Given the evolving landscape of COVID-19 in the UK and globally, where new variants will continue to emerge and the profile of population immunity will shift over time, it remains crucial to account for the role of hybrid immunity and consider the benefit of adapting future vaccination campaigns. Since the emergence of SARS-CoV-2, studies have used mathematical modelling to investigate population dynamics and the effectiveness of public health interventions to control the COVID-19 pandemic [33,34,35,36,37]. While the effect of vaccination on COVID-19 transmission has been modelled in the UK before [14, 15], this study builds on the dynamics of the disease and vaccination strategy by additionally considering hybrid immunity to COVID-19. A growing body of evidence strongly suggests that hybrid population immunity profiles, acquired from both prior infection and vaccination, are potentially more protective against surges in COVID-19 infections and healthcare demand [38,39,40,41].

This study provides novel insights into the impact of the autumn 2022 booster campaign in the UK, explicitly accounting for the role of hybrid immunity in the population. Beyond the impact on averting infections and hospital admissions, this study quantified the impact of boosters in averting long COVID and patient productivity—two outcomes that are considered to have serious clinical and societal implications [42, 43]. The model assessed the impact of vaccination across several health outcomes for six vaccination strategies, compared to a counterfactual no booster campaign. Under conservative assumptions, the most beneficial results across all health outcomes were observed when coverage was expanded to all adults and children aged 12 and over, during both the autumn 2022 and spring 2023 campaigns with improved vaccine uptake rates observed during the campaigns. Enhanced uptake may be achieved by public health information campaigns driven by government and public health authorities [44].

Previous studies indicate booster vaccination has been consistently advantageous, with an estimated 100,000 hospitalizations and 23,000 deaths due to COVID-19 averted in England between October 2021 and December 2022 when compared with a counterfactual no booster scenario [45]. This study looked at a different time horizon, focusing on the current landscape of the COVID-19 pandemic dominated by the Omicron lineage of subvariants. Published evidence has shown that initial doses of the vaccine provided high levels of protection from severe disease, which is now diminished via waning and immune escape of the Omicron variant. Administration of a booster vaccine developed against the Omicron variant versus the ancestral vaccine designed for previous strains has shown that booster vaccines targeting specific and current circulating variants result in substantially more reductions in the number of severe outcomes associated with disease, demonstrating the long-term applicability of COVID booster vaccines for public health [46].

Current JCVI eligibility criteria recommend booster vaccinations for those at highest risk of developing severe COVID-19 or those with a higher risk of contracting and transmitting SARS-CoV-2 through occupational exposure, such as healthcare workers [47]. Expanding vaccine eligibility criteria to all adults and children over the age of 12 may indirectly protect those of advanced age and at risk of severe complications from COVID-19, which aligns with previous studies [48, 49]. Therefore, our study supports a growing argument that expanding the eligibility for COVID-19 booster vaccination would provide direct and indirect protection against serious COVID-19-related outcomes in the most vulnerable members of society, as well as the wider population [50].

COVID-19 is associated with reduced productivity [51, 52], which has widespread effects such as workplace absenteeism or inability to care for others. A recent study demonstrated that vaccinated individuals with COVID-19 had lower absenteeism rates compared to unvaccinated individuals (45.6% versus 65.0%) [51]. By increasing eligibility criteria to include standard risk adults for the autumn and spring booster campaigns, this study predicts a substantial reduction in productivity loss associated with COVID-19, which could result in significant economic benefits.

Further benefits of the booster campaign on long COVID have been modelled here. Long COVID has a considerable detriment to productivity and has a significant societal burden with 3.6% of individuals in the UK self-reporting as experiencing long COVID. Over half (57.0%) of those individuals reported that the condition negatively affected their well-being, and one-third reported that long COVID impacted their work [53]. Long COVID has a substantial clinical burden with an increased cost of £23.4 million attributed to primary care consultations per year in the UK [54]. Self-reported long COVID has been described to affect people across all age groups, with women most susceptible to the condition [53, 55]. Further, there is a 25% lower risk of long COVID in those aged > 70 years, and 6.0% lower risk in those aged 30–39 compared with those aged 18–30, indicating that long COVID may disproportionately affect younger generations [56]. In a study of 672 individuals with long COVID, defined as experiencing symptoms 12 months post-infection, a reduction in work ability scores was reported compared with those without long COVID symptoms [57]. Here, expansion of the eligibility criteria to include the wider population and increasing uptake rates for the 2022 autumn and 2023 spring booster campaign was estimated to substantially offset long COVID cases and could further restore productivity. Further analyses are required to examine this full productivity gain and the wider societal benefits associated with booster vaccination.

A reduction in hospital bed days by the administration of the autumn and spring booster demonstrates the potential benefit that vaccination may have on healthcare resources and bed capacity. Even small changes in occupancy translate to large differences in real-world hospital settings, particularly in the post-pandemic era. The NHS has experienced overwhelming pressures since the COVID-19 pandemic [58] and measures to alleviate such pressures, particularly during a winter season, could have substantial benefits. The potential to save 18,921 admissions over winter may have enhanced value, given the significant bed capacity limits of NHS during this time [59,60,61,62,63]. Vaccination reduces severe consequences of COVID-19 that result in hospitalization and mortality [64], and so, vaccination strategies that directly and indirectly protect those at risk may have substantial benefits for the NHS.

Our NNV estimates are aligned with those estimated by the UKHSA for the autumn 2023 COVID-19 booster campaign [65]. Our analysis shows that increasing the eligibility of the COVID-19 booster campaign has the potential to reduce the NNV to avoid one symptomatic case from 64 to 34, demonstrating that the impact of the booster campaign could be improved by widening the current eligibility criteria.

The model presented here accounts for real world dynamics and complexities of disease transmission. As with any modelling analysis, there are some limitations. Firstly, it should be noted that the model does not account for the various sublineages of the Omicron strain and variants that may influence transmission and hospitalizations. This is reflected using the transmission parameters obtained during the calibration period for predictions. As the model was calibrated to data from the 9 months prior to the prediction period, projections over time may be impacted as fewer people have severe reactions to COVID-19 owing to the combined effect of vaccine and prior infection, and a higher number of people would have been infected with different subvariants of Omicron with different risk profiles and waned protection over time, which may not be reflected in the calibrated parameters. The observed hospital admissions (until March 2023) and deaths (until July 2023) were 133,623 and 13,144 respectively [66, 67]. Using the point estimates of parameters and conservative assumptions for VE, the model underpredicts both hospitalizations (109,460) and deaths (8344). Further, any disparities in disease transmission due to differences in the microenvironment, such as hospital or community care, are not explicitly modelled.

There is no specific vaccine considered in the model, nor are the differences between vaccines investigated between outcomes. Productivity gains due to prevention of long COVID by vaccination have not been accounted for in this model; therefore, the societal benefits of the booster campaign are underestimated. We have not modelled adverse events because of the rarity of severe adverse events after vaccination [68, 69]. Adverse events after COVID-19 vaccination are mostly mild and their risk is largely outweighed by the benefit of vaccination in reducing the risk of severe disease [68, 69]. Guidance from the NHS supports seasonal vaccination to protect against severe COVID-19 [70] with the Medicines and Healthcare products Regulatory Agency (MHRA) continually reviewing suspected adverse events associated with vaccination via the Yellow Card reporting scheme. Reviews following the autumn 2022 booster campaign did not acknowledge any new safety concerns associated with vaccination [71]. A full description of model limitations is provided in the Supplementary Material.