Immunization in the United States

History

In the United States, immunizations are provided through the private and public sectors. The public sector consists primarily of health departments, but also includes other clinics, such as community and migrant health centers and public hospital–based clinics supported by public funds. There was no federal support specifically for immunization activities until 1955, when inactivated polio vaccine was licensed. Through the Polio Vaccination Assistance Act, Congress appropriated funds in 1955 and 1956 to the Communicable Disease Center (now the CDC) to help states and local communities purchase and administer vaccine.

In 1962, President John F. Kennedy signed into law the Vaccination Assistance Act. The central thrust of this legislation was to allow the CDC to support mass immunization campaigns and to initiate maintenance programs. The first grants, authorized under Section 317 of the Public Health Service Act, were made in June 1963. During the more than 50 years since the Vaccination Assistance Act was signed into law, this grant program has thrived. There are now 64 grantees under what has become known as the “317” Immunization Grant Program, including all 50 states, six large cities (including the District of Columbia), and eight territories and former territories. The level of grant funding has varied greatly over the years. When the grant program began in 1963, the only vaccines available were diphtheria and tetanus toxoids and whole-cell pertussis (DTP), polio, and smallpox. In 1966, a national effort to eradicate measles began. The focus on measles vaccine administration ultimately led to adoption of many critical components of the program that have had impact on multiple vaccine-preventable diseases and funding has since expanded to support the infrastructure underpinning the addition of new vaccines to the immunization schedule, as outlined in the NVAC report “Protecting the Public's Health: Critical Functions of the Section 317 Immunization Program.”

In the early 1970s, immunization programs around the country were in varying states of implementation. In 1977, a national childhood immunization initiative was announced with two goals: attainment of immunization levels of 90% in the nation's children by October 1979 and establishment of a permanent system to provide comprehensive immunization services to all infants born each year in the United States (currently 3.7 million children). At that time, it was estimated that nearly 20 million U.S. children were in need of at least one dose of a vaccine to be fully immunized. The poor and minority populations were disproportionately represented among the persons needing additional vaccine doses. Efforts were expended to enact school immunization requirements in states that did not have them and to enforce those already in existence. As a result of these efforts, all 50 states soon had, and were enforcing, school-entry immunization laws. Since 1981, approximately 95% or more of children entering school have documentation of required vaccinations. Given these levels, even with lower levels in preschoolers, the overall immunization uptake in children of all ages in this country was 90% or greater. Thus, the first goal of the initiative was met.

Unfortunately, the second goal of the 1977 initiative was not met. Although the overall level of support for immunization grants rose rapidly in the late 1970s and throughout the 1980s (reaching US$126.8 million in 1989), almost all the increase was used to meet the increasing cost of vaccines or the addition of new vaccines or additional doses of existing vaccines. In the 1980s, the level of federal support to grantees to carry out maintenance immunization did not increase significantly. Over the years, the federal government provided more existing and new vaccines within a delivery system that had remained static in the face of increasing demands. Investigations of the measles epidemics of 1989 to 1991, which especially affected unvaccinated preschool children, made it clear that the public sector delivery system was inadequate for the challenge and that it required substantial financial assistance.

Part of the problem was that immunization grant funds were only permitted to be used to purchase vaccines and to carry out surveillance, investigation, education, and coordination, but not to support vaccine delivery (e.g., salaries of nurses, clinic supplies, expenses associated with increasing clinic hours). In 1991, President George H.W. Bush announced the federal government's support to accomplish a major health goal—to raise immunization levels by the year 2000 so that 90% or more of the nation's children routinely completed their basic series of vaccinations by their second birthday. The President announced that model immunization plans would be developed in several areas of the country as a beginning for the national effort to ensure adequate and timely immunization of infants and young children. This began a process that ultimately resulted in the preparation of immunization action plans by all 50 states and 28 metropolitan areas. Although there was great variation in the reported needs around the country, one theme was common to most plans and that was to increase the availability of immunization services. Consequently, for the first time, use of federal immunization grant funds for the actual provision of immunization services was permitted.

