Immunizations are widely considered one of the leading public health achievements of the 20th century. Mandatory immunization programs have eradicated polio and smallpox in the United States and reduced the number of deaths from several childhood diseases, such as measles, to near zero. A consistent supply of many different vaccines is needed to support this effort. CDC currently recommends routine immunizations against 11 childhood diseases: diphtheria, tetanus, pertussis (whooping cough), Haemophilus influenzae type b (most commonly meningitis), hepatitis B, measles, mumps, rubella (German measles), invasive pneumococcal disease, polio, and varicella (chicken pox). By combining antigens (the component of a vaccine that triggers an immune response), a single injection of a combination vaccine can protect against multiple diseases. The federal government, primarily through agencies of the Department of Health and Human Services (HHS), has a role both as a purchaser of vaccines and as a regulator of the industry. The federal government is the largest purchaser of vaccines in the country. CDC negotiates large purchase contracts with manufacturers and makes the vaccines available to public immunization programs under the Vaccines for Children (VFC) program. Under VFC, vaccines are provided for certain children, including those who are eligible for Medicaid or uninsured. Participating public and private health care providers obtain vaccines through VFC at no charge. A second program, established under section 317, of the Public Health Service Act, provides project grants for preventive health services, including immunizations. Currently, CDC supports 64 state, local, and territorial immunization programs (for simplicity, we refer to them as state immunization programs). In total, about 50 percent of all the childhood vaccines administered in the United States each year are obtained by public immunization programs through CDC contracts. The federal government is also responsible for ensuring the safety of the nation’s vaccine supply. FDA regulates the production of vaccines. It licenses all vaccines sold in the United States, requiring clinical trials to demonstrate that vaccines are safe and effective, and reviews the manufacturing process to ensure that vaccines are made consistently in compliance with current good manufacturing practices. Once vaccines are licensed, FDA also conducts periodic inspections of production facilities to ensure that manufacturers maintain compliance with FDA manufacturing requirements. States also have an important role in immunization efforts. Policies for immunization requirements, including minimum school and day care entry requirements are made almost exclusively at the state level, although cities occasionally impose additional requirements. Each state also established an immunization infrastructure to monitor infectious disease outbreaks, administer federal immunization grants, manage centralized supplies of vaccine, and otherwise promote immunization policies. Recent vaccine shortages have necessitated temporary modifications to the recommended immunization schedule and have caused states to scale back immunization requirements. In our survey of 64 state immunization programs, administered through the Association for State and Territorial Health Officials (ASTHO), all 52 responding programs indicated that they had experienced shortages of two or more vaccines and had taken some form of action to deal with the shortages. Vaccine shortages experienced at the state level have, in turn, prompted cutbacks in immunization requirements for admission to day care or school. Thirty-five states reported putting into effect new, less stringent immunization requirements that allow children who have received fewer than the recommended number of vaccinations to attend school. In general, these states have reduced the immunization requirements for day care and/or school entry or have temporarily suspended enforcement of those requirements until vaccine supplies are replenished. For example, the Minnesota Department of Health suspended the school and postsecondary immunization laws for Td vaccine for the second year in a row, with the suspension extending through the 2002-2003 school year. Other states, including South Carolina and Washington, reported allowing children to attend day care or school even if they were not in compliance with immunization requirements, under the condition that they be recalled for vaccinations when supplies became available. While it is too early to measure the effect of deferred vaccinations on immunization rates, a number of states reported that vaccine shortages and missed make-up vaccinations may take a toll on coverage and, therefore, increase the potential for infectious disease outbreaks. The full impact of vaccine shortages is difficult to measure for several reasons. For example, none of the national immunization coverage surveys measures vaccination coverage of children under the age of 18 months—the age cohort receiving the majority of vaccinations. While immunization experts generally agree that the residual effects of historically high immunization rates afford temporary protection for underimmunized children, missed immunizations could make susceptible children vulnerable to disease outbreaks. For example, a CDC analysis of a 1998 outbreak of measles in an Anchorage, Alaska, school showed that only 51 percent of the 2,186 children exposed had received the requisite two doses of measles vaccine. No single reason explains the rash of recent vaccine shortages; rather, multiple factors coincided that affected both the supply of and demand for vaccines. We identified