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.

Pharmaceutical and Non-Pharmaceutical Interventions for Controlling the COVID-19 Pandemic

“…With the development of COVID-19 vaccines, policy makers started vaccination campaigns with the aim to protect individuals and relax NPIs. Vaccines became the most important 1 intervention for mitigating disease severity and spread, allowing the return of social and economic activities [4][5][6][7][8].…”

A Scoping Review of Three Dimensions for Long-Term COVID-19 Vaccination Models: Hybrid Immunity, Individual Drivers of Vaccinal Choice, and Human Errors

“…Again, vaccine hesitancy was modeled population-wide by assuming either a high booster intake (85% to 98%) of individuals 50 years of age or higher with comorbidities, or modeled with a reduced intake of 50%. These models concluded that a high adherence to frequent booster doses is necessary to avoid future outbreaks and prevent a burden on the healthcare system—a conclusion shared by studies using differential equations [ 37 , 38 ]. Since adherence to frequent boosters is the key but it was only modeled by assuming a percentage across wide sub-populations, it is important for future ABMs to address this research gap by detailing how individuals decide to get vaccinated as a function of sociodemographic, health, psychological, and information-related variables.…”