President Bill Clinton's announcement of the Childhood Immunization Initiative in 1993 and the leadership and infusion of funds associated with that initiative, coupled with the VFC program that followed, have brought the United States to the point that it is now, finally achieving 90% coverage in most vaccines recommended for preschool children. A critical part of the Childhood Immunization Initiative was to eliminate financial barriers to vaccination and ensure children could be vaccinated at their site of usual care (“medical home”), typically a private provider's office. The Childhood Immunization Initiative also established the National Immunization Survey (NIS) in 1994 to assess vaccination coverage levels among young children. During the measles resurgence of 1989–1991, there was no national mechanism to assess vaccination coverage. NIS was designed to estimate national, state, and selected local area coverage levels of children, 19–35 months of age, and has been in continuous operation since its establishment.

Following successful achievement of the 1996 Childhood Immunization Initiative goals, funding for the Section 317 program declined. Concerned about the adequacy of immunization infrastructure in the United States, Congress requested that the Institute of Medicine (IOM) conduct a study on the level of need for federal infrastructure support. The IOM recognized the damaging impact that the ebb and flow of infrastructure funding was having on the nation's immunization system and recommended a combined, 5-year investment of US$1.5 billion, shared between both federal and state resources. Between the publication of the IOM's 2000 report, “Calling the Shots,” and 2010, federal infrastructure funding was increased by US$115 million to US$239 million. Colgrove reviewed the history of two critical components of U.S. immunization efforts—public financing and immunization mandates.

Three major pieces of legislation have helped solidify the success of the U.S. immunization program: the National Childhood Vaccine Injury Act, the VFC program, and the Patient Protection and Affordable Care Act (ACA). Their roles in vaccine successes are summarized below. In 1986, the National Childhood Vaccine Injury Act was enacted, which put in place a ­no-fault compensation program for persons who had been injured after receipt of a vaccine that was universally recommended for children (no matter the age of the recipient). This reduced the threat of liability for vaccine manufacturers and facilitated the availability of vaccines. The VFC program was created by the Omnibus Budget Reconciliation Act of 1993 and first implemented in 1994. The VFC program entitles eligible children 0–18 years of age to receive all ACIP recommended vaccines free of charge. The ACA was passed in 2010 and requires private insurers to cover all vaccines as indicated on the pediatric and adult immunization schedules. A comprehensive review of the U.S. immunization program has been published in the Morbidity and Mortality Weekly Report (MMWR) .

Assuring Vaccine Purchase, Supply, and Distribution

Except for a small amount of diphtheria and tetanus toxoids vaccine, all U.S. vaccines for routine use are manufactured by private sector pharmaceutical companies, with some produced by several companies and others by a single manufacturer. Manufacturers are the sole decision makers for which vaccines they will develop and bring to the U.S. market, including new vaccines and combinations of existing vaccines. However, the National Vaccine Plan of the U.S. Department of Health and Human Services proposes a government role to encourage the development of vaccines specific to U.S. public health needs. ^, This involves strengthening the science base for the development and licensure of new and innovative vaccines. The IOM, with support from National Vaccine Program Office, developed a new vaccine decision-support tool called the Strategic Multi-Attribute Ranking Tool for Vaccines, or SMART Vaccines tool to prioritize vaccines for development.

Approximately half of the childhood vaccine supply is purchased with federal and state government funding using the CDC's vaccine contracts. These contracts allow state and local immunization programs to obtain vaccines at reduced prices and without having to negotiate separate contracts with the vaccine manufacturers. Current CDC contract prices and comparative private sector catalog prices are available from the CDC. Fig. 74.3 shows the CDC vaccine contract cost to fully vaccinate a child at four points in time. The current policy allows multiple manufacturers to contract for any of the program vaccines. States and local immunization programs select which of the vaccines they will use if there is more than one manufacturer for a given vaccine. In turn, states generally permit their providers to select which vaccines they will use to vaccinate program-eligible persons. Vaccines purchased using the CDC contracts are not allowed to be resold, and parents cannot be charged for the cost of CDC-contracted vaccine, although a state-determined administration fee can be charged to parents who can afford this fee. Approximately 90% of vaccine funding is provided by the federal VFC entitlement program, with state/local funding and the federal Section 317 funding making up the remainder. State and local immunization programs are provided a credit line of federal vaccine purchase funding that is awarded through population-based formulas for VFC and Section 317 vaccines.

