Hilleman had for some time been pursuing his own investigations into a possible hepatitis B vaccine (Hilleman et al., 1975), and quickly secured several additional patents for improvements on Blumberg and Millman's method of purifying HBsAg (e.g. Bertland et al., 1977, McAleer and Wasmuth, 1977), thereby consolidating Merck's control over the final product. Here, PPIB is linked to hepatitis A virus infection.