1. Department of Biostatistics; Harvard University; Boston, Massachusetts, 02115 U.S.A.
2. Department of Preventive Medicine; Northwestern University; Chicago, Illinois 60611 U.S.A.
3. Department of Health Research and Policy; Stanford University; Stanford, California, 94305 U.S.A.
4. Division of Cardiovascular Medicine; Brigham and Women's Hospital; Boston, Massachusetts, 02115 U.S.A.
5. GlaxoSmithKline Vaccines; 89 Rue de I'Institut, 1330 Rixensart Belgium