The management of public-sector vaccine is accomplished with a comprehensive information technology system called VTrckS, for “Vaccine Tracking System.” Public sector vaccine is delivered by a CDC-contracted distributor directly to more than 40,000 end-user routine immunization providers. The provi­ders can order vaccine through VTrckS or through their state's immunization information system that, in turn, places the orders into VTrckS. State and local immunization programs have oversight and control of the vaccine orders placed by providers, and they can also enter the vaccine orders on behalf of their providers. VTrckS and its interfaces to manufacturers, the CDC-funded distributors, and state and local immunization programs now provide a unified, seamless system with complete visibility of all segments of the system. In addition to routine immunization, CDC currently uses VTrckS as its platform for all COVID-19 vaccine ordering. Jurisdictions, federal agencies receiving a direct vaccine allocation, and enrolled national provider organizations use VTrcks to view vaccine allocations, place and manage vaccine orders for their providers, generate reports throughout the vaccine distribution process, and track vaccine shipments. Information about VTrckS and centralized vaccine distribution is available from the CDC.

The CDC maintains stockpiles of childhood vaccines that are intended to serve as a buffer against short- and medium-term vaccine supply disruptions and to control emerging outbreaks of vaccine-preventable diseases. The stockpiles are maintained as storage and rotation inventories of vaccine so that the vaccine that enters the stockpile is used before its expiration. All the vaccines that are routinely recommended for children and are included in the VFC program are included in the stockpiles. The stockpiles include a 6-month supply of each of the CDC-contracted pediatric vaccines purchased through the public sector. Because shortages of vaccine and outbreaks of vaccine-preventable diseases span private and public sectors, the stockpile also supports the private sector vaccine supply system by allowing vaccine manufacturers to borrow vaccine from the stockpile to sell to their private customers. Because the public sector purchases half of the childhood vaccines, CDC's stockpiles can manage a complete vaccine supply disruption of at least 3 month's duration or longer if disruptions are incomplete (e.g., a vaccine with more than one manufacturer).

Assuring an uninterrupted supply of vaccine is especially challenging during manufacturing production problems. A staged response is used to manage such supply disruptions. If there is more than one manufacturer the disruption can often be managed by increasing the use of another manufacturer's vaccine. If there is only one manufacturer, then the shortage is, by definition, a national shortage. The first step in managing such a shortage is to limit vaccine purchase in the public and private sectors so that excessive inventory is halted. If these measures are unable to address the shortage, the next step is to use CDC's vaccine stockpile to maintain the vaccine supply. If the stockpile is exhausted or is insufficient to manage the projected scope and duration of the shortage, temporary recommendations are often made to reduce vaccine use. For example, during the Haemophilus influenzae type b (Hib) vaccine shortage from 2007 to 2009, interim recommendations eliminated the booster dose for children unless they were at high risk such as Native Americans or Native Alaskans. Once the shortage resolved, routine recommendations were reinstated. Following the resolution of a severe vaccine shortage, additional catch-up vaccination is often needed to ensure adequate protection. The much greater visibility of public sector vaccine with centralized vaccine distribution and VTrckS has greatly enhanced the CDC's ability to manage shortages. Current information on U.S. vaccine shortages and delays is available from the CDC ( https://www.cdc.gov/vaccines/hcp/clinical-resources/shortages.html ).

