Population immunity is key to prevent outbreaks. Solves bioterror.

In low-income countries, infectious diseases still account for a large proportion of deaths, highlighting health inequities largely caused by economic differences. Vaccination can cut health-care costs and reduce these inequities. Disease control, elimination or eradication can save billions of US dollars for communities and countries. Vaccines have lowered the incidence of hepatocellular carcinoma and will control cervical cancer. Travellers can be protected against “exotic” diseases by appropriate vaccination. Vaccines are considered indispensable against bioterrorism. They can combat resistance to antibiotics in some pathogens. Noncommunicable diseases, such as ischaemic heart disease, could also be reduced by influenza vaccination. Immunization programmes have improved the primary care infrastructure in developing countries, lowered mortality in childhood and empowered women to better plan their families, with consequent health, social and economic benefits. Vaccination helps economic growth everywhere, because of lower morbidity and mortality. The annual return on investment in vaccination has been calculated to be between 12% and 18%. Vaccination leads to increased life expectancy. Long healthy lives are now recognized as a prerequisite for wealth, and wealth promotes health. Vaccines are thus efficient tools to reduce disparities in wealth and inequities in health. Eradication Unless an environmental reservoir exists, an eradicated pathogen cannot re-emerge, unless accidentally or malevolently reintroduced by humans, allowing vaccination or other preventive measures to be discontinued. While eradication may be an ideal goal for an immunization programme, to date only smallpox has been eradicated, allowing discontinuation of routine smallpox immunization globally. Potentially, other infectious diseases with no extrahuman reservoir can be eradicated provided an effective vaccine and specific diagnostic tests are available. Eradication requires high levels of population immunity in all regions of the world over a prolonged period with adequate surveillance in place.9 The next disease targeted for eradication is polio, which is still a global challenge.10 Although high coverage with oral polio vaccine (OPV) has eliminated type 2 poliovirus globally, transmission of types 1 and 3 continues in limited areas in a few countries. OPV-caused paralytic disease, directly or by reversion to virulence, and persistent vaccine-virus excretion in immunodeficient individuals are problems yet to be solved. Global use of monovalent type 1 and type 3 OPV and inactivated polio vaccine (IPV) may eventually be required.10 Elimination Diseases can be eliminated locally without global eradication of the causative microorganism. In four of six WHO regions, substantial progress has been made in measles elimination; transmission no longer occurs indigenously and importation does not result in sustained spread of the virus.11 Key to this achievement is more than 95% population immunity through a two-dose vaccination regimen. Combined measles, mumps and rubella (MMR) vaccine could also eliminate and eventually eradicate rubella and mumps.11 Increasing measles immunization levels in Africa, where coverage averaged only 67% in 2004, is essential for eradication of this disease. Already, elimination of measles from the Americas, and of measles, mumps and rubella in Finland has been achieved, providing proof in principle of the feasibility of their ultimate global eradication.12 It may also be possible to eliminate Haemophilus influenzae type b (Hib) disease through well implemented national programmes, as experience in the West has shown.13 Local elimination does not remove the danger of reintroduction, such as in Botswana, polio-free since 1991, with importation of type 1 poliovirus from Nigeria in 2004,14 and in the United States of America (USA) with measles reintroduced to Indiana in 2005 by a traveller from Romania.15 For diseases with an environmental reservoir such as tetanus, or animal reservoirs such as Japanese encephalitis and rabies, eradication may not be possible, but global disease elimination is a feasible objective if vaccination of humans (and animals for rabies) is maintained at high levels. Control of mortality, morbidity and complications For the individual Efficacious vaccines protect individuals if administered before exposure. Pre-exposure vaccination of infants with several antigens is the cornerstone of successful immunization programmes against a cluster of childhood diseases. Vaccine efficacy against invasive Hib disease of more than 90% was demonstrated in European, Native American, Chilean and African children in large clinical studies in the 1990s.13 In the United Kingdom, no infant given three doses developed Hib disease in the short-term (boosters may be required for long-term protection), and recent postmarketing studies have confirmed the high effectiveness of vaccination of infants against Hib in Germany and pertussis in Sweden.13,16,17 Many vaccines can also protect when administered after exposure – examples are rabies, hepatitis B, hepatitis A, measles and varicella.18 For society Ehreth estimates that vaccines annually prevent almost 6 million deaths worldwide.19 In the USA, there has been a 99% decrease in incidence for the nine diseases for which vaccines have been recommended for decades,20 accompanied by a similar decline in mortality and disease sequelae. Complications such as congenital rubella syndrome, liver cirrhosis and cancer caused by chronic hepatitis B infection or neurological lesions secondary to measles or mumps can have a greater long-term impact than the acute disease. Up to 40% of children who survive meningitis due to Hib may have life-long neurological defects.13 In field trials, mortality and morbidity reductions were seen for pneumococcal disease in sub-Saharan Africa and rotavirus in Latin America.21,22 Specific vaccines have also been used to protect those in greatest need of protection against infectious diseases, such as pregnant women, cancer patients and the immunocompromised.