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Successful vaccination resulting in the reduction of a variety of bacterial and viral diseases is recognized as one of the top public health achievements of the 20th century.1 This success has continued into the 21st century with the introduction of later-generation vaccines that protect against shingles, human papillomavirus (HPV), pneumococcal pneumonia, meningococcal disease, and pertussis. As a result, the implementation of routine immunization protocols for children and adults has led to documented, dramatic reductions in the incidence of vaccine-preventable diseases in the United States (Table 1 and Table 2).2,3 On a cautionary note, however, although the incidence of many vaccine-preventable diseases has been sharply reduced by childhood vaccination, they have not been eliminated.
As the number of vaccine-preventable infections has been reduced through widespread vaccination, an increasing percentage of the population has neither experienced nor even seen many diseases that are prevented by this public health measure. Ideally, the demonstrated success shown in Table 1 and Table 2 should bolster people's confidence in vaccines prescribed for children, adolescents, and adults, ensuring that the general population's "herd immunity" remains high. Unfortunately, a puzzling and dangerous pattern is emerging in a small segment of the population. In the absence of hundreds of thousands to millions of annual infections and documented deaths from vaccine-preventable diseases, there are some who are currently focusing more on the danger posed by the possible adverse effects of vaccination than on the protection that widespread immunization provides against diseases that still pose global health problems. This thinking has already become an emerging public health problem. Despite the availability of safe and effective vaccines, many people die each year from complications of preventable infections. Using the current epidemic of measles in the United States and other countries as an example, the World Health Association reported 110,000 global measles deaths for 2017, most of which were among young children.4
Why do some individuals fail to receive or refuse acceptance of routine immunizations? Possible reasons include the lack of a perceived risk to vaccine-preventable diseases; a mistaken perception that vaccine-preventable diseases have been virtually eliminated from developed countries, especially the United States; and fears concerning the safety of vaccines and the potential for adverse reactions after vaccination. The following discussion will specifically focus on the United States to address the final and arguably most problematic reason—the perception that vaccines are dangerous and should be guaranteed to be safe for all persons who receive them.
Properties of an Ideal Vaccine
Developers of new vaccines strive to meet certain criteria for what is termed an "ideal preparation" (Table 3).5 First and foremost, a vaccine must be proven to be safe for human use. Scientists and the US Food and Drug Administration (FDA) apply this basic standard during testing and regulatory activity to help ensure a vaccine's safety, potency, and effectiveness before it is licensed for use in the United States. This approach is consistent with the basic rule that applies to all health professionals when providing patient care, which is to do no harm. Immunogenicity is another fundamental criterion; a vaccine must be shown to stimulate an immune response that is sufficient to protect an immunized host from disease. Other major considerations include the number of times that a vaccine must be administered in order to immunize the recipient and the duration of protection. The provision of lifelong immunity is definitely a goal, and fortunately, that goal has been achieved with many childhood vaccines.
Despite the best efforts of researchers to meet the properties and requirements of an ideal vaccine and documented success in the prevention of many diseases, no vaccine is perfect, and adverse effects do occasionally occur. The overwhelming majority of immunization adverse reactions are minor and include injection site soreness, temporary myalgia, fatigue, and fever. It is important to note here that the most common adverse reactions are identified in clinical studies before a vaccine is licensed. However, even with these safeguards, the possibility exists for very rare adverse effects to not be detected until after a vaccine has been licensed and used in the population. That is why clinical data and other reportable information concerning documented or suspected adverse effects of vaccines are continuously collected by the Centers for Disease Control and Prevention (CDC), FDA, and other national health agencies. This is part of an ongoing effort to weigh the benefits of vaccines against their risks. Unfortunately, in recent years, some individuals have promoted the mistaken perception that vaccines are never recalled—even when safety problems are discovered. The reality is that, in some instances, vaccines have been removed from the marketplace, and when possible, they have been reformulated in an effort to significantly reduce real or perceived adverse reactions. For example, this was the case in 1999 with the rotavirus vaccine. Rare instances of intestinal obstruction were reported among young immunized children after the rotavirus vaccine passed the trial period, and it was subsequently withdrawn within 2 years of being licensed.6