Assuring Access to Vaccines

A critically important step in the ensuring vaccine delivery is to make vaccines available with as few barriers to immunization as possible and with a minimum of missed opportunities to vaccinate. Cost of vaccines to the patient is a widely studied and recognized barrier, and one that is amenable to government programs that support the purchase of vaccine for financially vulnerable persons and government laws and regulations that require private insurance coverage for immunization—two solutions to the cost barrier that are cornerstones of the U.S. immunization system.

The 2010 ACA requires private health insurers to cover CDC/ACIP-recommended vaccines on the pediatric and adult immunization schedules without cost sharing (i.e., first dollar coverage) as a benefit for persons of all ages when administered by a provider in the person's health plan network. Vaccines are required to be covered within 1 year of the publication of a new ACIP vaccine recommendation in the MMWR . Although health plans not covering immunization before the ACA are exempt from the immunization requirement, their exemption (grandfathered status) disappears if the plan makes any changes in coverage. Before ACA, the general practice of the health insurance industry had been to cover vaccines, however, 11% of young children and 20% of teens had commercial insurance that did not cover vaccines (underinsured). ^, The net effect of the ACA has been a decline in the number of persons with private health insurance that does not cover immunizations and a reduction in the number of persons without health insurance.

The VFC program has been critical in assuring access to vaccine for children by providing financially vulnerable children with government-purchased vaccine up to a child's 19th birthday. The VFC program is an entitlement to eligible children that has mandatory federal funding of ACIP-recommended vaccines. Four groups of children are entitled to VFC vaccine: children who are 18 years of age or younger who are either (a) eligible for Medicaid health insurance (a federal–state means-tested health insurance program), (b) have no health insurance, (c) Native American or Alaska Native children, or (d) have insurance but are not insured for recommended vaccines (underinsured) and are vaccinated at a Federally Qualified Health Center or Rural Health Clinic. The combination of VFC and the ACA's requirement that private health insurance cover ACIP-recommended vaccines helps to ensure that U.S. children have access to vaccine without out-of-pocket costs for the vaccine.

The VFC program has important characteristics that make it effective at assuring access to vaccine with a minimum of missed vaccination opportunities. First and foremost, VFC is an entitlement with mandatory federal funding that is not subject to the annual appropriations process. State and local VFC immunization programs determine the size of the VFC-eligible population in their jurisdiction, and the CDC uses this information to determine the vaccine purchase funding needed to meet the vaccination needs of all VFC-entitled children.

Second, the ACIP is the sole authority for the list of vaccines that are included in the VFC program. The ACIP indicates to the CDC which vaccines are included by passing a VFC resolution that specifies the number of doses and VFC age groups entitled to VFC vaccine. Unlike ACIP's vaccine recommendations, VFC resolutions are not subject to approval by the CDC director and become effective immediately after a passing vote. For a new vaccine to become available to VFC children, the CDC negotiates a contract with the manufacturers of the vaccine.

Enrolled VFC providers establish eligibility to the program at the point of service by asking parents whether their child meets any of the eligibility categories. By VFC statute, independent verification is not required. Providers are required to indicate eligibility status in the child's medical record. Having point-of-service eligibility determination reduces missed immunization opportunities because parents are not required to sign their children up for VFC before visiting their child's healthcare provider. This provision is intended to prevent missed opportunities seen in the measles resurgence of 1989 to 1991 where mandatory referrals to public health department clinics were required. VFC providers order and receive their vaccine without charge. The VFC program pays for the vaccine and its distribution to VFC providers, in contrast to the reimbursement model for private insurance where the provider pays for vaccine upfront at a financial risk.

The CDC also administers the Section 317 program to state and local immunization programs. Although smaller than VFC in total funding, immunization program grantees can use Section 317 vaccine to vaccinate populations that are not captured by private insurance or the VFC program (e.g., uninsured adults) and to vaccinate rapidly in response to a vaccine-preventable disease outbreak. The ACA authorized Section 317 permanently added the authority to allow states to purchase vaccine for adults using the CDC's vaccine contracts.

