By: Samira Zantout, PharmD Candidate 2021; UMKC School of Pharmacy
Mentors: Jamie Rouch, PharmD, BCPPS and Jean Freudenthal, PharmD; Children’s Mercy Kansas City
How many vaccines have you received in your lifetime? Following the Centers for Disease Control and Prevention (CDC) recommendations, the average person receives over 30 doses of vaccinations by the age of 18 and that is not counting the annual influenza vaccine.10 Completing and staying up to date on different vaccination series is an investment into building long-term immunity against vaccine-preventable diseases. However, it can be a lengthy time commitment, especially for patients that have to redo their entire vaccination schedules. This is the reality for patients that receive a hematopoietic stem cell transplant (HSCT). 2,4
HSCT is a procedure in which hematopoietic stem cells are harvested from a donor (termed allogeneic) or from the patient (termed autologous) and then transfused intravenously into the patient. Ultimately, the newly transfused cells will travel and reach the bone marrow, where they can completely take over or enhance the functionality of the patient’s bone marrow to produce new blood cells.6 With almost 84,000 stem cell transplants completed worldwide every year, the number of living HSCT recipients is on the rise.8 A growing number of those living recipients are pediatric patients.
The role of HSCT in the pediatric population has widely expanded over the past two decades to go beyond conventional, oncological indications like leukemias and lymphomas.4,5,6 In fact, one-third of pediatric patients undergo HSCT to treat a variety of non-oncological disorders, such as sickle cell anemia, metabolic disorders, and immune deficiencies like beta thalassemia.4,5,6 While the procedure aims to replace defective cell lines and ultimately improve the patient’s impaired immune system, it comes with numerous risks and complications the transplant team must address. For example, serious infections have been shown to account for 4- 20% of late deaths after receiving an HSCT and infections are the leading cause of posttransplant complications.6 Due to the complete dissolution of the recipient’s current immune system and the slow reconstitution process over 1 to 2 years to recover those immune defenses, the risk of developing an infection or contracting communicable diseases is the highest in those first 2 years after the transplant.4,5,6 This is especially true for vaccine-preventable diseases.
HSCT-recipients lose their antibody titers to vaccine-preventable diseases like tetanus, polio, measles, mumps, rubella, and others within months to 10 years post-transplant if they are not revaccinated.2,3,7 Moreover, this gradual loss of immunity is affected and
can be accelerated by several factors, including chemotherapy regimens, immunosuppressive regimens, the type of HSCT, and the source of the received stem cells, among other factors.2,3,7 Previous studies have noted that HSCT recipients have a nearly 50-fold increased incidence of invasive pneumococcal disease compared to the general population, with a case-fatality rate of 10-20%.5 Similarly, studies on influenza infections have noted significant morbidity and mortality rates as high as 15- 17% in HSCT-recipients.5 While this reality impacts HSCT-recipients of all ages, the pediatric population is particularly vulnerable.
Pediatric HSCT recipients may not have completed many of their vaccination series prior to the transplant due to young age or disease-associated morbidity and, as a result, may not have built up any pre-transplant immunity.3,5,6 Combined with post-transplant immunosuppressive regimens that can further diminish vaccine-induced immunity, recent occurrence of vaccine-preventable disease outbreaks, a decline in vaccine coverage among the pediatric population, and the exposure risks that naturally accompany daycare or school entry, pediatric HSCT-recipients are at a significant risk of contracting vaccine-preventable diseases.3,5,6,7 Working proactively to minimize this risk is imperative for this population. The most effective, proactive measure health care providers can take to combat vaccine-preventable diseases is quite simple: revaccination. This is a paramount initiative where pharmacists can lead a phenomenal impact.
While the concept of revaccinating all HSCT-recipients seems simple enough in theory, many HSCT recipients remain unvaccinated, are vaccinated late, or undergo an incomplete schedule of vaccinations after their transplant.1,4,6 The most common reasons for these gaps in care include poor documentation of vaccine administration, loss to follow-up with other providers, and conscious deferral by clinicians.1 HSCT recipients follow a unique vaccination schedule that pharmacists can familiarize themselves with and utilize to identify the optimal timing and the best candidate for each vaccination (see IDSA’s Clinical Practice Guideline for Vaccination of the Immunocompromised Host).9 As pharmacists, we are optimally placed in healthcare institutions and in community settings to identify these vulnerable patients and recommend appropriate vaccinations. We can be their vocal advocates to prevent deviating from the recommended vaccination schedule. Moreover, we can be the safety net to catch those missed vaccination opportunities so that they do not fall through the cracks.