Impact of conjugate 7-valent vaccination in Belgium: addressing methodological challenges., Hanquet, Germaine, Lernout Tinne, Vergison Anne, Verhaegen Jan, Kissling Esther, Tuerlinckx David, Malfroot Anne, Swennen Béatrice, and Sabbe Martine , Vaccine, 2011 Apr 05, Volume 29, Issue 16, p.2856-64, (2011)