State and local immunization programs are responsible for enrolling providers into VFC. States also enroll non-VFC providers who can receive state or locally purchased vaccine or Section 317–purchased vaccine for the state/local priority populations. These networks of providers are vital to the success of government vaccine programs. The majority of VFC providers are private clinicians who use a combination of VFC vaccine and privately purchased vaccine to immunize children in their practices. VFC has approximately 40,000 providers enrolled, and three quarters of these providers are private clinicians. In total, VFC providers vaccinate approximately 90% of U.S. children. ^,

Maintaining the capacity and quality of networks of providers is also a responsibility of state and local immunization programs. Federal funding is made available through VFC and Section 317 to enable state and local programs to enroll new providers and conduct quality improvement activities among existing providers in accordance with the standards of immunization practices. Section 317 grant funds are also available to provide direct service delivery in public health department clinics.

The community of immunization providers is large and growing in diversity. Many state and local immunization programs are expanding access to vaccines through pharmacists and community vaccinators who can vaccinate in schools and other settings. ^,

The role of governmental programs in assuring access to vaccines for adults has been more limited than its role in assuring access to vaccines for children. Medicare Part B covers pneumococcal, COVID-19, and influenza vaccination services for all persons 65 years of age or older enrolled in Medicare Part B (>95% of the older-than-65 population) and hepatitis B vaccine for people at medium-to-high risk for hepatitis B, including people of any age with end-stage renal disease. State Medicaid programs, supported by federal contributions, may cover recommended vaccines for younger adults enrolled in Medicaid, but there is not a requirement that Medicaid cover vaccines for adults. In 2005, Medicare part D was enacted to cover prescription drugs, including vaccines other than those included in Part B, such as zoster vaccine. Thus, all vaccines recommended for the Medicare population are covered by Medicare Part B or Medicare Part D, for persons who pay to participate. Provider billing for vaccines under Medicare Part D is more complicated than under Medicare Part B. The Centers for Medicare & Medicaid Services provides additional information on Part D coverage for vaccines.

The clinical work of vaccinating is supported with a fee-for-service payment mechanism in the private and public sectors. Private insurance and Medicaid pay vaccine administration fees; Medicaid pays administration fees for Medicaid-enrolled VFC children and adults, but reimbursement varies from state to state. VFC providers charge parents of non-Medicaid VFC child­ren a fee for the administration of VFC vaccine, although the VFC statute prohibits VFC providers from denying VFC vaccine to an eligible child solely because the parent cannot pay the adminis­tration fee. The importance of the vaccine administration fee is apparent from its prominence in the NVAC's vaccine financing recommendations. The NVAC made five recommendations to improve the vaccine administration fee, including that “the American Medical Association Relative Value Scale Update Committee should review its relative value unit coding to ensure that it reflects accurately the administrative costs of vaccination, inclu­ding the potential costs and savings with the use of combination vaccines.”

Control and Prevent Infectious Diseases

The desired outcome of immunization is reduction, elimination, or eradication of disease or disability and prevention or control of outbreaks and their associated social disruption. The goals of an immunization program determine how intensively a vaccine-preventable disease is monitored ( Table 74.2 ). When programs aim to achieve or sustain disease elimination, intensive efforts are needed surrounding each suspected case. Control programs may also require rapid identification and reporting of cases so that interventions can be offered promptly to reduce complications or limit transmission. Recognition of disease outbreaks is important to limit their scope. Outbreaks may also serve as sentinel events because they may provide the first indication of important program deficiencies or unanticipated limitations in vaccine performance, as was seen in the measles resurgence in 1989-1991. While some vaccine-preventable diseases manifest as relatively distinct clinical syndromes (e.g., paralytic poliomyelitis, measles, mumps, and congenital rubella syndrome), others may require laboratory testing to differentiate the illness from non–vaccine-preventable causes of the same syndrome (e.g., gastroenteritis or pneumonia). Vaccine-preventable causes of pneumonia (e.g., Streptococcus pneumoniae and Haemophilus influenzae ) are difficult to diagnose even when optimal laboratory testing is available. Monitoring trends in the epidemiology of these diseases relies on targeted tracking of more specific manifestations of the disease, such as bacteremia and meningitis.