Those involved in vaccine development also endeavor to reduce the potential of vaccines to cause hypersensitivity (ie, allergic) reactions as much as possible. However, even with the best efforts of scientists working with manufacturers, hypersensitivity reactions can occur as a result of one or more preparation components, including residual proteins, preservatives, or stabilizers. The most common allergenic vaccine component has been shown to be egg protein, which is found in vaccines that are prepared using chicken embryo tissue cultures (eg, measles, mumps, influenza). Other less common allergic reactions have also resulted from the preservative thimerosal, which is used in multidose vaccine vials. Although thimerosal has been successfully used in inactivated vaccines as a preservative since the 1930s, it was removed from children's vaccines in 2001 as a precautionary measure.7
Safeguarding Vaccine Development and Use
The United States maintains the highest safety standards for the development, testing, licensure, and evaluation of human vaccines. Every licensed vaccine must go through years of development and clinical testing before being recommended for use. In addition, the FDA closely monitors manufacturing procedures to ensure compliance with established standards for high purity, potency, and safety. Excellent reviews of vaccine safety have been published that provide more detailed clinical-based evidence.8-11
The initial goal of vaccine development is to determine if a preparation is safe in animals, and the suggestion that it may also be safe in humans is an additional consideration.12 Clinical trials are conducted after these criteria are met. Studies with human volunteers involve three distinct phases:
Phase 1. This initial trial, which involves 20 to 100 healthy people, investigates vaccine effectiveness as well as the effects of vaccine dosage on potential adverse effects.
Phase 2. Encouraging results from phase 1 lead to the involvement of a larger volunteer group (ie, several hundred people), who are used to further study the efficacy of the vaccine dose. Also, more information can be collected concerning the occurrence of common vaccination adverse effects.
Phase 3. These clinical trials are the most extensive test studies. Hundreds or thousands of volunteers are required. These individuals are divided into two groups; one is administered the vaccine, and the other receives a placebo or a different vaccine preparation. The use of significantly more test subjects allows researchers to learn about safety and any possible adverse effects on a much larger scale. Throughout all clinical trials, collected data are evaluated in conjunction with other information, including the efficacy and safety of vaccine manufacturing and the vaccine's physical, chemical, and biological properties. As a result of these efforts, consistent quality control is a routine component of vaccine evaluation.
Tracking Vaccines After Licensure and Routine Administration
The FDA, CDC, and other federal agencies continue to monitor vaccines after they are approved and licensed for commercial release. This is accomplished by the integration of information from multiple reporting and data collection systems to assess continued vaccine safety.13
Vaccine Adverse Event Reporting System
The Vaccine Adverse Event Reporting System (VAERS) was established in 1990 as a national early warning system to detect possible safety problems. Although anyone can report a potential problem with a vaccine, including patients and parents, both healthcare professionals and vaccine manufacturers are required to report adverse events. A review of the most recent update from CDC indicates that VAERS receives approximately 30,000 reports annually. Most describe known temporary and mild adverse effects, such as fever.
Vaccine Safety Datalink
The Vaccine Safety Datalink (VSD) conducts vaccine safety studies based on concerns that are raised from the medical literature and reports to the VAERS. Since its inception in 1990, VSD has published the results of many studies involving vaccine safety issues. Topics include the safety of vaccines that contain additives and/or preservatives deemed safe for children, the safety of rotavirus vaccines for children, and the potential for vaccines to cause febrile illness.
Post-Licensure Rapid Immunization Safety Monitoring Program
The Post-Licensure Rapid Immunization Safety Monitoring (PRISM) program is a component of FDA's Sentinel Initiative—a national system for monitoring the safety of medical products after they are licensed for use. PRISM focuses on vaccine safety by using a database of health insurance claims to identify and evaluate possible safety issues for licensed vaccines.
Clinical Immunization Safety Assessment Project
The Clinical Immunization Safety Assessment (CISA) Project is a national network of vaccine safety experts from the CDC, medical research centers, and other partners. It addresses vaccine safety issues, conducts high-quality clinical research, and assesses complex clinical adverse events following vaccination. In addition, CISA provides consultation to healthcare providers and public health partners regarding vaccine safety issues.
What Can Dentists and Other Oral Healthcare Providers Do?
Oral healthcare providers are a valuable, knowledgeable resource concerning their patients' health. Beginning with the initial examination and during subsequent treatment, they screen for signs and symptoms of systemic and infectious diseases as well as the presence of oral cancer, take blood pressures, and provide appropriate referrals to physicians when indicated.
