In a recent study published in PLoS Computational Biology, researchers used mathematical modelling to evaluate whether prioritizing older individuals for coronavirus disease 2019 (COVID-19) booster vaccinations consistently leads to optimal public health outcomes across diverse socio-economic settings.

In the early stages of the COVID-19 pandemic, non-pharmaceutical interventions (NPIs) were crucial in reducing transmission and protecting healthcare systems.

The introduction of vaccines, such as those from Pfizer-BioNTech, Oxford-AstraZeneca, and Moderna, significantly altered the pandemic’s trajectory by reducing severe cases and relaxing NPIs.

However, immunity from these vaccines wanes over time, necessitating booster vaccinations, especially with emerging variants.

Further research is needed to refine booster vaccination strategies to address varying population structures and socio-economic contexts globally, ensuring the most effective public health outcomes.

About the study

The study extends a previously published age-structured compartmental model of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) transmission and vaccination.

This deterministic model, governed by ordinary differential equations, divides individuals into compartments based on age, current infection status, and immune status before any infection.

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 employs gamma distributions, which provide a realistic representation of epidemiological periods.

The model also tracks immune status, categorizing individuals into groups such as Vaccinated, Boosted, Partially Waned, Fully Waned, and Unvaccinated, reflecting their infection and vaccination history.

The effectiveness of vaccination is incorporated into the model, accounting for reduced risks of infection, symptoms, hospitalization, and death.

Using this model, the study examines the impacts of six different age-based booster vaccination strategies across eight countries. Strategy 1 prioritizes the oldest individuals first, while Strategy 6 explores vaccinating those with the most contacts to provide indirect protection to the elderly.

Strategies 2-5 test variations of these approaches, generally focusing on prioritizing older individuals for booster vaccination. The analysis assumes limited vaccine availability, with a 10% population coverage baseline and a 90% maximum uptake per age group.

Study results

The population’s age structure varies significantly across countries, with high-income countries generally having a larger proportion of older individuals compared to low-income countries.

This demographic difference influences the distribution of booster vaccines under different vaccination strategies. In the UK, for instance, strategies 1-4 that prioritize older individuals for booster vaccination result in a substantial number of individuals aged 50-74 receiving vaccines. However, the oldest individuals (75+) may not be fully covered due to the limited vaccine supply.

On the other hand, Strategies focused on vaccinating younger individuals, particularly those aged 20-49, who typically have more contacts and, therefore, could potentially contribute to indirect protection of older populations.

However, the age distribution of vaccinated individuals under each strategy varies between countries. For example, in Sierra Leone, where the proportion of older individuals is relatively small, Strategies 1-4 effectively vaccinate almost all older individuals, leading to identical outcomes for these strategies.

This contrasts with the situation in the UK, where a larger elderly population means that not all older individuals can be vaccinated under these strategies due to vaccine constraints.

When considering the impact of these strategies on public health outcomes during a wave of infections caused by a novel SARS-CoV-2 variant, the model projections indicated that Strategy 1 consistently resulted in the fewest deaths across all countries analyzed.

This outcome is particularly evident in high-income countries, where a large proportion of older individuals contribute to a higher projected number of deaths if they are not prioritized for booster vaccination.

In contrast, in countries like Sierra Leone, the small number of older individuals allows nearly all of them to receive booster vaccines under Strategy 1, leading to fewer deaths overall.

The analysis also examined the projected Years of Life Lost (YLL), which accounts for the number of deaths and the ages at which they occur.

While it might be expected that vaccinating younger individuals could reduce YLL, the model consistently showed that Strategy 1, which prioritizes the oldest individuals, was optimal in minimizing YLL. This is due to the higher risk of severe outcomes and mortality associated with SARS-CoV-2 infection in older populations.

Sensitivity analyses further supported these findings, showing that Strategy 1 led to fewer deaths and YLL under various assumptions, including different levels of vaccine availability, uptake rates, and timing of booster administration.

Even when the outbreak began 150 days after booster vaccines were administered, resulting in some immunity waning, Strategy 1 remained the most effective approach in reducing deaths and YLL.

Conclusions

To summarize, future SARS-CoV-2 transmission will be influenced by novel variants, booster vaccinations, and immunity dynamics. Unlike the pandemic’s early stages, many individuals globally are now infected or vaccinated, impacting immunity and transmission.

In this evolving scenario, it is essential to reassess the effectiveness of past interventions and explore new strategies.

This study evaluated different age-based booster vaccination strategies and found that prioritizing older individuals consistently leads to better public health outcomes, regardless of varying population structures.

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