four key factors, as follows. Production Problems - Manufacturing production problems contributed to the shortage of certain vaccines. In some cases, production slowdowns or interruptions occurred when planned maintenance activities took longer than expected; in other cases, production was affected as manufacturers addressed problems identified in FDA inspections. Changes over the last several years in FDA inspection practices may have resulted in the identification of more or different instances of manufacturers’ noncompliance with FDA manufacturing requirements. For example, prior to these changes, biologics inspections tended to focus primarily on scientific or technical issues and less on compliance with good manufacturing practices and documentation issues. FDA did take some steps to inform manufacturers about its inspection program changes; however, some manufacturers reported problems related to how well the changes were communicated. FDA issued a compliance program guidance manual detailing the new protocol for conducting inspections intended for FDA staff. However, the information in it could have provided manufacturers a better understanding of the scope of the inspections, but the manual was not made widely available—only upon request. Removal of Thimerosal - Calls for the removal of the preservative thimerosal from childhood vaccines illustrate the effect that policy changes can have on the supply of vaccine. As a precautionary measure, in July 1999, the American Academy of Pediatrics (AAP) and the U.S. Public Health Service (PHS) issued a joint statement advising that thimerosal in vaccines be eliminated or reduced as soon as possible. While thimerosal was present in several vaccines, removing it from some vaccines was more complex than for others. For example, one manufacturer of the diphtheria- tetanus-acellular pertussis vaccine (DTaP) had to switch its packaging from multidose to single-dose vials due to the removal of the preservative. This process reduced the manufacturer’s output of vaccine by 25 percent, according to the manufacturer. Manufacturer’s Decision to Discontinue Production - Another major factor in the shortage of DTaP, and also Td, was the decision of one manufacturer to discontinue production of all products containing tetanus toxoid. With little advance warning, the company announced in January 2001 that it had ceased production of these vaccines. According to the manufacturer, prior to its decision, it produced approximately one-quarter of all Td and 25 to 30 percent of all DTaP distributed in the United States, so the company’s departure from these markets was significant. In the previous year, another manufacturer that supplied a relatively small portion of DTaP also had stopped producing this vaccine. Together these decisions decreased the number of major manufacturers of DTaP from four to two and of Td from two to one. Unanticipated Demand - The addition of new vaccines to the recommended immunization schedule can also result in shortages if the demand for vaccine outstrips the predicted need and production levels. This was the case with a newly licensed vaccine, pneumococcal conjugate vaccine (PCV), which protects against invasive pneumococcal diseases in young children. PCV was licensed by FDA in February 2000 and formally added to the recommended schedule in January 2001. Company officials said an extensive education campaign prior to its availability resulted in record-breaking initial demand for the vaccine. CDC reported shortages of PCV existed through most of 2001, and the manufacturer was only able to provide about half the needed doses during the first 5 months of 2002. Ongoing manufacturing problems limit production, exacerbating the shortage. While the recent shortages have been largely resolved, the vaccine supply remains vulnerable to any number of disruptions that could occur in the future—including those that contributed to recent shortages and other potential problems, such as a catastrophic plant fire. One key reason is that the nature of vaccine manufacturing prevents the quick production of more vaccine when disruptions occur. Manufacturing a vaccine is a complex, highly controlled process, involving living biological organisms, that can take several months to over a year. Another underlying problem is the limited number of manufacturers—five of the eight recommended childhood vaccines have only one major manufacturer each. Consequently, if there are interruptions in supply or if a manufacturer ceases production, there may be few or no alternative sources of vaccine. One situation that may help add to the supply of existing vaccines is the development of new vaccines. A recent example is a new formulation of DTaP that recently received FDA approval and has helped ease the shortage of DTaP. We identified 11 vaccines in development that could help meet the current recommended immunization schedule. These vaccines, some of which are already licensed for use in other countries, are in various stages of development, but all must undergo a rather lengthy process of clinical testing and FDA review. While FDA has mechanisms available to shorten the review process, they are not used for most vaccines under development. FDA policies generally restrict the use of its expedited review processes to vaccines that offer protection against diseases for which there are no existing vaccines. Because childhood vaccines under development often involve new forms or combinations of existing vaccines, they typically do not qualify for expedited FDA review. Federal efforts to strengthen the nation’s vaccine supply have taken on greater urgency with the recent incidents of shortages.