COVID-19 Booster Vaccination in Older Adults: Who, Why, and How

William H. Hung, MD, MPH


November 16, 2021

William H. Hung, MD, MPH

The rapid development and deployment of effective COVID-19 vaccines in populations have mitigated the spread of the disease in communities, particularly those with high rates of vaccination. Given concerns that the vaccine's effectiveness wanes over time, COVID-19 booster vaccination has been discussed and investigated. A recent article highlights the potential of a COVID-19 vaccine booster to reduce the incidence of infections and risk for infection, and two others consider how to deploy vaccinations to those in greatest need Specifically, these articles provide evidence for the effectiveness of boosters, which patients should be targeted, and lessons for policy-makers and health system leaders charged with deploying the booster vaccines effectively and efficiently.

Booster Vaccine Effectiveness in Older Adults

This study used a nationwide health database in Israel to examine the effect of a booster vaccine (Pfizer-BioNTech) among adults aged 60 years or older who had received a second dose of vaccine at least 5 months prior. The third dose (booster) vaccine was approved on July 30, 2021, and the study period ran from July 30 to August 31, 2021. A total of 1,137,804 individuals were included in the analysis.

The primary analysis compared the rate of confirmed COVID-19 infection and the rate of severe disease in those who received a booster dose 12 days prior and those who did not receive the booster. The rate of confirmed infection in the booster group was 11.3 times lower than in the nonbooster group, and the rate of severe infection in the booster group was 19.5 times lower. The rate of severe infection was 29 cases in 6,265,361 person-days at risk in the booster group and 294 cases in 4,574,439 person-days at risk in the nonbooster group. In a secondary analysis among the booster group, the rate of confirmed infection was lower by a factor of 5.4 at 12 days or more after the booster vaccine compared with 4-6 days after the booster vaccine.

The study was not a randomized trial, and there may be differences between the groups other than receiving the vaccination, such as infection control behavior. However, the authors mitigated these potential biases with the secondary analysis, which showed that, even among those who received the booster, the booster is more effective at 12 days and after, when immunity builds up. This study demonstrates that the booster vaccine is effective in preventing COVID-19 infection and severe infection and probably restores the protection afforded by vaccination that wanes over time.

SARS-CoV-2 Infection After Immunization

Although vaccines are effective in preventing SARS-CoV-2 infection, breakthrough COVID-19 cases do occur. This study using a community-based database in the United Kingdom to examine factors associated with COVID-19 after vaccination. A free mobile phone application was developed to ask individuals to report their symptoms, COVID-19 test results, and vaccination information. A total of 4.5 million participants used the app and provided data through self-report or proxy report. Each individual reported their age, height and weight for body mass index, comorbidities, frailty as assessed by the PRISMA-7 scale (with ≥ 3 defined as frail), local area index of multiple deprivation using geographic location, and other factors.

Using a nested case-control design from the prospective cohort data, the investigators defined cases as those who had received a first or second dose of COVID-19 vaccine between December 2020 and July 2021, had positive COVID-19 tests either ≥ 14 days after first dose or ≥ 7 days after the second dose, and had no positive test before. Controls were defined as those who had received a first or second dose of COVID-19 vaccine in the timeframe matching the case definitions and reported a negative test ≥ 14 days after the first dose or ≥ 7 days after second dose.

Among persons who reported a first vaccine dose, 0.5% (6030 of 1,240,009) tested positive for COVID-19, and among those who reported a second vaccine dose, 0.2% (2370 of 971,504) tested positive for COVID-19. Frailty was associated with postvaccination infection in adults aged 60 years or older. Individuals living in highly deprived areas had increased odds of postvaccination infection after the first dose compared with those living in areas with an intermediate deprivation index; those living in areas with a low deprivation index had a lower likelihood of COVID-19 infection compared with those in intermediate areas. Individuals without obesity had a lower likelihood of COVID-19 infection compared with those who are obese.

These findings highlighting risk factors associated with breakthrough infections after vaccination suggest which populations may benefit most from booster vaccination and who could be targeted by vaccination efforts.

Vaccine Delivery in Older Adults: A Call to Action

Now that there is evidence of COVID-19 booster vaccine effectiveness, governments and health systems need to enhance vaccine delivery, especially to high-risk groups, such as older adults.

An article published in Vaccine describes a vaccine-delivery framework designed by a multidisciplinary group of experts from the International Council on Adult Immunization, consisting of vaccine experts, healthy aging advocates, social scientists, gerontologists, economists, and providers. The group noted that clear messaging on the value of adult immunization is needed along with platforms —which include the people, institutions, systems, and resources — to deliver vaccines. Delivery strategies must account for barriers to implementation of immunization programs, such as policies, political will, funding, monitoring, ease of access, and communication.

Efforts to address vaccine hesitancy can include education and engagement of the healthcare workforce. Also, registries can be used to help identify areas of low vaccine uptake, whereas established networks and health systems, such as health and wellness centers, can be leveraged to target these areas. Understanding the levers controlling vaccination delivery to older adults helps policy-makers devise policy to overcome barriers to immunization, such as vaccine hesitancy, and disparities in vaccine availability and administration, reducing morbidity and mortality for vaccine-preventable diseases like COVID-19.

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