Immunogenicity of intradermal hepatitis B vaccination in renal transplant recipients

The influence of selected factors on the immunogenicity of preventive vaccinations against hepatitis A, B and influenza in solid organ transplant recipients undergoing immunosuppressive therapy - a review

INTRODUCTION

Immunization is the most effective form of the primary prevention of infectious diseases. Knowledge on the efficacy and immunogenicity of vaccinations in the group of organ transplant patients taking chronic immunosuppressive treatment remains incomplete.

AREAS COVERED

The aim of this paper was to analyze factors influencing the post-vaccination response in patients undergoing chronic immunosuppressive therapy based on a literature review. Only publications that evaluated the immunogenicity of influenza, HAV and HBV vaccinations in patients on immunosuppressive therapy were reviewed.

EXPERT OPINION

The following methods are used to potentially increase the immunogenicity of vaccinations against HAV and HBV amongst post-transplantation patients: increasing the number of doses, increasing dose volumes, the method of administering as well as the addition of adjuvant. Immunogenicity is also impacted by the immunosuppression mechanism. Overall, vaccination has been concluded to be safe for post-transplantation patients and adverse events following immunization (AEFI) have typically been rated as mild or moderate. The instances of transplant rejections as observable in the long term have not been related to administered vaccinations. The data shows certain correlations of some factors with increased immunogenicity, however it is necessary to repeat the studies on a more representative group of patients.

Vaccination in patients with chronic kidney disease-Review of current recommendations and recent advances

Hepatitis B virus (HBV), influenza, pneumococcus and herpes zoster are important infections which could result in significant morbidity and mortality in patients with chronic kidney disease (CKD). While seroconversion rates after vaccination are often lower in CKD patients compared with healthy adults due to impaired innate and adaptive immunity, vaccinations for HBV, influenza, pneumococcus and herpes zoster are generally effective in reducing the transmission and/or severity of these infections. Practical issues that have an impact on the efficacy of vaccination in the CKD population include the timing, dose, schedule of vaccination, the route of administration, and adjuncts applied at time of vaccination. This review discusses the vaccination regimens and the efficacy of HBV, influenza, pneumococcus and zoster vaccines in CKD patients, and highlights recent advances in enhancing vaccine seroconversion rates.

Travel vaccination recommendations and endemic infection risks in solid organ transplantation recipients

BACKGROUND

Solid organ transplant (SOT) recipients are often heavily immunosuppressed and consequently at risk of serious illness from vaccine preventable viral and bacterial infections or with endemic fungal and parasitic infections. We review the literature to provide guidance regarding the timing and appropriateness of vaccination and pathogen avoidance related to the immunological status of SOT recipients.

METHODS

A PUBMED search ([Vaccination OR vaccine] AND/OR ["specific vaccine"] AND/OR [immunology OR immune response OR cytokine OR T lymphocyte] AND transplant was performed. A review of the literature was performed in order to develop recommendations on vaccination for SOT recipients travelling to high-risk destinations.

RESULTS

Whilst immunological failure of vaccination in SOT is primarily the result of impaired B-cell responses, the role of T-cells in vaccine failure and success remains unknown. Vaccination should be initiated at least 4 weeks prior to SOT or more than 6 months post-SOT. Avoidance of live vaccination is generally recommended, although some live vaccines may be considered in the specific situations (e.g. yellow fever). The practicing physician requires a detailed understanding of region-specific endemic pathogen risks.

CONCLUSIONS

We provide a vaccination and endemic pathogen guide for physicians and travel clinics involved in the care of SOT recipients. In addition, recommendations based on timing of anticipated immunological recovery and available evidence regarding vaccine immunogenicity in SOT recipients are provided to help guide pre-travel consultations.

Immunizations following solid-organ transplantation

PURPOSE OF REVIEW

To highlight the latest evidence for the use of key vaccines that are recommended in organ transplant candidates and recipients.

