In a study recently published in the journal PLoS Computational Biology, researchers used mathematical modeling to evaluate whether prioritizing older adults for booster COVID-19 vaccinations consistently leads to optimal public health outcomes across diverse socioeconomic contexts.
Study: Prioritizing older adults for booster COVID-19 vaccinations optimizes public health outcomes across socioeconomic settings. Image credit: BaLL LunLa/Shutterstock.com
background
During the early stages of the COVID-19 pandemic, non-pharmaceutical interventions (NPIs) were crucial in containing the spread of infection and protecting health systems.
The introduction of vaccines such as Pfizer-BioNTech, Oxford-AstraZeneca and Moderna has significantly altered the trajectory of the pandemic, reducing severe cases and relaxing NPIs.
However, the immunity provided by these vaccines fades over time, making booster shots necessary, especially in the case of new variants.
Further research is needed to refine booster vaccination strategies to address different demographics and socioeconomic circumstances around the world and ensure the most effective public health outcomes.
About the Research
This study extends a previously published age-structured compartment model of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission and vaccination.
This deterministic model is governed by ordinary differential equations and divides individuals into segments based on age, current infection status, and pre-infection immune status.
The model includes 16 age groups and categorizes individuals as susceptible, exposed, infectious (symptomatic or asymptomatic) and recovered.
To more accurately represent time spent in an exposure class, the model employs a gamma distribution that realistically represents epidemiological periods.
The model also tracks immune status, classifying individuals into groups such as vaccinated, immune-enhanced, partially immune-compromised, fully immune-compromised and unvaccinated, reflecting their infection and vaccination history.
The effectiveness of vaccinations is built into the model, taking into account the reduction in risk of infection, symptoms, hospitalization and death.
Using this model, the study examines the impact of six different age-based booster vaccination strategies in eight countries: strategy 1 prioritises the oldest living people, while strategy 6 looks at vaccinating those with the most contacts to provide indirect protection to older people.
Strategies 2–5 test variations on these approaches, generally focusing on prioritizing older adults for booster vaccinations.This analysis assumes limited vaccine availability, a baseline population coverage of 10%, and a maximum vaccination rate of 90% per age group.
Research findings
The age structure of the population varies greatly from country to country, with high-income countries generally having a larger proportion of older people than low-income countries.
This demographic difference impacts the distribution of booster vaccines in different vaccination strategies. For example, in the UK, strategies 1–4 prioritizing older people for booster vaccinations will result in a significant number of people aged 50–74 years being vaccinated. However, due to limited vaccine supply, the oldest old people (ages 75 and over) may not be fully covered.
Meanwhile, the strategy focuses on vaccinating younger people, especially those aged 20-49, who generally have more contact and may contribute indirectly to protecting the older population.
However, the age distribution of those vaccinated under each strategy varies from country to country. For example, in Sierra Leone, where the proportion of elderly people is relatively small, strategies 1–4 effectively vaccinated almost all elderly people, and the outcomes of these strategies are identical.
This contrasts with the situation in the UK, where a large elderly population means that these strategies would not be able to vaccinate all older people due to vaccine constraints.
Considering the impact of these strategies on public health outcomes during a wave of infections caused by new SARS-CoV-2 variants, model projections showed that strategy 1 consistently resulted in the lowest number of deaths in all countries analyzed.
This finding is particularly pronounced in high-income countries, where projected deaths would be higher if many older people were not prioritized for booster vaccinations.
In contrast, a country like Sierra Leone, with its small number of elderly people, could receive almost everyone's booster vaccinations under strategy 1, resulting in fewer overall deaths.
The analysis also looked at expected life years lost (YLLs) taking into account the number of deaths and age at death.
While vaccination of younger populations is expected to potentially reduce YLLs, the models consistently showed that strategy 1, prioritizing the oldest age group, was optimal for minimizing YLLs, as older populations are at higher risk of severe outcomes and death associated with SARS-CoV-2 infection.
Sensitivity analyses further supported these findings, showing that strategy 1 reduced deaths and YLLs under different assumptions, including vaccine availability, coverage, and timing of booster doses.
Even when the epidemic began 150 days after booster vaccination and immunity waned, strategy 1 remained the most effective approach to reduce deaths and YLLs.
Conclusion
In summary, future SARS-CoV-2 transmission will be affected by new variants, booster vaccinations, and immunity. Unlike in the early stages of the pandemic, many people around the world are now infected or vaccinated, affecting immunity and transmission.
In this evolving scenario, it is imperative to re-evaluate the effectiveness of past interventions and explore new strategies.
The study evaluated different age-based booster vaccination strategies and found that prioritizing older adults consistently improved public health outcomes, regardless of demographic changes.
