In a recently published study PLoS Computational Biology, The researchers used mathematical models to evaluate whether prioritizing older people for booster vaccinations against coronavirus disease 2019 (COVID-19) consistently leads to optimal public health outcomes across different socioeconomic situations.
Study: Prioritizing older people for COVID-19 booster vaccination leads to optimal public health outcomes in a range of socioeconomic situationsPhoto credit: BaLL LunLa/Shutterstock.com
background
In the early stages of the COVID-19 pandemic, non-pharmaceutical interventions (NPIs) were critical to reducing transmission and protecting health systems.
The introduction of vaccines, such as those from Pfizer-BioNTech, Oxford-AstraZeneca and Moderna, significantly changed the course of the pandemic by reducing the number of severe cases and relaxing NPIs.
However, immunity from these vaccines wanes over time, so booster vaccinations are necessary, especially for emerging variants.
Further research is needed to refine booster vaccination strategies and adapt them to different population structures and socioeconomic contexts around the world to achieve optimal public health outcomes.
About the study
The study extends a previously published age-structured compartment model of transmission and vaccination against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).
This deterministic model, based on ordinary differential equations, divides people into groups based on their age, current infection status, and pre-infection immune status.
The model includes 16 age groups, with individuals classified as susceptible, exposed, infectious (symptomatic or asymptomatic), or recovered.
To more accurately represent the time spent in the exposed class, the model uses gamma distributions that allow a realistic representation of epidemiological time periods.
The model also tracks immune status and categorizes individuals into groups such as “vaccinated,” “boostered,” “partially attenuated,” “fully attenuated,” and “unvaccinated,” reflecting their infection and vaccination history.
The effectiveness of vaccination is included in the model and takes into account reduced risks of infection, symptoms, hospitalizations and deaths.
Using this model, the study examines the impact of six different age-based booster vaccination strategies in eight countries. Strategy 1 vaccinates the oldest people first, while Strategy 6 vaccinates those with the most contacts to provide indirect protection to older people.
Strategies 2 to 5 test variations of these approaches and generally focus on prioritizing older people for booster vaccination. The analysis assumes limited vaccine availability, with baseline coverage of 10% of the population and maximum uptake of 90% per age group.
Study results
The age structure of the population varies considerably from country to country. High-income countries generally have a larger proportion of older people than low-income countries.
This demographic difference influences the distribution of booster doses under different vaccination strategies. In the UK, for example, strategies 1 to 4, which prioritise older people for booster vaccination, result in a significant number of people aged 50 to 74 being vaccinated. However, the oldest people (75+) may not be fully protected due to limited vaccine supply.
On the other hand, strategies focused on vaccinating younger people, especially those aged 20 to 49, who typically have more contacts and could therefore potentially contribute to indirectly protecting the older population.
However, the age distribution of people vaccinated under each strategy varies across countries. For example, in Sierra Leone, where the proportion of older people is relatively low, Strategies 1 to 4 effectively vaccinate almost all older people, resulting in identical outcomes across these strategies.
This contrasts with the situation in the UK, where due to the larger older population, not all older people can be vaccinated under these strategies due to vaccine shortages.
When considering the public health impact of these strategies during a wave of infections caused by a new variant of SARS-CoV-2, model projections showed that Strategy 1 consistently resulted in the fewest deaths in all countries analyzed.
This result is particularly evident in high-income countries, where a large proportion of older people contribute to a higher projected number of deaths if they are not prioritized for booster vaccination.
In contrast, in countries such as Sierra Leone, due to the small number of older people, almost everyone can receive a booster vaccination under Strategy 1, resulting in fewer deaths overall.
The analysis also examined predicted years of life lost (YLL), which take into account the number of deaths and the age at which they occur.
Although one might expect that vaccinating younger individuals might reduce YLL, the model consistently showed that Strategy 1, which prioritizes the oldest-old, is optimal for minimizing YLL. This is due to the higher risk of severe disease and mortality associated with SARS-CoV-2 infection in older populations.
These results were supported by sensitivity analyses, which showed that Strategy 1 resulted in fewer deaths and fewer new infections under various assumptions, including different vaccine availability, uptake rates and timing of booster vaccinations.
Even when the outbreak began 150 days after booster vaccinations were administered, causing some waning of immunity, Strategy 1 continued to be the most effective approach in reducing deaths and YLL.
Conclusions
In summary, future transmission of SARS-CoV-2 will be influenced by new variants, booster vaccinations and immunity dynamics. Unlike in the early stages of the pandemic, many people around the world are now infected or vaccinated, which impacts immunity and transmission.
Given this evolving scenario, it is crucial to reassess the effectiveness of previous interventions and explore new strategies.
This study examined different age-based booster vaccination strategies. The result: Prioritizing older people consistently leads to better public health outcomes, regardless of different population structures.
