Global smallpox cases from 1920 to 2010. Source: WHO (2011) Vaccine development and approval Just like any medication or procedure, no vaccine can be 100% safe or effective for everyone because each person's body can react differently. While minor side effects, such as soreness or low grade fever, are relatively common, serious side effects are very rare and occur in about 1 out of every 100,000<unk> s and typically involve allergic reactions that can cause hives or difficulty breathing. However, vaccines are the safest they ever have been in history and each vaccine undergoes rigorous clinical trials to ensure their safety and efficacy before approval by authorities such as the US FDA. Prior to human testing, vaccines are tested on cell cultures and the results modelled to assess how they will interact with the immune system. During the next round of testing, researchers study vaccines in animals, including mice, rabbits, guinea pigs, and monkeys. Vaccines that pass each of these stages of testing are then approved by the public health safety authority (FDA in the United States) to start a three-phase series of human testing, advancing to higher phases only if they are deemed safe and effective at the previous phase. The people in these trials participate voluntarily and are required to prove they understand the purpose of the study and the potential risks. During phase I trials, a vaccine is tested in a group of about 20 people with the primary goal of assessing the vaccine's safety. Phase II trials expand the testing to include 50 to several hundred people. During this stage, the vaccine's safety continues to be evaluated and researchers also gather data on the effectiveness and the ideal dose of the vaccine. Vaccines determined to be safe and efficacious then advance to phase III trials, which focuses on the efficacy of the vaccine in hundreds to thousands of volunteers. This phase can take several years to complete and researchers use this opportunity to compare the vaccinated volunteers to those who have not been vaccinated to highlight any true reactions to the vaccine that occur. If a vaccine passes all of the phases of testing, the manufacturer can then apply for license of the vaccine through the FDA. Before the FDA approves use in the general public, they extensively review the results to the clinical trials, safety tests, purity tests, and manufacturing methods and establish that the manufacturer itself is up to government standards in many other areas. However, safety testing of the vaccines never ends. After FDA approval, the FDA continues to monitor the manufacturing protocols, batch purity, and the manufacturing facility itself. Additionally, most vaccines also undergo phase IV trials, which monitors the safety and efficacy of vaccines in tens of thousands of people, or more, across many years. This allows for delayed or very rare reactions to be detected and evaluated. Side effects The Centers for Disease Control and Prevention (CDC) has compiled a list of vaccines and their possible side effects. The risk of side effects varies from one vaccine to the next, but below are examples of side effects and their approximate rate of occurrence with the diphtheria, tetanus, and acellular pertussis (DTaP) vaccine, a common childhood vaccine. Mild side effects (common) * Mild fever (1 in 4) * Redness, soreness, swelling at the injection site (1 in 4) * Fatigue, poor appetite (1 in 10) * Vomiting (1 in 50) Moderate side effects (uncommon) * Seizure (1 in 14,000) * High fever (over 105 °F) (1 in 16,000) Severe side effects (rare) * Serious allergic reaction (1 in 1,000,000) * Other severe problems including long-term seizure, coma, brain damage have been reported, but are so rare that it is not possible to tell if they are from the vaccine or not. Certain vaccines have had adverse outcomes identified after being used in mass<unk>  programs: *In 1976 in the United States, a mass a swine flu<unk>  programme was discontinued after 362 cases of Guillain–Barré syndrome among 45 million vaccinated people. William Foege of the CDC estimated that the incidence of Guillain-Barré was four times higher in vaccinated people than in those not receiving the swine flu vaccine. *Dengvaxia, the only approved vaccine for Dengue fever, was found to increase the risk of hospitalization for Dengue fever by 1.58 times in children of 9 years or younger, resulting in the suspension of a mass<unk>  program in the Philippines in 2017. *Pandemrix a vaccine for the H1N1 pandemic of 2009 given to around 31 million people was found to have a higher level of adverse events than alternative vaccines resulting in legal action. In a response to the narcolepsy reports following immunization with Pandemrix, the CDC carried out a population-based study and found the FDA-approved 2009 H1N1 flu shots were not associated with an increased risk for the neurological disorder. Ingredients The ingredients of vaccines can vary greatly from one to the next and no two vaccines are the same. The CDC has compiled a list of vaccines and their ingredients that is readily accessible on their website. Aluminium Aluminium is an adjuvant ingredient in some vaccines. An adjuvant is a type of ingredient that is used to help the body's immune system create a stronger immune response after receiving the<unk> . Aluminium is in a salt form (the ionic version of an element) and is used in the following compounds: aluminium hydroxide, aluminium phosphate, and aluminium potassium sulfate. For a given element, the ion form has different properties from the elemental form. Although it is possible to have aluminium toxicity, aluminium salts have been used effectively and safely since the 1930s when they were first used with the diphtheria and tetanus vaccines. Although there is a small increase in the chance of having a local reaction to a vaccine with an aluminium salt (redness, soreness, and swelling), there is no increased risk of any serious reactions. Mercury Certain vaccines once contained a compound called thiomersal or thimerosal, which is an organic compound containing mercury. Organomercury is commonly found in two forms. The methylmercury cation (with one carbon atom) is found in mercury-contaminated fish and is the form that people might ingest in mercury-polluted areas (Minamata disease), whereas the ethylmercury cation (with two carbon atoms) is present in thimerosal, linked to thiosalicylate. Although both are organomercury compounds, they do not have the same chemical properties and interact with the human body differently. Ethylmercury is cleared from the body faster than methylmercury and is less likely to cause toxic effects. Thimerosal was used as a preservative to prevent the growth of bacteria and fungi in vials that contain more than one dose of a vaccine. This helps reduce the risk of potential infections or serious illness that could occur from contamination of a vaccine vial. Although there was a small increase in risk of injection site redness and swelling with vaccines containing thimerosal, there was no increased risk of serious harm or autism. Even though evidence supports the safety and efficacy of thimerosal in vaccines, thimerosal was removed from childhood vaccines in the United States in 2001 as a precaution. Monitoring CDC Immunization Safety Office initiatives Government organizations Non-government organizations Vaccine Adverse Event Reporting System (VAERS) Food and Drug Administration (FDA) Center for Biologics Evaluation and Research (CBER) Immunization Action Coalition (IAC) Vaccine Safety Datalink (VSD) Health Resources and Service Administration (HRSA) Institute for Safe Medication Practices (ISMP) Clinical Immunization Safety Assessment (CISA) Project National Institutes of Health (NIH) Emergency preparedness for vaccine safety National Vaccine Program Office (NVPO) The administration protocols, efficacy, and adverse events of vaccines are monitored by organizations of the US federal government, including the CDC and FDA, and independent agencies are constantly re-evaluating vaccine practices. As with all medications, vaccine use is determined by public health research, surveillance, and reporting to governments and the public.
Vaccination
Vaccine
Immunization
Mass vaccination
Active immunization
Vaccine bundling
Booster dose
Therapeutic vaccines