Title:Vaccines for Aging Populations
By - Margaret A. Liu, M.D.
Many of us who work in the vaccine field think of vaccines by categories: pediatrics, global health, emerging and epidemic diseases, travelers, or even bioterrorism. Some of us often ignore a type of vaccine that we may need from a personal perspective sooner than we would admit: vaccines for older adults. In July of this year, The Alliance for Aging Research issued a White Paper addressing certain aspects of underutilization of vaccines by this demographic, and provided recommendations for how to increase vaccination rates for older adults in the US. The white paper was focused on four existing vaccines: influenza, pneumococcal, shingles, and tetanus.
The challenges, however, are not simply underutilization, not simply in the US, and not simply for these four diseases in older people (note that I’m avoiding the use of the word “elderly”!) The diseases targeted are clearly important for protection of this age group. However immunizing older individuals is also important because of their role in transmission of disease, even for diseases such as pertussis, which we think of as a pediatric disease. When grandparents and grandchildren spend time together, they transmit pathogens in both directions.
So what are the issues and the challenges for developing and utilizing vaccines for adults in general, and for aging adults in particular? What diseases do we need to target, and what is the immunologic milieu for older adults?
Once maternal antibodies wane, infants must develop their own immunity (preferably by vaccines rather than through illness) to the many pathogens they may encounter. We tend to think of adults as having already encountered many of those pathogens either in nature or from immunizations. Of course an adult can travel, a new disease may emerge, etc. resulting in exposure to new pathogens. But in addition to such de novo encounters, key issues include both waning immunologic memory and the decreased capability of the immune system known as immune senescence. The latter involves both innate and adaptive immune responses, and in particular T cells. Immune senescence has a host of implications for fighting, or recovery from, infections. In addition, the elderly may respond less well to immunizations. To illustrate, let us examine the disease known as Shingles.
Shingles, also known as herpes zoster, occurs in nearly a third of people in the US, arising from a reactivation of the virus that causes chicken pox years after the person has recovered from chicken pox or had the (live) varicella vaccine. While some people get zoster due to immunosuppression (either from immunosuppressive drugs, certain cancers, or HIV), many cases arise in older individuals, with the incidence increasing with age. The first vaccine developed to prevent shingles had an efficacy of about 50% for preventing shingles in people 60 years of age or older. However, the efficacy declined from a high of 64% in the youngest decade vaccinated (the 60s) to 18% for people ≥80 years of age. A newer vaccine has a higher overall efficacy rate (97.2%) that remained the same in all age groups, ranging from 50 years of age to a tier of people ≥70. Nevertheless, the earlier vaccine illustrates the challenge of stimulating the aging immune system. As another example, a high dose influenza vaccine is made each year, specifically for use in people aged 65 or older.
Another challenge for vaccine usage in the elderly is how to make sure that people get their needed immunizations. Most infants receive their vaccinations via immunizations performed neonatally (when they are still in a hospital for birthing) and at subsequent periodic pediatric exams. In addition, in many places documentation of immunizations is mandatory in order for students to enroll in schools. The elderly may not have such specific interactions with health care providers to ensure regular immunizations, and since some vaccines are given at long boosting intervals, (such as tetanus with a minimal interval of 5 years to the next dose) individuals may not remember when it is time for a boost.
For those of us working to develop new vaccines, let us remember to keep the aging population in mind, both in terms of the specific disease targets, and in terms of the specific challenges of the aging immune system.