. By contrast, infections in older children become chronic only around 30% of the time, and in adults only 5%.
In 1983, these latter two treatments were still under investigation, but at twelve months of age, and again around his second birthday, Chan’s son tested negative for HBV. Three years later, her daughter received the same treatment and likewise avoided infection. The success of vaccination so far is, however, with regional variations, thanks largely to a global immunization programme in which infants are given an HBV vaccine along with inoculations against four other diseases. Crucially, such immunization commences at 6–8 weeks of age — too late to reverse an infection seeded at birth.
The key issue now, if the world is to respond effectively to the evolving epidemiology of HBV, is implementation. There are logistical and financial challenges to delivering MTCT prevention measures, and the unmet need is often most acute in countries with constrained resources. “Hepatitis B is endemic in low- and middle-income countries, more than it is in higher-income countries,” says Chloe Thio, an immunologist at the Johns Hopkins University School of Medicine in Baltimore, Maryland.
Maternal viral load is important. HBV infections can be either dormant, with low circulating virus levels, or active, generating abundant blood-borne virus particles. It is now established that perinatal MTCT occurs predominately in mothers with active infections and viral loads of greater than 200,000 international units per millilitre. Now, vaccination at birth is recommended for all newborns by the WHO.