Measuring the impact of COVID-19 vaccination and immunity waning: A modelling study for Portugal

Abstract

Vaccination strategies to control COVID-19 have been ongoing worldwide since the end of 2020. Understanding their possible effect is key to prevent future disease spread. Using a modelling approach, this study intends to measure the impact of the COVID-19 Portuguese vaccination strategy on the effective reproduction number and explore three scenarios for vaccine effectiveness waning. Namely, the no-immunity-loss, 1-year and 3-years of immunity duration scenarios. We adapted an age-structured SEIR deterministic model and used Portuguese hospitalisation data for the model calibration. Results show that, although the Portuguese vaccination plan had a substantial impact in reducing overall transmission, it might not be sufficient to control disease spread. A significant vaccination coverage of those above 5 years old, a vaccine effectiveness against disease of at least 80% and softer non-pharmaceutical interventions (NPIs), such as mask usage and social distancing, would be necessary to control disease spread in the worst scenario considered. The immunity duration scenario of 1-year displays a resurgence of COVID-19 hospitalisations by the end of 2021, the same is observed in 3-year scenario although with a lower magnitude. The no-immunity-loss scenario presents a low increase in hospitalisations. In both the 1-year and 3-year scenarios, a vaccination boost of those above 65 years old would result in a 53% and 38% peak reduction of non-ICU hospitalisations, respectively. These results suggest that NPIs should not be fully phased-out but instead be combined with a fast booster vaccination strategy to reduce healthcare burden.

Keywords: COVID-19; Compartmental models; Epidemiology; SEIR model; Vaccination; Vaccine effectiveness waning.

Fig. 1

Effective reproduction number as a function of vaccine coverage (by age-group), vaccine effectiveness and basic reproduction number. Two scenarios were considered: no NPIs in place (left) and overall effective contacts reduced by 47% (right) to take into account social distancing and mask usage . The vaccination coverage was assigned to each age-group according to the final targeted coverage in the Portuguese vaccination plan, presented in Table 1. Coverage of the age group 5–11 was assumed 80%.

Fig. 2

Number of COVID-19 non-ICU hospitalised cases for the 3 considered scenarios: no loss of vaccine induced protection (green), vaccine effectiveness duration of 1 and 3 years (blue and red respectively). Dashed lines depict the model calibration for each immunity loss scenario, the solid lines represent the trajectory of the total number of non-ICU hospitalisation cases in each scenario. The dotted lines represent the trajectories where a third dose of the vaccine was administered to those above 65 years old and assuming that this group achieves 99% coverage with this new dose within one month of October 11, 2021. It is also assumed that no NPIs are in place after September 30, 2021 and that vaccine/infection granted immunity lasts an equal amount of time and is the same for each age-groups. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Number of COVID-19 ICU hospitalised cases for the 3 considered scenarios: no loss of vaccine granted protection (green), vaccine effectiveness duration of 1 and 3 years (blue and red respectively). Dashed lines depict the model calibration for each immunity loss scenario, the solid lines represent the trajectory of the total number of hospitalisation cases in each scenario. The dotted lines represent the trajectories where a third dose of the vaccine was administered to those above 65 years old and assuming that this group achieves 99% coverage with this new dose within one month of October 11, 2021. It is also assumed that no NPIs are in place after September 30th 2021 and that vaccine/infection granted immunity lasts an equal amount of time and is the same for age-group. The horizontal dashed line depicts the ICU capacity of COVID-19 hospitalisations in Portugal. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)