The Lancet
May 02, 2009  Volume 373  Number 9674  Pages 1495 – 1576
http://www.thelancet.com/journals/lancet/issue/current

Immunisation timing: the protective layer in vaccine coverage
Jim P Buttery, Stephen M Graham
Vaccination coverage has long been a traditional measure of the success and efficiency of immunisation programmes. Encouragingly, crude coverage rates, measured as three doses of diphtheria, tetanus, and pertussis vaccine by 1 year of age, have risen in most surveys over the past two decades. However, populations at greatest risk of vaccine-preventable disease are least likely to be immunised. Timely immunisation is crucial for direct protective effects during infancy and early childhood, when disease and mortality are highest.