comp rom ised , ▁or ▁have ▁severe ▁all erg ies ▁to ▁the ▁ing red ients ▁in ▁the ▁v acc ine . ▁The ▁outcome ▁for ▁people ▁with ▁comprom ised ▁imm une ▁systems ▁who ▁get ▁inf ected ▁is ▁often ▁worse ▁than ▁that ▁of ▁the ▁general ▁population . ▁ ▁Cost - effect iveness ▁ ▁Comm only ▁used ▁v acc ines ▁are ▁a ▁cost - effect ive ▁and ▁prevent ive ▁way ▁of ▁prom oting ▁health , ▁compared ▁to ▁the ▁treatment ▁of ▁ac ute ▁or ▁chron ic ▁disease . ▁In ▁the ▁US ▁during ▁the ▁year ▁ 2 0 0 1 , ▁routine ▁child hood ▁imm un izations ▁against ▁seven ▁dise ases ▁were ▁estimated ▁to ▁save ▁over ▁$ 4 0 ▁billion ▁per ▁birth - year ▁coh ort ▁in ▁overall ▁social ▁costs , ▁including ▁$ 1 0 ▁billion ▁in ▁direct ▁health ▁costs , ▁and ▁the ▁soci etal ▁benefit - cost ▁ratio ▁for ▁these ▁v acc in ations ▁was ▁estimated ▁to ▁be ▁ 1 6 . 5 . ▁ ▁N ec ess ity ▁When ▁a ▁v acc ination ▁program ▁successfully ▁reduces ▁the ▁disease ▁threat , ▁it ▁may ▁reduce ▁the ▁perce ived ▁risk ▁of ▁disease ▁as ▁cultural ▁mem ories ▁of ▁the ▁effects ▁of ▁that ▁disease ▁fade . ▁At ▁this ▁point , ▁parents ▁may ▁feel ▁they ▁have ▁nothing ▁to ▁lose ▁by ▁not ▁v acc in ating ▁their ▁children . ▁If ▁enough ▁people ▁hope ▁to ▁become ▁free - rid ers , ▁gain ing ▁the ▁benefits ▁of ▁her d ▁imm unity ▁without ▁v acc ination , ▁v acc ination ▁levels ▁may ▁drop ▁to ▁a ▁level