Vaccine hesitancy

SOURCE:  Wikipedia, captured 2020-06-16

Vaccine hesitancy, also known as anti-vaccination or anti-vax [anti-vaxx], is a reluctance or refusal to be vaccinated or to have one's children vaccinated against contagious diseases despite the availability of vaccination services. It is identified by the World Health Organization as one of the top ten global health threats of 2019. The term encompasses outright refusal to vaccinate, delaying vaccines, accepting vaccines but remaining uncertain about their use, or using certain vaccines but not others. Arguments against vaccination are contradicted by overwhelming scientific consensus about the safety and efficacy of vaccines.

Hesitancy primarily results from public debates around the medical, ethical and legal issues related to vaccines. Vaccine hesitancy stems from multiple key factors including a person's lack of confidence (mistrust of the vaccine and/or healthcare provider), complacency (the person does not see a need for the vaccine or does not see the value of the vaccine), and convenience (access to vaccines). It has existed since the invention of vaccination, and pre-dates the coining of the terms "vaccine" and "vaccination" by nearly 80 years. The specific hypotheses raised by anti-vaccination advocates have been found to change over time. Vaccine hesitancy often results in disease outbreaks and deaths from vaccine-preventable diseases.

Bills for mandatory vaccination have been considered for legislation, including California Senate Bill 277 and Australia's No Jab No Pay, all of which have been strenuously opposed by anti-vaccination activists. Opposition to mandatory vaccination may be based on anti-vaccine sentiment, concern that it violates civil liberties or reduces public trust in vaccination, or suspicion of profiteering by the pharmaceutical industry.


Scientific evidence for the effectiveness of large-scale vaccination campaigns is well established. Two to three million deaths are prevented each year worldwide due to vaccination and an additional 1.5 million deaths could be prevented each year if all recommended vaccines were used. Vaccination campaigns helped eradicate smallpox, which once killed as many as one in seven children in Europe, and have nearly eradicated polio.As a more modest example, infections caused by Haemophilus influenzae (Hib), a major cause of bacterial meningitis and other serious diseases in children, have decreased by over 99% in the US since the introduction of a vaccine in 1988. It is estimated that full vaccination, from birth to adolescence, of all US children born in a given year would save 33,000 lives and prevent 14 million infections.

Some argue that these reductions in infectious disease are a result of improved sanitation and hygiene (rather than vaccination), or that these diseases were already in decline before the introduction of specific vaccines. These claims are not supported by scientific data; the incidence of vaccine-preventable diseases tended to fluctuate over time until the introduction of specific vaccines, at which point the incidence dropped to near zero. A Centers for Disease Control and Prevention website aimed at countering common misconceptions about vaccines argued, "Are we expected to believe that better sanitation caused incidence of each disease to drop, just at the time a vaccine for that disease was introduced?"

Other critics argue that the immunity granted by vaccines is only temporary and requires boosters, whereas those who survive the disease become permanently immune. As discussed below, the philosophies of some alternative medicine practitioners are incompatible with the idea that vaccines are effective.

Population health

Incomplete vaccine coverage increases the risk of disease for the entire population, including those who have been vaccinated, because it reduces herd immunity. For example, the measles vaccine is given to children between the ages of 9 and 12 months, and the short window between the disappearance of maternal antibody (before which the vaccine often fails to seroconvert) and natural infection means that vaccinated children are frequently still vulnerable. Herd immunity lessens this vulnerability if all the children are vaccinated. Increasing herd immunity during an outbreak or risk of outbreak is perhaps the most widely accepted justification for mass vaccination. When a new vaccine is introduced mass vaccination helps increase coverage rapidly.

If enough of a population 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.


Commonly used vaccines are a cost-effective and preventive way of promoting health, compared to the treatment of acute or chronic disease. In the US during the year 2001, routine childhood immunizations against seven diseases were estimated to save over $40 billion per birth-year cohort in overall social costs, including $10 billion in direct health costs, and the societal benefit-cost ratio for these vaccinations was estimated to be 16.5.


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.

Common themes

While some anti-vaccinationists openly deny the improvements vaccination has made to public health, or succumb to 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 expected. 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 this 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 no longer appreciate how serious the illness is due to a lack of familiarity with it and become complacent. The lack of personal experience with these diseases reduces the perceived danger and thus reduces the perceived benefit of immunization. Conversely, certain illnesses (e.g., influenza) remain so common that vaccine-hesitant people mistakenly perceive the illness to be non-threatening despite clear evidence that the illness poses a significant threat to human health. Omission and disconfirmation biases also contribute to vaccine hesitancy.

