Plans-Rubió 50% (95% CI: 28%–65%), and mortality by 68% (95% CI: 56%–76%) (Mullooly et al 1994). Several case-control studies have shown the effective- ness of inﬂuenza vaccination to prevent hospitalizations. One study carried out in the UK estimated that vaccination reduced inﬂuenza, pneumonia, bronchitis, and emphysema by 63% (Ahmed et al 1997). Another study carried out in Spain showed that vaccination reduced serologically conﬁrmed pneumonia by 79% (Ohmitt and Monto 1995). One major factor inﬂuencing on the vaccine effectiveness is the antigenic similarity between vaccine viral strains and circulating strains (Edwards et al 1994; Nichol et al 1995; Grotto et al 1998; Pyhala et al 2001). When vaccine and circulating viruses are well matched, vaccine can reduce laboratory-conﬁrmed inﬂuenza illness in adults aged <65 years by 70%–90% (Demichelei et al 2000), but in one study carried out when vaccine and circulating strains were not well matched, laboratory-conﬁrmed inﬂuenza illness was reduced by 52% in healthy persons and by 38% in patients with high-risk conditions (CDC 2006). In another study in persons with high-risk conditions, vaccine effectiveness ranged from 43% to 56% when antigens from vaccine and circulating strains were similar and from 21% to 42% when they were not well matched (Nichol et al 1998). In working population, inﬂuenza vaccination can reduce upper respiratory illness by 25%, laboratory-conﬁrmed inﬂu- enza illness by 86%–88% and febrile respiratory illness by 29%–34%. Physician visits and illness-related work absentee- ism can be reduced by 42% and 32%–52%, respectively (Patri- arca et al 1986; Mullooly et al 1994; Demicheli et al 2000). Children aged 6 months can develop protective levels of anti-inﬂuenza antibody against speciﬁc inﬂuenza virus strains after inﬂuenza vaccination. One study in children with asthma aged 2–6 years and 7–14 years obtained a vaccine efﬁcacy of 22%–54% and 60%–78%, respectively (Sugaya et al 1994). A 2-year randomized study of children aged 6–24 months determined that 89% of children seroconverted to all three vaccine strains during both years (Hoberman et al 2003). Other studies report that inactivated TIV decreases the incidence of inﬂuenza-associated otitis media among young children by approximately 30% (Heikkinen et al 1991; Cle- ments et al 1995). Cost-effectiveness of inﬂuenza vaccine All evaluative studies assessing the cost-effectiveness of inﬂuenza vaccination obtained favorable cost-effectiveness ratios or cost savings (Nichol et al 2003; Rothberg 2005). 48 In one 6-year study of elderly, members of a managed care organization obtained a net cost saving of US$73 per vacci- nated person (Nichol et al 1998). An evaluative study carried out in The Netherlands obtained a cost-effectiveness ratio of €1820 per life year gained in elderly persons and cost savings in chronically ill elderly persons (Postma et al 1999). In one study assessing the cost quality adjusted life year gained (QALY) that included all age groups, cost savings were obtained in persons aged 65 years, while in younger individuals the cost-effectiveness ratio ranged from US$23 to US$256 per QALY (Bridges et al 2000). Cost-effectiveness of inﬂuenza vaccination is lower in healthy individuals aged <64 years because healthcare costs from inﬂuenza are lower in younger individuals. Inﬂuenza vaccination can reduce healthcare costs with inﬂuenza illness. Economic studies of inﬂuenza vaccination of persons aged 65 years conducted in the US have reported overall societal cost savings and substantial reductions in hospitalization and death (Harper et al 2005). Studies of adults aged <65 years have reported that vaccination can reduce both direct medical costs and indirect costs from work absenteeism (Patriarca et al 1986; Mullooly et al 1994; Gross et al 1995; Bridges et al 2000; Nichol 2001). Reductions of 13%–44% in healthcare provider visits, 18%–45% in lost workdays, 18%–28% in days working with reduced effec- tiveness, and 25% in antibiotic use for inﬂuenza-associated illnesses have been reported (Patriarca et al 1986; Mullooly et al 1994; Demicheli et al 2000; Nichol et al 2003, 2004). The cost-effectiveness of inﬂuenza vaccination depends on the following factors: 1) efﬁcacy and effectiveness of the vaccine, 2) incidence of inﬂuenza, 3) vaccination costs, and 4) healthcare costs from inﬂuenza. A higher inﬂuenza vaccine efﬁcacy and effectiveness is associated with a higher reduction in healthcare costs and a higher cost-effectiveness. For this reason, in evaluative studies taking into account results obtained in clinical trails and cohort studies, the cost-effectiveness is higher when vaccine and circulating viral strains are well matched. Vaccine-related reduction in healthcare costs from inﬂuenza is higher when attack rate and healthcare costs per case are higher. One cost-effectiveness analysis estimated a cost of US$60 to US$4000 per illness averted among healthy persons aged 18–64 years, depending on inﬂuenza attack rates and vaccine effectiveness (Demicheli et al 2000). Cost-effectiveness of inﬂuenza vaccination depends also on vaccination costs. In one evaluative study assessing the cost-effectiveness of vaccinating elderly people in a low cost vaccination centre, a net cost saving of US$117 per individual was obtained (Nichol et al 1998). International Journal of COPD 2007:2(1)