Reports of the occurrence of vaccine-preventable diseases have been the means of evaluating the impact of most of the immunization disease programs. Information is obtained through state health departments, which have authority to mandate reporting of selected diseases by physicians and other healthcare providers to the state level. The Council of State and Territorial Epidemiologists, in consultation with the CDC, establishes the list of nationally notifiable diseases. In the United States, most vaccine-preventable diseases of childhood are reported to the National Notifiable Diseases Surveillance System.

For all nationally notifiable diseases, data collected weekly include date of report and locating information (e.g., state and, when appropriate, county). For many of the vaccine-­preventable diseases, supplemental information (e.g., date of onset, age, sex, race/ethnicity, vaccination status, whether the case was confirmed by a laboratory, and disease complications) is forwarded to the CDC. Reports by state of cases notified that week and the cumulative total for the year of many of the vaccine-preventable diseases are published in the MMWR. Case definitions have been established with the Council of State and Territorial Epidemiologists that detail criteria for confirmation of a given illness as a case of a vaccine-preventable disease.

The National Notifiable Diseases Surveillance System is supplemented by other surveillance systems operated by the CDC for several vaccine-preventable diseases. Some of these systems involve laboratory-based methods and others rely on death certificate data or other administrative data systems. Where data are not available nationally, special sentinel surveillance systems in selected states and communities may be established. Laboratory-confirmed, population-based surveillance systems in selected geographic areas are used to monitor causes of invasive bacterial disease, including H. influenzae type b, meningococcal disease, and pneumococcal disease. The largest of these is the Active Bacterial Core surveillance within the Emerging Infections Program network, which tracks disease and assesses vaccine impact in parts or all of 10 states.

The Emerging Infections Program has also been used to evaluate the incidence of hospitalization for laboratory-confirmed influenza, as well as vaccine effectiveness; the burden of invasive pneumococcal disease, and more recently the Emerging Infections Program network's HPV Impact has conducted active population-based surveillance of cervical intraepithelial neoplasia grades 2 and 3 and adenocarcinoma in situ, as well as associated HPV types in women 18 years of age and older.

The New Vaccine Surveillance Network was established in 2000 to determine the impact of various vaccines. It provides population-based laboratory confirmed surveillance in seven geographically diverse sites to establish the burden of influenza, respiratory viruses, acute gastroenteritis, and recently COVID-19 in children and the impact of various vaccines on this burden. As new vaccines are developed, licensed, and made available for use, surveillance and monitoring are critical to address their impact on communities and populations. The New Vaccine Surveillance Network supports broad-based laboratory confirmed surveillance and research projects for acute gastroenteritis in areas with a population base of at least 500,000.

In addition, a sentinel counties surveillance system was used for tracking hepatitis B infections, and varicella active surveillance was conducted in three sites. ^, Administrative data sources have provided information on varicella, rotavirus, and pneumococcal hospitalizations. The National Health and Nutrition Examination Survey provides periodic reports on population immunity to various vaccine-preventable diseases, including antibody levels against pertussis, HPV, and other infectious agents. ^, Reporting completeness for each of the surveillance systems varies, and the active systems are substantially more sensitive than the passive National Notifiable Diseases Surveillance System. Completeness of reporting varies by source of report and type of disease, with more complete reporting from hospitals and laboratories than from individual physicians for diseases such as H. influenzae type b.