Various concerns about immunization have been raised. They have been addressed and the concerns are not supported by evidence. Concerns about immunization safety often follow a pattern. First, some investigators suggest that a medical condition of increasing prevalence or unknown cause is an adverse effect of vaccination. The initial study and subsequent studies by the same group have inadequate methodology—typically a poorly controlled or uncontrolled case series. A premature announcement is made about the alleged adverse effect, resonating with individuals suffering from the condition, and underestimating the potential harm of forgoing vaccination to those whom the vaccine could protect. Other groups attempt to replicate the initial study but fail to get the same results. Finally, it takes several years to regain public confidence in the vaccine. Adverse effects ascribed to vaccines typically have an unknown origin, an increasing incidence, some biological plausibility, occurrences close to the time of vaccination, and dreaded outcomes. In almost all cases, the public health effect is limited by cultural boundaries: English speakers worry about one vaccine causing autism, while French speakers worry about another vaccine causing multiple sclerosis, and Nigerians worry that a third vaccine causes infertility.


  • Main article: Vaccines and autism

    The idea of a link between vaccines and autism has been extensively investigated and conclusively shown to be false. The scientific consensus is that there is no relationship, causal or otherwise, between vaccines and incidence of autism, and vaccine ingredients do not cause autism.

    Nevertheless, the anti-vaccination movement continues to promote myths, conspiracy theories, and misinformation linking the two. A developing tactic appears to be the "promotion of irrelevant research an active aggregation of several questionable or peripherally related research studies in an attempt to justify the science underlying a questionable claim."


  • Main article: Thiomersal and vaccines

    Thiomersal (spelled "thimerosal" in the US) is an antifungal preservative used in small amounts in some multi-dose vaccines (where the same vial is opened and used for multiple patients) to prevent contamination of the vaccine. Despite thiomersal's efficacy, the use of thiomersal is controversial because it contains mercury (specifically ethylmercury). As a result, in 1999, the Centers for Disease Control (CDC) and the American Academy of Pediatrics (AAP) asked vaccine makers to remove thiomersal from vaccines as quickly as possible on the precautionary principle. Thiomersal is now absent from all common US and European vaccines, except for some preparations of influenza vaccine. Trace amounts remain in some vaccines due to production processes, at an approximate maximum of 1 microgramme, around 15% of the average daily mercury intake in the US for adults and 2.5% of the daily level considered tolerable by the WHO. The action sparked concern that thiomersal could have been responsible for autism. The idea is now considered disproven, as incidence rates for autism increased steadily even after thiomersal was removed from childhood vaccines. Currently there is no accepted scientific evidence that exposure to thiomersal is a factor in causing autism. Since 2000, parents in the United States have pursued legal compensation from a federal fund arguing that thiomersal caused autism in their children. A 2004 Institute of Medicine (IOM) committee favored rejecting any causal relationship between thiomersal-containing vaccines and autism. The concentration of thiomersal used in vaccines as an antimicrobial agent ranges from 0.001% (1 part in 100,000) to 0.01% (1 part in 10,000). A vaccine containing 0.01% thiomersal has 25 micrograms of mercury per 0.5 mL dose, roughly the same amount of elemental mercury found in a three-ounce can of tuna. There is robust peer-reviewed scientific evidence supporting the safety of thiomersal-containing vaccines.

    MMR vaccine

  • Main article: MMR vaccine and autism

    In the UK, the MMR vaccine was the subject of controversy after the publication in The Lancet of a 1998 paper by Andrew Wakefield and others reporting case histories of 12 children mostly with autism spectrum disorders with onset soon after administration of the vaccine. At a 1998 press conference, Wakefield suggested that giving children the vaccines in three separate doses would be safer than a single vaccination. This suggestion was not supported by the paper, and several subsequent peer-reviewed studies have failed to show any association between the vaccine and autism. It later emerged that Wakefield had received funding from litigants against vaccine manufacturers and that he had not informed colleagues or medical authorities of his conflict of interest; had this been known, publication in The Lancet would not have taken place in the way that it did. Wakefield has been heavily criticized on scientific grounds and for triggering a decline in vaccination rates (vaccination rates in the UK dropped to 80% in the years following the study), as well as on ethical grounds for the way the research was conducted. In 2004, the MMR-and-autism interpretation of the paper was formally retracted by 10 of Wakefield's 12 coauthors, and in 2010 The Lancet's editors fully retracted the paper. Wakefield was struck off the UK medical register, with a statement identifying deliberate falsification in the research published in The Lancet, and is barred from practicing medicine in the UK.