RECENT FINDINGS

Influenza vaccine is the best studied vaccine; factors affecting immunogenicity of this vaccine include time from transplant, use of mycophenolate mofetil and type of transplant. Newer formulations of influenza vaccine are available, but data for these are limited. Updated recommendations include giving conjugated pneumococcal vaccine to adult transplant candidates and recipients followed by the polysaccharide vaccine to increase serotype coverage. Human papillomavirus vaccine should also be given to transplant recipients, although the immunogenicity may be suboptimal. Quadrivalent meningococcal conjugate vaccine needs to be given in special circumstances such as to patients who are starting eculizumab therapy. Live vaccines in general are contraindicated, although increasing safety data are emerging for Varicella vaccine. Herpes Zoster vaccine may be offered prior to transplant, although the utility of this strategy regarding protection from shingles after transplant is not known. Newer vaccines such as inactivated zoster vaccine and vaccines for the prevention of cytomegalovirus are under study.

SUMMARY

Immunization for organ transplant recipients is an important part of pretransplant evaluation and the long-term care of the transplant recipient.

Prevalence of occult hepatitis B virus infection in kidney transplant recipients

In this cross-sectional study, 207 hepatitis B surface antigen (HBsAg)-negative kidney transplant recipients were evaluated based on demographic and epidemiological data and on the levels of serological markers of hepatitis B virus (HBV) and hepatitis C virus infection and liver enzymes. Patients with HBV or human immunodeficiency virus infection were excluded. Sera were analysed for the presence of HBV-DNA. HBV-DNA was detected in two patients (1%), indicating occult hepatitis B (OHB) infection (the HBV-DNA loads were 3.1 and 3.5 IU/mL in these patients). The results of the liver function tests were normal and no serological markers indicative of HBV infection were detected. The prevalence of OHB infection was low among kidney transplant recipients, most likely due to the low HBsAg endemicity in the general population of the study area.

Serological response to one intradermal or intramuscular hepatitis B virus vaccine booster dose in human immunodeficiency virus-infected nonresponders to standard vaccination

Purpose:

Hepatitis B virus (HBV) vaccination is recommended for all human immunodeficiency virus (HIV)-infected patients without HBV immunity. However, serological response to standard HBV vaccination is frequently suboptimal in this population and the appropriate strategy for revaccination of HIV-infected nonresponders remained controversial. We aimed to determine the serological response to one booster dose of HBV vaccine given by intradermal (ID) or intramuscular (IM) route in HIV-positive nonresponders to standard HBV vaccination.

Materials and Methods:

In this study, 42 HIV-infected nonresponders were enrolled. We randomized them to receive either 10 μg (0.5 mL) for ID (20 cases) or 20 μg (1 mL) for IM (22 cases) administration of HBV vaccine as a one booster dose. After 1 month, anti-HBs titer was checked in all cases. A protective antibody response (seroconversion) defined as an anti-HBs titer ≥10 IU/L.

Results:

Seroconversion was observed in 47.6% of subjects after 1 ID dose. A total of 30% showed antibody titers above 100 IU/L. Except one case, all responders had CD4⁺ >200 cells/mm³. Mean anti-HBs titer was 146.5 ± 246 IU/L. After the one IM booster dose, seroconversion was observed in 50% of cases. A total of 36.3% of subjects had anti-HBs ≥100 IU/L. All responders had CD4⁺ >200 cells/mm³, except one case. Mean anti-HBs titer was 416.4 ± 765.6 IU/L. Responders showed significantly higher CD4⁺ cell counts, in comparison to nonresponders (P < 0.001).

Conclusions:

One booster dose administered IM or ID to HIV-infected nonresponders resulted in similar rates of seroconversion, overall response rate 50%. However, higher anti-HBs titers observed more frequently in IM group.

Evolution of hepatitis B management in kidney transplantation

Chronic hepatitis B virus (HBV) infection adversely influences the clinical outcomes of renal transplant recipients owing to increased hepatic complications. Management of HBV infection in kidney transplant recipients presents a challenge to clinicians, especially in endemic regions. Interferon precipitates renal allograft dysfunction. Treatment with lamivudine, the first oral nucleoside analogue available, resulted in effective viral suppression, reduced liver-related complications, and improved patient survival so that medium-term data showed comparable patient survival rates between hepatitis B surface antigen-positive and HBsAg-negative kidney transplant recipients in the era of effective antiviral therapies. Entecavir has replaced lamivudine as first-line therapy for treatment-naïve subjects in view of the propensity for drug resistance with the latter. Management of HBV infection in kidney transplant patients needs to take into consideration the nephrotoxicity of nucleoside/tide analogues such as adefovir and tenofovir. Prevention of HBV-related complications in kidney transplant recipients starts much earlier prior to transplantation, with vaccination of patients with chronic kidney disease and donor-recipient matching with regard to HBV status. In addition to anti-viral treatment, patients with chronic HBV infection must have regular surveillance for liver cancer and assessment for the development of cirrhosis.

