants had HBV markers by age 1 [2], but those numbers reflect widespread maternal carriage, not the risk to a child of a negative mother.

My call: for a child of an HBsAg-negative mother in a low-endemicity setting, the chance of chronic HBV without vaccination is well under 1 in 100, likely far lower. Confidence: moderate, limited by the absence of a direct modern study.