    The CDC, the IOM of the National Academy of Sciences, Australia's Department of Health, and the UK National Health Service have all concluded that there is no evidence of a link between the MMR vaccine and autism. A Cochrane review concluded that there is no credible link between the MMR vaccine and autism, that MMR has prevented diseases that still carry a heavy burden of death and complications, that the lack of confidence in MMR has damaged public health, and that the design and reporting of safety outcomes in MMR vaccine studies are largely inadequate. Additional reviews agree, with studies finding that vaccines are not linked to autism even in high risk populations with autistic siblings.

    In 2009, The Sunday Times reported that Wakefield had manipulated patient data and misreported results in his 1998 paper, creating the appearance of a link with autism. A 2011 article in the British Medical Journal described how the data in the study had been falsified by Wakefield so that it would arrive at a predetermined conclusion. An accompanying editorial in the same journal described Wakefield's work as an "elaborate fraud" that led to lower vaccination rates, putting hundreds of thousands of children at risk and diverting energy and money away from research into the true cause of autism.

    A special court convened in the United States to review claims under the National Vaccine Injury Compensation Program ruled on February 12, 2009 that parents of autistic children are not entitled to compensation in their contention that certain vaccines caused autism in their children.

    Vaccine overload

    Vaccine overload, a non-medical term, is the notion that giving many vaccines at once may overwhelm or weaken a child's immature immune system and lead to adverse effects. Despite scientific evidence that strongly contradicts this idea, some parents of autistic children believe that vaccine overload causes autism. The resulting controversy has caused many parents to delay or avoid immunizing their children. Such parental misperceptions are major obstacles towards immunization of children.

    The concept of vaccine overload is flawed on several levels. Despite the increase in the number of vaccines over recent decades, improvements in vaccine design have reduced the immunologic load from vaccines; the total number of immunological components in the 14 vaccines administered to US children in 2009 is less than 10% of what it was in the 7 vaccines given in 1980. A study published in 2013 found no correlation between autism and the antigen number in the vaccines the children were administered up to the age of two. Of the 1,008 children in the study, one quarter of those diagnosed with autism were born between 1994 and 1999, when the routine vaccine schedule could contain more than 3,000 antigens (in a single shot of DTP vaccine). The vaccine schedule in 2012 contains several more vaccines, but the number of antigens the child is exposed to by the age of two is 315. Vaccines pose a very small immunologic load compared to the pathogens naturally encountered by a child in a typical year; common childhood conditions such as fevers and middle-ear infections pose a much greater challenge to the immune system than vaccines, and studies have shown that vaccinations, even multiple concurrent vaccinations, do not weaken the immune system or compromise overall immunity. The lack of evidence supporting the vaccine overload hypothesis, combined with these findings directly contradicting it, has led to the conclusion that currently recommended vaccine programs do not "overload" or weaken the immune system.

    Any experiment based on withholding vaccines from children is considered unethical, and observational studies would likely be confounded by differences in the health care-seeking behaviors of under-vaccinated children. Thus, no study directly comparing rates of autism in vaccinated and unvaccinated children has been done. However, the concept of vaccine overload is biologically implausible, as vaccinated and unvaccinated children have the same immune response to non-vaccine-related infections, and autism is not an immune-mediated disease, so claims that vaccines could cause it by overloading the immune system go against current knowledge of the pathogenesis of autism. As such, the idea that vaccines cause autism has been effectively dismissed by the weight of current evidence.

    Prenatal infection

    There is evidence that schizophrenia is associated with prenatal exposure to rubella, influenza, and toxoplasmosis infection. For example, one study found a sevenfold increased risk of schizophrenia when mothers were exposed to influenza in the first trimester of gestation. This may have public health implications, as strategies for preventing infection include vaccination, simple hygiene, and, in the case of toxoplasmosis, antibiotics. Based on studies in animal models, theoretical concerns have been raised about a possible link between schizophrenia and maternal immune response activated by virus antigens; a 2009 review concluded that there was insufficient evidence to recommend routine use of trivalent influenza vaccine during the first trimester of pregnancy, but that the vaccine was still recommended outside the first trimester and in special circumstances such as pandemics or in women with certain other conditions. The CDC's Advisory Committee on Immunization Practices, the American College of Obstetricians and Gynecologists, and the American Academy of Family Physicians all recommend routine flu shots for pregnant women, for several reasons:

    Despite this recommendation, only 16% of healthy pregnant US women surveyed in 2005 had been vaccinated against the flu.