Vaccination of immunocompromised patients

Vaccination of immunocompromised patients is challenging both regarding efficacy and safety. True efficacy data are lacking so existing recommendations are based on immune responses and safety data. Inactivated vaccines can generally be used without risk but the patients who are most at risk for infectious morbidity and mortality as a result of their severely immunosuppressed state are also those least likely to respond to vaccination. However, vaccination against pneumococci, Haemophilus influenzae and influenza are generally recommended. Live vaccines must be used with care because the risk for vaccine-associated disease exists.

Future prevention and treatment of chronic hepatitis B infection

Vaccination for hepatitis B virus (HBV) infection and treatment for chronic hepatitis B, while effective for primary prevention and control of the disease, still have their limitations. Global coverage of HBV immunization needs improvement. Several patient populations are noted to have suboptimal seroprotective rates after HBV vaccination. There are currently several potential new vaccines undergoing animal and human studies, most notably vaccines containing immunostimulatory DNA sequences. Long-term nucleoside analogue therapy is necessary in achieving permanent virologic suppression. Potential new treatments explore new mechanisms of action, including the inhibition of hepatitis B surface antigen release, targeting antifibrotic mechanism, and immunomodulation through novel interferons and therapeutic vaccines. The clinical application of potential new vaccines and therapies would enhance the prevention of HBV infection and treatment of chronic hepatitis B.

Vaccination in HIV-infected individuals

Vaccinations are key to limiting the increased risk of severe infectious diseases in HIV-infected patients for whom the risk–benefit ratio has been re-evaluated. Vaccine safety and immunogenicity depend on both vaccine type and immune deficiency, while vaccine-induced immune activation promotes a transient increase in viral load. Vaccine immunogenicity is reduced and wanes more rapidly, strengthening the need for revaccination. While inactivated vaccines are safe, attenuated vaccines are theoretically contraindicated, but the risk of infectious diseases outweighs the risks of severe adverse events in endemic areas, where the majority of HIV-infected individuals live, thus allowing their use when immune deficiency is moderate. Immune reconstitution with HAART has improved vaccine immune response, highlighting the importance of global access to and early initiation of therapy.

Hepatitis B vaccination in HIV-infected adults: current evidence, recommendations and practical considerations

Immunization with hepatitis B (HBV) vaccine is recommended for all HIV-infected individuals without immunity to HBV. This patient population, however, has relatively poor HBV vaccine responses. Factors associated with this impaired HBV vaccine response in HIV-infected individuals may include older age, uncontrolled HIV replication, and low nadir CD4 cell count. Postvaccination testing for HBV surface antibody is recommended and vaccine non-responders should undergo repeat immunization with a full series. The benefit of double dosage, the appropriate strategy for HIV-infected patients with isolated HBV core antibody and the timing and number of vaccinations in persons with advanced immunosuppression on highly active antiretroviral therapy remain controversial areas.

[Recommendations before travelling for renal transplant patients]

Travel is now a reasonable objective of CKD patients after renal transplantation. However, immunosuppressive treatment makes them particularly susceptible to infections and may interfere with vaccinations and other drugs. Travel in countries with low health level should be strongly discouraged in the first six months after transplantation or following an acute event. Otherwise, specific consultations should be arranged to prepare the patient as soon as possible. Vaccinations should be started early before departure. Specific immunisations include vaccines against hepatitis A, typhoid, meningococcus and rabies in some cases. Living vaccines are formally contra-indicated. Particular attention should be paid for protection against insects because this is the only effective measure against diseases. In the case of malaria, it should be complemented by adapted chemoprophylaxis that should be started 15 days before the departure date. Advice on hygiene measures should be clarified because this can prevent numerous infections, especially of the digestive tract. Advice on the management of diarrhoea is essential, especially in terms of preventing dehydration. Finally, advice about transport and physical risks, especially those related to sun exposure, should also be addressed.

