is vaccinated, herd immunity takes effect, decreasing risk to people who cannot receive vaccines because they are too young or old, immunocompromised, or have severe allergies to the ingredients in the vaccine. The outcome for people with compromised immune systems who get infected is often worse than that of the general population. Cost-effectiveness Commonly used vaccines are a cost-effective and preventive way of promoting health, compared to the treatment of acute or chronic disease. In 2001, the United States spent approximately $2.8 billion to promote and implement routine childhood immunizations against seven diseases. The societal benefits of those vaccinations were estimated to be $46.6 billion, yielding a benefit-cost ratio of 16.5. Necessity When a vaccination program successfully reduces the disease threat, it may reduce the perceived risk of disease as cultural memories of the effects of that disease fade. At this point, parents may feel they have nothing to lose by not vaccinating their children. If enough people hope to become free-riders, gaining the benefits of herd immunity without vaccination, vaccination levels may drop to a level where herd immunity is ineffective. According to Jennifer Reich, those parents who believe vaccination to be quite effective but might prefer their children to remain unvaccinated, are those who are the most likely to be convinced to change their mind, as long as they are approached properly. Safety concerns While some anti-vaccinationists openly deny the improvements vaccination has made to public health or believe in conspiracy theories, it is much more common to cite concerns about safety. As with any medical treatment, there is a potential for vaccines to cause serious complications, such as severe allergic reactions, but unlike most other medical interventions, vaccines are given to healthy people and so a higher standard of safety is demanded. While serious complications from vaccinations are possible, they are extremely rare and much less common than similar risks from the diseases they prevent. As the success of immunization programs increases and the incidence of disease decreases, public attention shifts away from the risks of disease to the risk of vaccination, and it becomes challenging for health authorities to preserve public support for vaccination programs. The overwhelming success of certain vaccinations has made certain diseases rare, and, consequently, has led to incorrect heuristic thinking in weighing risks against benefits among people who are vaccine-hesitant. Once such diseases (e.g., Haemophilus influenzae B) decrease in prevalence, people may