To help combat the misinformation being spread by antivaccination campaigns and improve vaccination rates, some have suggested expanding the number of vaccine providers to include dentists. On April 25, 2019, the Oregon state legislature approved a bill that will add the prescription and administration of vaccines into the scope of dental practice following its implementation in 2020.14 Many individuals remain critical of this policy's ability to achieve widespread implementation due to certain barriers, including the need for training on preservation, handling, communication, and administration. However, the idea at its core, which is that dentists represent an untapped source of primary care providers whose actions can support vaccination, is important to embrace.
Most patients will not have questions about vaccines, but some will, and others may have already received misinformation about immunizations. The fact that some patients ask questions means that they value the opinion of their oral healthcare providers as a trusted source of information, so be prepared to answer and offer evidence that supports the importance of vaccination. Multiple approaches are available. For example, many clinicians routinely inquire about a patient's status for recommended immunizations when taking a medical history. In particular, dentists may want to inquire about their patients' HPV vaccination status. In 2018, the American Dental Association adopted a policy encouraging dentists to support the use and administration of the HPV vaccine because it could help prevent the vast majority of oropharyngeal cancer cases, of which 70% to 80% are estimated to be attributable to HPV.15
Reiterating the information provided to patients by their physicians can also serve as useful guidance. An oral healthcare provider's recommendation that is supported by overwhelming scientific and clinical evidence can further reinforce the acceptance of vaccination facts as opposed to unproven myths. This positive message can be further reinforced by having vaccine information brochures from the CDC and the US Public Health Service available in the practice. There are also numerous information pieces available to assist dentists in preparation for questions that may arise. These discuss multiple topics of interest, including specific vaccine recommendations, scientific and clinical evidence debunking vaccine myths, and answers to questions that parents may have about vaccine safety and adverse effects for their children.16 One of the important messages to get across is that the occasional adverse effects of vaccination, such as injection site soreness, fever, and muscle aches, are far milder than and preferable to having the actual vaccine-preventable disease.
Systems with multiple checks are in place to provide monitoring of vaccines. Concerted efforts by the FDA, CDC, and federal and local health agencies continue to scrutinize the variety of recommended vaccines for children and adults. However, as with other administered medications, occasional adverse reactions can occur. Although the overwhelming majority of these are minor and resolve relatively quickly, unfortunately, some rare effects can be serious. However, these may be seen in only one individual out of hundreds of thousands to millions of vaccine recipients and may be so rare that they are almost impossible to appropriately investigate. In working to make vaccines as safe as possible, one overriding feature that vaccine monitoring systems have in common is to evaluate the population's risk of effects from a disease versus its risk of adverse sequelae from vaccination. Although evidence collected over decades strongly reinforces the fact that vaccination continues to be highly effective in preventing many diseases, it must still be emphasized that the FDA will only license a vaccine if the benefits of immunization far outweigh its potential risks.
About the Author
John A. Molinari, PhD
University of Detroit Mercy
School of Dentistry
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13. The College of Physicians of Philadelphia. Vaccine development, testing, and regulation. The College of Physicians of Philadelphia website. https://www.historyofvaccines.org/content/articles/vaccine-development-testing-and-regulation. Updated January 17, 2018. Accessed November 21, 2019.
14. Solana K. ADA News. Oregon passes bill allowing dentists to administer vaccines. American Dental Association website. https://www.ada.org/en/publications/ada-news/2019-archive/april/oregon-passes-bill-allowing-dentists-to-administer-vaccines20190426t142836. Published April 26, 2019. Accessed January 22, 2020.
15. American Dental Association. ADA adopts policy on HPV vaccination for the prevention of oral HPV infection. American Dental Association website. https://www.ada.org/en/press-room/news-releases/2018-archives/october/ada-adopts-policy-on-hpv-vaccination-for-the-prevention-of-oral-hpv-infection. Published October 25, 2018. Accessed January 22, 2020.
16. Centers for Disease Control and Prevention. Preparing for questions parents may ask about vaccines. Centers for Disease Control and Prevention website. https://www.cdc.gov/vaccines/hcp/conversations/downloads/prepare-infants-508.pdf. Reviewed January 2019. Accessed November 21, 2019.