    Ingredient concerns

    Aluminum compounds are used as immunologic adjuvants to increase the effectiveness of many vaccines. The aluminum in vaccines simulates or causes small amounts of tissue damage, driving the body to respond more powerfully to what it sees as a serious infection and promoting the development of a lasting immune response. In some cases these compounds have been associated with redness, itching, and low-grade fever, but the use of aluminum in vaccines has not been associated with serious adverse events. In some cases, aluminum-containing vaccines are associated with macrophagic myofasciitis (MMF), localized microscopic lesions containing aluminum salts that persist for up to 8 years. However, recent case-controlled studies have found no specific clinical symptoms in individuals with biopsies showing MMF, and there is no evidence that aluminum-containing vaccines are a serious health risk or justify changes to immunization practice. Infants are exposed to greater quantities of aluminum in daily life in breastmilk and infant formula than in vaccines. In general, people are exposed to low levels of naturally occurring aluminum in nearly all foods and drinking water. The amount of aluminum present in vaccines is small, less than 1 milligram, and such low levels are not believed to be harmful to human health.

    Vaccine hesitant people have also voiced strong concerns about the presence of formaldehyde in vaccines. Formaldehyde is used in very small concentrations to inactivate viruses and bacterial toxins used in vaccines. Very small amounts of residual formaldehyde can be present in vaccines but are far below values harmful to human health. The levels present in vaccines are minuscule when compared to naturally-occurring levels of formaldehyde in the human body and pose no significant risk of toxicity. The human body continuously produces formaldehyde naturally and contains 50–70 times the greatest amount of formaldehyde present in any vaccine. Furthermore, the human body is capable of breaking down naturally occurring formaldehyde as well as the small amount of formaldehyde present in vaccines. There is no evidence linking the infrequent exposures to small quantities of formaldehyde present in vaccines with cancer.

    Sudden infant death syndrome

    Sudden infant death syndrome (SIDS) is most common in infants around the time in life when they receive many vaccinations. Since the cause of SIDS has not been fully determined, this led to concerns about whether vaccines, in particular diphtheria-tetanus toxoid vaccines, were a possible causal factor. Several studies investigated this and found no evidence supporting a causal link between vaccination and SIDS. In 2003, the Institute of Medicine favored rejection of a causal link to DTwP vaccination and SIDS after reviewing the available evidence. Additional analyses of VAERS data also showed no relationship between vaccination and SIDS. In fact, evidence is mounting that vaccination may protect children against SIDS.

    Anthrax vaccines

    When the U.S. military began requiring its troops to receive the anthrax vaccine, multiple US military troops refused to do so, which led to threats of military courts martial.

    Swine flu vaccine

    During the 2009 flu pandemic, significant controversy broke out regarding whether the 2009 H1N1 flu vaccine was safe in, among other countries, France. Numerous different French groups publicly criticized the vaccine as potentially dangerous.

    Other safety concerns

    Other safety concerns about vaccines have been promoted on the Internet, in informal meetings, in books, and at symposia. These include hypotheses that vaccination can cause epileptic seizures, allergies, multiple sclerosis, and autoimmune diseases such as type 1 diabetes, as well as hypotheses that vaccinations can transmit bovine spongiform encephalopathy, hepatitis C virus, and HIV. These hypotheses have been investigated, with the conclusion that currently used vaccines meet high safety standards and that criticism of vaccine safety in the popular press is not justified. Large well-controlled epidemiologic studies have been conducted and the results do not support the hypothesis that vaccines cause chronic diseases. Furthermore, some vaccines are probably more likely to prevent or modify than cause or exacerbate autoimmune diseases. Another common concern parents often have is about the pain associated with administering vaccines during a doctor's office visit. This may lead to parental requests to space out vaccinations; however, studies have shown a child's stress response is not different when receiving one vaccination or two. The act of spacing out vaccinations may actually lead to more stressful stimuli for the child.


    In Pakistan, there have been several attacks and deaths among vaccination workers. Several Islamist preachers and militant groups, including some factions of the Taliban, view vaccination as a plot to kill or sterilize Muslims. This is part of the reason Pakistan and Afghanistan are the only countries where polio still remained endemic as of 2015.

