In Northern California, 2,024 children younger than 5 lived with someone who had an active hepatitis B infection—and 58.2% of those children had an infected household contact who was not their birth mother.
The finding points to a gap in how families may think about hepatitis B exposure. The virus, known as HBV, can spread from any person with an active infection, not only from a mother to her baby at birth. Knowing this matters because children may share a home with relatives or other household members whose infection status is not known.
Karen Jacobson, a physician and public health researcher with Kaiser Permanente’s Division of Research in Pleasanton, California, and her colleagues reviewed household health records in Kaiser Permanente Northern California. Their retrospective study included 158,169 children under age 5, living in 131,800 households that had at least one other Kaiser Permanente member. The researchers counted children who were younger than 5 on June 16, 2023.
Among those households, 1,689 had a person with active HBV. In total, 2,024 children—1.3% of all children in the study—lived in a household with someone who had an active infection. Of those exposed children, 41.8% were exposed at birth, while 58.2% were exposed through another infected household member.
The study also found differences between infected mothers and other infected household contacts. The nonmaternal contacts were older on average: 44 years compared with 38 years for HBV-positive mothers. More than three-quarters of the nonmaternal contacts, 76.5%, were male.
Vaccination patterns varied, too. Every child exposed at birth received a hepatitis B vaccine within seven days of birth. The same was true for 93.5% of children exposed through another household member and 86.7% of children who were not exposed to an infected household member. These figures describe the children in this study; they do not show whether any child became infected.
Jacobson said the results underline why protection at birth can matter even when a baby’s mother is not known to have HBV. “A child can be exposed to hepatitis B from anyone with an active infection, not just their mother,” she said. She added that families cannot know the infection status of everyone a child may encounter, and described the vaccine as a safe and effective way to protect against possible sources of transmission.
The findings were presented at the annual meeting of the Infectious Diseases Society of America, known as IDWeek, held Oct. 21 to 24 in Washington, D.C. The study does not establish how infections occurred or measure vaccine effectiveness. But it offers a clear reminder: household exposure can come from beyond the birth parent, and early vaccination is one way to protect infants from risks that may not be visible to their families.