Hepatitis B vaccination: The key towards elimination and eradication of hepatitis B

Hepatitis B virus infection is a global health problem. Worldwide, about 360 million people are chronically infected with the virus. They continue to spread the virus to others and are themselves at risk of chronic liver diseases and hepatocellular carcinoma. The infection can now be treated by antivirals or interferons and the transmission route can be interrupted. Nevertheless, the most effective means is to immunize all susceptible individuals, especially young children, with safe and efficacious vaccines. The combined efforts of vaccination, effective treatment and interruption of transmission make elimination of the infection plausible and may eventually lead to eradication of the virus. Because hepatitis B vaccination has a key role in the control of hepatitis B, properties of this vaccine, its effectiveness in pre-exposure and post-exposure settings, duration of protection after vaccination and the need of booster doses are discussed. Mass hepatitis B vaccination in children decreases the carriage of the virus, and the diseases associated with acute and chronic infection, including hepatocellular carcinoma. Challenges that need to be solved to expand mass vaccination, and the strategies towards elimination and eventual eradication of hepatitis B in the world are also discussed.

Cutaneous delivery of prophylactic and therapeutic vaccines: historical perspective and future outlook

The skin has long been recognized as an attractive target for vaccine administration. A number of clinical studies have tested the epidermal and dermal routes of delivery using a variety of vaccines over the years. In many cases, cutaneous administration has been associated with immunological benefits, such as the induction of greater immune responses compared with those elicited by conventional routes of delivery. Furthermore, there is a growing body of evidence to suggest that such benefits may be particularly important for certain higher-risk populations, such as the elderly, the immunocompromised and cancer patients. Despite the potential advantages of vaccination via the skin, results have sometimes been conflicting and the full benefits of this approach have not been fully realized, partly due to the lack of delivery devices that accurately and reproducibly administer vaccines to the skin. The 5-year outlook, however, appears quite promising as new cutaneous delivery systems advance through clinical trials and become available for more widespread clinical and commercial use.

Vaccination of the solid organ transplant recipient

Active immunization is the most important way to protect immunocompromised patients from vaccine-preventable infectious diseases. Although live vaccines are contraindicated for most immunocompromised patients, many inactivated or conjugate vaccines are safe and generally recommended. Some vaccines are known to be of suboptimal immunogenicity in transplant recipients. As a consequence, this may be associated with an impaired ability to mount protective immunity. Nevertheless, even partial protection has been shown to confer significant benefit to this vulnerable patient group. To increase efficacy in generating protective immunity, patients should complete the full complement of recommended vaccinations early in the course of disease before transplantation. This review summarizes the general recommendations for vaccinations of adult transplant recipients and candidates including special considerations for household contacts and health care workers.

Poor efficacy of intradermal administration of recombinant hepatitis B virus immunization in HIV-infected individuals who fail to respond to intramuscular administration of hepatitis B virus vaccine

OBJECTIVE

It is recommended that hepatitis B virus (HBV)-susceptible, HIV-infected persons be immunized for HBV. However, 44-76% of HIV-infected persons fail to respond to a standard series of recombinant HBV vaccine. Intradermal (i.d.) administration of HBV vaccine has been effective in nonresponders to intramuscularly administered vaccine among healthcare workers, haemodialysis patients and renal transplant recipients. We evaluated the immunogenicity of HBV vaccine given by the intradermal route in HIV-infected individuals who failed to respond to two series of HBV vaccine given intramuscularly.

METHODS

Recombinant HBV vaccine [10 microg HBV surface antigen (HBsAg)/mL] was administered as 0.25 mL i.d. every 2 weeks for four doses in 12 HIV-infected adults who failed to respond to six doses of HBV vaccine administered by the intramuscular route. Anti-HBs was tested at least 2 weeks following the fourth dose of i.d. administered vaccine, and if the anti-HBs titre was negative or <30 IU/L, a second series of four i.d. doses were administered every 2 weeks. Anti-HBs was measured at least 2 weeks following the second series of i.d. administered HBV vaccine and 6 and 12 months after the last dose.

RESULTS

Protective levels of anti-HBs (>10 IU/L) were achieved in six subjects (50%) after four doses. Administration of four additional i.d. doses to the six nonresponders did not result in any additional seroconverters. Five of the six responders had no detectable anti-HBs at 12 months after the last dose of i.d. administered vaccine.

CONCLUSIONS

The i.d. route of administration of recombinant HBV vaccine does not appear to be immunogenic in HIV-infected adults who fail to respond to six doses of intramuscularly administered vaccine.