    In India, a 3-minute doctored clip has been circulating among Muslims claiming that the MR-VAC vaccine against measles and rubella was a "Modi government-RSS conspiracy" to stop the population growth of Muslims. The clip was taken from a TV show that exposed the baseless rumors. Hundreds of madrassas in the state of Uttar Pradesh have refused permission to health department teams to administer vaccines because of rumors spread using WhatsApp.

    Other vaccine myths

    There are several other vaccination myths that contribute to parental concerns and vaccine hesitancy. These include the alleged superiority of natural infection when compared to vaccination, questioning whether the diseases vaccines prevent are dangerous, whether vaccines pose moral or religious dilemmas, suggesting that vaccines are not effective, proposing unproven or ineffective approaches as alternatives to vaccines, and conspiracy theories that center on mistrust of the government and medical institutions.

    Vaccination during illness

    Many parents are concerned about the safety of vaccination when their child is sick. Moderate to severe acute illness with or without a fever is indeed a precaution when considering vaccination. Vaccines remain effective during childhood illness. The reason vaccines may be withheld if a child is moderately to severely ill is because certain expected side effects of vaccination (e.g., fever or rash) may be confused with progression of the illness. It is safe to administer vaccines to well-appearing children who are mildly ill with the common cold.

    Natural Infection

    Another common anti-vaccine myth is that natural infection produces better immune protection against contracting the illness in the future when compared to vaccination. In some cases, actual infection with the illness may produce lifelong immunity; however, natural disease carries a higher risk of harming a person's health than vaccines. For example, natural varicella infection carries a higher risk of bacterial superinfection with Group A streptococci.

    HPV vaccine

    The idea that the HPV vaccine is linked to increased sexual behavior is not supported by scientific evidence. A review of nearly 1,400 adolescent girls found no difference in teen pregnancy, incidence of sexually transmitted infection, or contraceptive counseling regardless of whether they received the HPV vaccine. Thousands of Americans die each year from cancers preventable by the vaccine.

    Vaccine schedule

    Other concerns have been raised about the vaccine schedule recommended by the Advisory Committee on Immunization Practices (ACIP). The immunization schedule is designed to protect children against preventable diseases when they are most vulnerable. The practice of delaying or spacing out these vaccinations increases the amount of time the child is susceptible to these illnesses. Receiving vaccines on the recommended ACIP schedule is not linked to autism or developmental delay.

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    Vaccine hesitancy is challenging and optimal strategies for approaching it remain uncertain. Many interventions designed to address vaccine hesitancy have been based on the information deficit model.This model assumes that vaccine hesitancy is due to a person lacking the necessary information and attempts to provide them with that information to solve the problem. Despite many educational interventions attempting this approach, ample evidence indicates providing more information is often ineffective in changing a vaccine-hesitant person's views and may, in fact, have the opposite of the intended effect and reinforce their misconceptions.

    Several communication strategies are recommended for use when interacting with vaccine-hesitant parents. These include establishing honest and respectful dialogue; acknowledging the risks of a vaccine but balancing them against the risk of disease; referring parents to reputable sources of vaccine information; and maintaining ongoing conversations with vaccine-hesitant families. The American Academy of Pediatrics recommends healthcare providers directly address parental concerns about vaccines when questioned about their efficacy and safety. Additional recommendations include asking permission to share information; maintaining a conversational tone (as opposed to lecturing); not spending excessive amounts of time debunking specific myths (this may have the opposite effect of strengthening the myth in the person's mind); focusing on the facts and simply identifying the myth as false; and keeping information as simple as possible (if the myth seems simpler than the truth, it may be easier for people to accept the simple myth). Storytelling and anecdote (e.g., about the decision to vaccinate one's own children) can be powerful communication tools for conversations about the value of vaccination. The perceived strength of the recommendation, when provided by a healthcare provider, also seems to influence uptake, with recommendations that are perceived to be stronger resulting in higher vaccination rates than perceived weaker recommendations.

    Limited evidence suggests that a more paternalistic or presumptive approach ("Your son needs three shots today.") is more likely to result in patient acceptance of vaccines during a clinic visit than a participatory approach ("What do you want to do about shots?") but decreases patient satisfaction with the visit. A presumptive approach helps to establish that this is the normative choice. Similarly, one study found that the way in which physicians respond to parental vaccine resistance is important. Nearly half of initially vaccine-resistant parents accepted vaccinations if physicians persisted in their initial recommendation. The Centers for Disease Control and Prevention has released resources to aid healthcare providers in having more effective conversations with parents about vaccinations.

