July 30, 2021
Dr. Wilmark N. Gular, MD
The importance of vaccination cannot be overemphasized. Edward Jenner’s providential inoculation of the smallpoxvirus started an era of eradicating one of the world’s deadliest infections.1Today, there are over twenty vaccines used to help prevent specific infections.
Majority of vaccines are geared toward protecting infants and children from vaccine-preventable illnesses. Due to their immature immunity, children are more prone to contract infections and suffer from complications. Most countries have a health program that covers vaccination of infants and children.
It is not only children who are prone to vaccine-preventable illnesses. The adult population, particularly those 60 years and above (older adult), are at risk for diseases such as pneumonia and herpes zoster. The phenomenon that predisposes the older adults to these illnesses is known as immunosenescence.2It is basically an age-related deterioration of the immune system.
Most developed countries have national programs for immunizing the adult population but there seems to be a lower coverage compared to infants and children. This lower coverage, despite the availability of a program, contributes to a higher failure rate in herd immunity.
The Centers for Disease Control recommends a schedule for adult vaccination aged 19 years and above.3 It comes with a pocket-sized version to help individuals and healthcare workers ensure that immunizations are up to date. Moreover, this serves as a reminder of the importance and availability of the said vaccines.
World Health Day 2018advocates for Universal Health Coverage.4 Vaccination is an essential part of this and must beincluded in the national healthcare of every country.This technical article identifies the different factors associated with problems in adult vaccination. Recognizing key areas in vaccine failure can help build an effective and sustainable adult vaccination program.
The concept of healthy aging includes a healthy triad.5 This consists of healthy diet, exercise, and vaccination. These three are essential primary prevention strategies to counter diseases. Vaccination in the adult population seems to be lower in coverage compared to children. This may be due to lack of education regarding the importance of vaccines in the adult population as well as the costs involved in availing such service. Moreover, immunosenescence contributes to higher failure rate of some vaccines among older adults. Strategies such as the life-course immunization approach and development of newer vaccine technologies may help increase the coverage and success rate of vaccines in adults.
The review article identifies the physiologic, demographic, technical, economic, and political factors associated with vaccine failure in the adult population. It also addresses strategies to make adult immunization effective and sustainable.
The Aging Population
It is estimated that by the year 2020, the number of older adults or those aged 65 and above would be greater than children below 5 years. Moreover by the year 2050, the number of adults aged 60 years and above is expected to rise to 22 to 25 % of the total global population and 400 million aged 80 years and above.5
This rise of the adult population, particularly older adults, would result to an increase in the prevalence of diseases associated with aging such as non-communicable diseases as well as infectious diseases. Recognizing this possibility would help prepare better measures in addressing the diseases specifically implementing primary preventive strategies such as vaccination.
Noncommunicable and Infectious Diseases in Adults
There is a rise of non-communicable and lifestyle related diseases among adults. Hypertension, diabetes, cancer, and chronic lung disease still remain to be the top diseases of adults in a global scale. These diseases, however, have been reduced by healthy nutrition, exercise, and appropriate medical care. With the management and control of these diseases, the lifespan of adults is expected to be longer.
Infectious diseases among adults, however, have become somewhat difficult to manage. There has only been modest reductions in cases of influenza and pneumonia in adults despite the availability of vaccines. These diseases also contribute to a significant percentage of morbidity and mortality in the said population.
Immunosenescence and Age Related Immune Responsiveness
An individual’s response to vaccine is affected by his or her age. Extremes of ages exhibit an immunocompromised state. The mechanisms for explaining such state differ for both age groups. Infants and children are thought to have lower immunogenicity because of immature immunity particularly cell mediated immunity. Older adults, on the other hand, exhibit immunosenescence.
Immunosenescence can be explained by a greater differentiation of immune cells into myeloid progenitors compared to lymphoid ones. Lower antibody production results from a relative shift to a lower percentage of naïve cells and a higher number of memory B cells and T cells. This is illustrated by the lower success rate of vaccination against herpes zoster for individuals aged 70 and above.
The timing of vaccination also contributes to effective vaccination. This is demonstrated by a higher success rate of the combination of Hepatitis A and B vaccine compared to its monovalent counterparts among older individuals not previously immunized.
Lower Vaccination Coverage among Adults
Reduced awareness on the benefits and schedule of vaccines as well as the costs of the vaccine contribute to lower vaccine coverage among the adult population. As such, the burden of vaccine-preventable infections such as pneumonia, influenza, hepatitis B and Human Papilloma Virus (HPV) remain high.
Table 1 shows the adult vaccination coverage in the United States in the year 2015.5
Vaccine coverage (%)
By age group (years)
Healthcare workers (HCW) with direct patient responsibilities.
bReported receipt of at least two doses of hepatitis A vaccine (ever). cReported receipt of at least three doses of hepatitis B vaccine (ever). dReported receipt of at least one dose of HPV vaccine (ever).
eFor the previous 2014–2015 season.
fReported receipt at least one dose of pneumococcal vaccine (ever). gAdults aged 19–64 years at increased risk of pneumococcal disease. hWithin past 10 years.
iReported receipt of herpes zoster vaccine (ever).
Tdap: tetanus, diphtheria and pertussis vaccine; HCW: health care worker; NHIS: National Health Interview Survey
Advances in Vaccine Technology
Vaccine efficacy is lower among adults due to immunosenescence. The success rate of unadjuvantedinfluenza vaccine among adults is about 17 to 50% which is low compared to the 70 to 90% success rate seen in children. Due to this lower rate, some adults are discouraged to have the vaccine. In addition, protection from influenza is short-lived due to antigenic shift. All these could mar perceptions of an adult regarding vaccine, negatively impacting vaccination.
Strategies to improve vaccines have been developed. The adjuvanted trivalent influenza vaccine has shown higher immune response among adults compared to the unadjuvanted vaccine. A herpes zoster subunit vaccine which also has undergone adjuvantation with a liposomal based formulation has been evaluated and has shown to have higher vaccine efficacy among adults and older adults.
Effective Vaccination Strategy
An effective vaccination strategy would depend on a variety of factors. There should be an increase in the public awareness on the importance of vaccination. Furthermore, healthcare workers, physicians in particular, must take bold steps in promoting vaccination among all adults. The vaccination schedule must also be arranged to fit in with the patient’s schedule and convenience. In addition, the national government must make moves and ensure that a vaccination program is available and running smoothly.
The economics involved in vaccination cannot be overlooked. Vaccination remains to be a cost-effective strategy in reducing morbidity and mortality from infectious diseases. It has been shown that life-course immunization costs are lesser than other primary prevention strategies. Thecost benefit of vaccination can be measured by cost optimization which looks broadly at the overall benefits on health, social, and economic aspects of the program.
There are a myriad of challenges hounding adult vaccination. From the concept of immunosenescence to problems in promoting adult vaccination, the list of problems seem to be unending. However, there is hope in developing and implementing strategies such as the life-course immunization program to increase overall adherence to vaccination.