Immunogenicity and safety of an intradermal boosting strategy for vaccination against influenza in lung transplant recipients

The immunogenicity of influenza vaccine is suboptimal in lung transplant recipients. Use of a booster dose and vaccine delivery by the intradermal rather than intramuscular route may improve response. We prospectively evaluated the immunogenicity and safety of a 2-dose boosting strategy of influenza vaccine. Sixty lung transplant recipients received a standard intramuscular injection of the 2006-2007 inactivated influenza vaccine, followed 4 weeks later by an intradermal booster of the same vaccine. Immunogenicity was assessed by measurement of geometric mean titer of antibodies after both the intramuscular injection and the intradermal booster. Vaccine response was defined as 4-fold or higher increase of antibody titers to at least one vaccine antigen. Thirty-eight out of 60 patients (63%) had a response after intramuscular vaccination. Geometric mean titers increased for all three vaccine antigens following the first dose (p < 0.001). However, no significant increases in titer were observed after the booster dose for all three antigens. Among nonresponders, 3/22 (13.6%) additional patients responded after the intradermal booster (p = 0.14). The use of basiliximab was associated with a positive response (p = 0.024). After a single standard dose of influenza vaccine, a booster dose given by intradermal injection did not significantly improve vaccine immunogenicity in lung transplant recipients.

Hepatitis B vaccination after living donor liver transplantation

BACKGROUND

The efficacy of hepatitis B vaccination after living donor liver transplantation (LDLT) in patients transplanted anti-HBc-positive grafts or in patients who underwent LDLT for fulminant hepatitis B remains unknown.

METHOD

A total of 11 recipients who underwent LDLT between October 1996 and October 2002 prospectively received hepatitis B vaccination three times within 6 months, starting a few weeks after the cessation of hepatitis B immunoglobulin (HBIG) prophylaxis. Serial quantification of the hepatitis B surface antibody (HBsAb) was performed.

RESULTS

At the last follow-up, six out of 11 patients (54.5%) had seroconversion and were free from HBIG thereafter. Four out of those six responders had a peak HBsAb level of more than 1000 IU/L, while the other two patients had peak HbsAb levels below 1000 IU/L. Five patients never responded to the treatment and were back to HBIG prophylaxis. The average age of the six responders was 25.5 years, which was significantly younger than that of non-responders (44.4 years, P<0.05). None had side effects or hepatitis B infection during the study period.

CONCLUSIONS

In conclusion, the use of this treatment modality could be used to reduce the cost of HBIG.

Primary high-dose intradermal hepatitis B vaccination in hemodialysis: cost-effectiveness evaluation at 2 years

Reinforced hepatitis B (HB) vaccination schedules have been tested in nonresponsive hemodialysis (HD) patients. Primary high-dose intradermal (ID) vaccination in HD has been proposed in one study with higher seroconversion rate, but no cost analysis was made. The aim of this prospective study was to confirm this previous report and focus on a cost-effectiveness evaluation of the thorough vaccination with a maintenance program. Thirty-five chronic incident HD patients received primary ID HB vaccination with a reinforced schedule (20 microg Engerix-B every 2 weeks). Revaccination with a monthly single ID dose of 20 microg was performed whenever anti-HBs titer fell under 20 IU/L and continued until a titer of 20 U/L was reached. Outcome measures were cumulative seroconversion rates, mean levels of anti-HBs, maintenance booster doses, rate of seroprotection at the end of the 2-year follow-up and subsequent costs. The present study was associated with an earlier peak of anti-HBs titer (3.9+/-1.7 months) and a higher cumulative seroconversion rate (96.9%) after 1 year. Moreover, a low-booster shot (17.4 microg) of ID Engerix-B/year/patient confers a 100% seroprotection for all responders for a second-year period. The mean cost of our schedule is 127.7 euro/patient for a 2-year period, revaccination included. This current study demonstrates that primary reinforced ID HB vaccination with a maintenance program for a 2-year period warrants the best cost-effectiveness ratio with rapid and sustained seroprotection in almost all HD patients.