    Parents may be hesitant to have their child vaccinated due to concerns about the pain of vaccination. There are several strategies that can be used to reduce the child's pain. Such strategies include distraction techniques (pinwheels); deep breathing techniques; breastfeeding the child; giving the child sweet-tasting solutions; quickly administering the vaccine without aspirating; keeping the child upright; providing tactile stimulation; applying numbing agents to the skin; and saving the most painful vaccine for last. As above, the number of vaccines offered in a particular encounter is related to the likelihood of parent vaccine refusal (the more vaccines offered, the higher the likelihood of vaccine deferral). The use of combination vaccines to provide protection against more diseases but with fewer injections may provide reassurance to parents. Similarly, reframing the conversation with less emphasis on the number of diseases the healthcare provider is immunizing against (e.g., "we will do two injections (combined vaccinations) and an oral vaccine") may be more acceptable to parents than "we're going to vaccinate against 7 diseases."

    It is unclear whether interventions intended to educate parents about vaccines improve the rate of vaccination. It is also unclear whether citing the reasons of benefit to others and herd immunity improves parents' willingness to vaccinate their children. In one trial, an educational intervention designed to dispel common misconceptions about the influenza vaccine decreased parents' false beliefs about the vaccines but did not improve uptake of the influenza vaccine. In fact, parents with significant concerns about adverse effects from the vaccine were less likely to vaccinate their children with the influenza vaccine after receiving this education. Multicomponent initiatives which include targeting undervaccinated populations, improving the convenience of and access to vaccines, educational initiatives, and mandates may improve vaccination uptake.

    It is recommended that healthcare providers advise parents against performing their own web search queries since many websites on the Internet contain significant misinformation. Many parents perform their own research online and are often confused, frustrated, and unsure of which sources of information are trustworthy. Additional recommendations include introducing parents to the importance of vaccination as far in advance of the initial well-child visit as possible; presenting parents with vaccine safety information while in their pediatrician's waiting room; and using prenatal open houses and postpartum maternity ward visits as opportunities to vaccinate.

    Internet advertising, on Facebook and elsewhere, is purchased by both public health authorities and anti-vaccination groups. In the United States, the majority of anti-vaccine Facebook advertising in December 2018 and February 2019 had been paid for one of two groups: Robert F. Kennedy Jr.'s Children's Health Defense and Stop Mandatory Vaccination. The ads targeted women and young couples and generally highlighted the alleged risks of vaccines, while asking for donations. Several anti-vaccination advertising campaigns also targeted areas where measles outbreaks were underway during this period. The impact of Facebook's subsequent advertising policy changes has not been studied.

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    Information warfare

  • Further information: Cyberwarfare by Russia and Propaganda in the Russian Federation

    An analysis of tweets from July 2014 through September 2017 revealed an active campaign on Twitter by the Internet Research Agency (IRA), a Russian troll farm accused of interference in the 2016 U.S. elections, to sow discord about the safety of vaccines. The campaign used sophisticated Twitter bots to amplify highly polarizing pro-vaccine and anti-vaccine messages, containing the hashtag #VaccinateUS, posted by IRA trolls.

    Confidence in vaccines varies over place and time and among different vaccines. The London School of Hygiene & Tropical Medicine's Vaccine Confidence Project in 2016 found that confidence was lower in Europe than in the rest of the world. Refusal of the MMR vaccine has increased in 12 European states since 2010. The project published a report in 2018 assessing vaccine hesitancy among the public in all the 28 EU member states and among general practitioners in ten of them. Younger adults in the survey had less confidence than older people. Confidence had risen in France, Greece, Italy, and Slovenia since 2015 but had fallen in the Czech Republic, Finland, Poland, and Sweden. 36% of the GPs surveyed in the Czech Republic and 25% of those in Slovakia did not agree that the MMR vaccine was safe. Most of the GPs did not recommend the seasonal influenza vaccine. Confidence in the population correlated with confidence among GPs. One study in the United States found that after vaccine-hesitant college students interviewed survivors of vaccine-preventable diseases, they were more likely to become pro-vaccine than a control group.

    Parties opposed to the use of vaccines frequently refer to data obtained from the US Vaccine Adverse Event Reporting System (VAERS). This is a database of reports of issues associated with vaccines. When used appropriately VAERS is a useful tool for investigation, but since anyone can make a claim and have it entered into the VAERS, by itself it is not a reliable source of information. Dubious claims about vaccines against hepatitis B, HPV and other diseases have been propagated based on misuse of data from VAERS.

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