Surveillance data are analyzed to determine whether disease reduction occurs with increasing vaccine coverage; greater-than-expected reductions and changes in incidence beyond the target age groups may signal important herd effects. ^, Surveillance data are also analyzed to evaluate potential changing epidemiology of disease, such as a shift to a predominance of adult cases where there had been a childhood focus or the emergence of disease caused by strains of the pathogen that are not targeted by the vaccine. Surveillance can also determine whether cases are a result of failure of vaccine or of failure to vaccinate. ^,

Vaccine Effectiveness

When a substantial proportion of cases of vaccine-preventable disease occur in persons with a history of vaccination, an investigation is often undertaken to determine whether the rates of vaccine failure are within expected ranges. Vaccine effectiveness is the term used for these observational studies, to contrast with vaccine efficacy, which is usually measured prospectively in randomized, double-blind, placebo-controlled trials. Several methods are used to evaluate vaccine effectiveness in the post-licensure setting, including cohort, case-control, secondary attack rates in families, and other techniques. ^, Vaccine effectiveness (VE) is calculated based on the attack rate in vaccinated persons (ARV) and the attack rate in unvaccinated persons (ARU); (VE%) = ([Attack Rate _unvaccinated – Attack Rate _vaccinated) ]/Attack Rate _unvaccinated ) × 100). When case-control studies are used, the odds ratio is used to approximate the relative risk of disease in the vaccinated compared with the unvaccinated population (ARV/ARU).

Of particular note, has been a relatively simple screening technique to help determine whether a more rigorous investigation of vaccine effectiveness is warranted. This screening analysis requires data on both the vaccination status of the cases and the vaccination status of the populations from which the cases are drawn. These data are often available from existing population-based surveys and other methods. Data must always be broken down into dichotomous variables, with unvaccinated persons compared with vaccinated individuals. For example, for a vaccine recommended in three doses, the population of cases vaccinated must be calculated excluding persons who received only one or two doses. Similar adjustments must be made for coverage assessment. Fig. 74.4 displays a nomogram that allows approximation of vaccine effectiveness.

Public health investigation of vaccine effectiveness can be facilitated in areas where comprehensive immunization information systems exist. One study of pentavalent rotavirus vaccine reported a vaccine effectiveness of 82% using data from an immunization information system (IIS) compared with 82–88% using data from providers. Vaccine effectiveness studies may use population-based control subjects obtained through telephone surveys or household visits, as well as control subjects obtained through inpatient or outpatient healthcare facilities who have conditions different from the vaccine-preventable disease under investigation. For example, influenza vaccine effectiveness studies often use control subjects who have sought medical care for respiratory illness but have laboratory tests negative for influenza virus. When vaccine effectiveness is within expected limits but disease transmission persists, new immunization strategies may be considered. For example, investigations showed that measles transmission could persist despite a vaccine effectiveness of greater than 90% and high levels of coverage with a single dose of vaccine. This information was critical in the decision to universally recommend a second dose of measles vaccine. When vaccine effectiveness is lower than expected, investigations also are triggered into potential technical causes, such as failure to maintain vaccine at the proper temperature (i.e., deficiencies in the cold chain).

When most cases occur in unvaccinated persons, investigations can identify geographic and demographic characteristics to help guide future vaccination efforts. For example, a resurgence of measles in the United States during 1989–1991 was found to be caused by a failure to vaccinate young preschool children and led to a major national initiative to improve coverage. ^, In contrast, relatively small outbreaks of measles in 2008 and 2011 were focused in communities where personal beliefs led parents to exempt their children from vaccination required for school entry. ^, In 2013–2014 and 2018–2019, several measles outbreaks were linked to local religious and cultural communities where immunization rates were very low.

Conduct Surveillance for Immunization Coverage

The U.S. immunization program uses a set of coverage surveys to monitor implementation of CDC/ACIP vaccination recommendations. These surveys provide an indirect measure of the population protection from vaccine-preventable diseases while assessing performance of the federal, state, and urban area immunization programs.