Response to an experimental HBV vaccine permits withdrawal of HBIg prophylaxis in fulminant and selected chronic HBV-infected liver graft recipients

Strategies using lamivudine and hepatitis B immunoglobulins (HBIg) for prevention of hepatitis B virus (HBV) reinfection after liver transplantation (LT) are expensive since life-long treatment is needed. We evaluated the possibility to obtain protective hepatitis B surface antigen (HBsAg) antibody (anti-HBs) titers after LT and to discontinue HBIg prophylaxis after a reinforced course of vaccination against HBV using an experimental adjuvant HBsAg / AS04 vaccine (GlaxoSmithKline Biologicals [GSK], Rixensart, Belgium) in patients transplanted for hepatitis B. Fifteen LT patients on stable low-level immunosuppression were vaccinated with a double dose of the vaccine at 0, 1, 2, 6, and 12 months: 5 patients were transplanted for nonviral diseases and 10 patients were transplanted for HBV on HBIg monotherapy. HBIg were continued during baseline vaccination (0, 1, and 2 months) and when anti-HBs titers determined every 6 weeks dropped below 150 IU/L. Overall follow-up was 18 months. Sustained long-term response to vaccination was defined as anti-HBs titers >500 IU/L without further need for HBIg administration during a follow-up period of at least 12 months. Overall sustained response to vaccination was 53% (8 / 15 patients); 80% (4 / 5 patients) in the nonviral disease group and 40% (4 / 10 patients) in the HBV group (2 /2 fulminant and 2/8 chronically infected patients) developed a sustained long-term response and were completely free of HBIg at the end of the 18-month follow-up. No HBV recurrence, rejection episodes, or side effects occurred during the follow-up. In conclusion, protective anti-HBs titers were obtained in a substantial number of LT patients following a reinforced course of HBV vaccination with vaccines containing new immunostimulating adjuvants. Vaccination seems well tolerated and safe and allows long-term discontinuation of HBIg.

Viral infection in the renal transplant recipient

Viruses are among the most common causes of opportunistic infection after transplantation and the most important. The risk for viral infection is a function of the specific virus encountered, the intensity of immune suppression used to prevent graft rejection, and other host factors governing susceptibility. Viral infection, both symptomatic and asymptomatic, causes the "direct effects" of invasive disease and "indirect effects," including immune suppression predisposing to other opportunistic infections and oncogenesis. Rapid and sensitive microbiologic assays for many of the common viruses after transplantation have replaced, for the most part, serologic testing and in vitro cultures for the diagnosis of infection. Furthermore, quantitative molecular tests allow the individualization of antiviral therapies for prevention and treatment of infection. This advance is most prominent in the management of cytomegalovirus, Epstein-Barr, hepatitis B, and hepatitis C viruses. Diagnostic advances have not been accompanied by the development of specific and nontoxic anti-viral agents or effective antiviral vaccines. Vaccines, where available, should be given to patients as early as possible and well in advance of transplantation to optimize the immune response. Studies of viral latency, reactivation, and the cellular effects of viral infection will provide clues for future strategies in prevention and treatment of viral infections.

Prevention of infection in adult travelers after solid organ transplantation

Increasing numbers of solid organ transplant recipients are traveling to the developing world. Many of these individuals either do not seek or do not receive optimal medical care prior to travel. This review considers risks of international travel to adult solid organ transplant recipients and the use of vaccines and prophylactic agents in this population.

Gynecologic issues of the adolescent female solid organ transplant recipient

Advances in the field of transplant medicine are providing adolescent recipients with continual improvements in health and quality of life. With expanding opportunities for normal social and sexual relationships, adolescents require careful attention to their gynecologic and reproductive health (Box 1). Medical considerations vary depending on the type of organ transplanted, underlying and comorbid conditions, and current medication use. Most adolescent girls achieve menarche, however, and irregular cycles should be evaluated and managed with the same considerations applied to healthy young women. The management of menstrual disorders frequently uses hormonal contraceptive methods. Many transplant recipients also are sexually active and require a contraceptive method to prevent a mistimed pregnancy. With careful attention to organ function, other medical problems, and concurrently prescribed medications, many transplant recipients can use safely the currently available methods of hormonal contraception.

Does antigenic overload exist? The role of multiple immunizations in infants

There is no evidence that currently recommended vaccines overload or weaken the infant immune system. Infants have an enormous capacity to respond safely and effectively to multiple vaccines. The schedule for the administration of childhood vaccines is tailored to the unique developmental pattern of the infant immune system. Childhood vaccines provide immediate protection from common childhood illness and establish the foundation for lifelong immunity that develops with subsequent vaccination or infection. Widespread vaccination of infants and children represents a public health triumph of the 20th century. This fact must be reinforced continually by health care workers and parent education to help maintain progress in the 21st century.