Coverage among young children has been comprehensively measured since 1960, except for 1985–1991. ^, Beginning in 1994, immunization of preschool children has been assessed through the National Immunization Survey (NIS), which consists of random-sample telephone surveys inquiring about immunization coverage by 24 months of age. In 2006, the NIS was expanded to the NIS-Teen to include children 13- through 17-year-old to assess coverage of vaccines recommended during adolescence. Descriptions of this family of national immunization surveys is available at the CDC website. In addition, the NIS Adult COVID Module (NIS-ACM) and Child COVID Module (NIS-CCM) were added in 2021 in response to the COVID-19 pandemic, to assess COVID-19 vaccination coverage. The website also provides information for both the public and healthcare professionals on the purpose, methods, instruments, and results of these surveys.

The NIS and NIS-Teen provide national estimates of coverage, usually with 95% confidence intervals of less than 3 percentage points, and comparable, statistically valid estimates for each of the 50 states, selected local areas, Guam, and Puerto Rico. Although response rates vary by survey area, the methods are identical, allowing for comparison among the survey areas. In addition to the jurisdictions sampled annually, states are allowed to purchase from the CDC additional coverage surveys for geographic areas of interest within their jurisdictions.

Since parental recall of vaccination is known to have poor concordance with provider-verified vaccination status, the NIS and NIS-Teen surveys only report verified vaccinations by the sampled child's healthcare provider(s). Parents are interviewed about their demographics and access to vaccines, and they are asked permission to contact their child's providers to report the child's immunization history. Survey results are adjusted for telephone nonresponse. Because of the increasing use of cell phones and a concomitant decline in landline-only households, a cell phone sample frame was added in 2011 for adjustment of telephone access, and starting in 2018, a cell phone only sample frame was used. The NIS and NIS-Teen results are reported annually in the CDC's MMWR and on the CDC's website. ^, The CDC makes NIS and NIS-Teen public-use data files available to researchers and others who request access to deidentified raw data. The also CDC evaluates its immunization surveys for evidence of bias or nonrepresentativeness, and conducts research on methods to improve the representativeness and value of the surveys.

The CDC estimates coverage for vaccinations such as influenza, pneumococcal, HPV, zoster, and Tdap in adults in the United States by using data from several nationally representative surveys: the Behavioral Risk Factor Surveillance System, the National Health Interview Survey, the Pregnancy Risk Assessment Monitoring System, the Centers for Medicare & Medicaid Services Minimum Data Set, and Internet panel surveys of adults, healthcare personnel, pregnant women, and long-term care facility residents. The CDC also provides ongoing vaccine effectiveness assessment during and after each influenza season through the National Immunization Survey-flu survey. The CDC's website provides reports, interactive figures, and data tables for these adult vaccination coverage surveys, including for influenza, at https://www.cdc.gov/vaccines/imz-managers/coverage/adultvaxview/index.html and http://www.cdc.gov/flu/fluvaxview/index.htm .

Surveillance for vaccination coverage among new kindergarten students is conducted by the states and reported annually to the CDC. These data are published on the CDC's SchoolVaxView website and in the MMWR. ^, Managed care health plan performance on immunization is measured annually by the National Committee on Quality Assurance using the Health Plan Employer Data and Information Set (HEDIS). The HEDIS currently estimates immunization coverage for 2-year-old children, 13-year-old adolescents, 50-year-old, and older adults. Results of HEDIS measures are published annually in a report called “The State of Healthcare Quality.” Separate measures are made for commercial plans and Medicaid or Medicare plans.

Techniques for measuring immunization coverage at the provider level were developed during the 1990s and have been integrated into the VFC program since 2000. Additionally, immunization programs can use their IISs for determining provider-level coverage. IISs are also an important way for immunization programs to assess coverage at the local level and target areas of undervaccination for outreach.

Sustain and Improve Immunization Coverage Levels

Sustaining and improving vaccination coverage levels requires a system that actively promotes vaccination of individuals targeted by recommendations. Simply making vaccines available will not result in coverage levels high enough to achieve immunization program goals and objectives. This section describes barriers to vaccination, key program activities to promote vaccination, and evidence-based strategies to improve coverage.