The following is a summary of “Modeling COVID-19 dynamics in the Basque Country: characterizing population immunity profile from 2020 to 2022,” published in the January 2025 issue of Infectious Disease by Naffeti et al. 

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus leading to Coronavirus Disease 2019 (COVID-19), caused a global pandemic, necessitating a critical public health response. While vaccination mitigated severe disease and mortality, waning immunity and the emergence of immune-evasive variants necessitated ongoing vaccination efforts, including booster doses, to sustain population-level immunity.  

Researchers conducted a retrospective study to model COVID-19 dynamics in the Basque Country, Spain, aiming to characterize the population’s immunity profile and evaluate its impact on outbreak severity from 2020 to 2022.  

They developed a Susceptible-Infected-Recovered-disease induced Death-Susceptible (SIR/DS) model to examine the interaction between virus-specific and vaccine-induced immunity. The model incorporated 3 immunity levels, considering the boosting effects from reinfection and/or vaccination. It was validated with daily case data from the Basque Country. The model tracked immunity status shifts and impact on disease dynamics over time.  

The results showed the COVID-19 epidemic in the Basque Country unfolded in 3 phases, influenced by virus transmission, public health measures, and vaccination. In the initial phase, cases surged rapidly, followed by a decline due to strict measures, with a seroprevalence of 1.3%. In the intermediate phase, smaller outbreaks occurred as restrictions eased and new variants like Alpha and Delta emerged, with reinfection rates reaching 20% and seroprevalence increasing to 32%. The final phase, including Omicron variant, rose in cases due to waning immunity and high transmissibility, with 34% of infections occurring in the naive population and seroprevalence peaking at 43%. Throughout all phases, infections in naive and unvaccinated individuals were key drivers of outbreak severity.  

Investigators concluded the monitoring and adaptive public health strategies to address the changing epidemiological and immunological landscape of COVID-19, with immunity levels, reinfections, and vaccinations playing a key role in shaping outbreak severity and guiding interventions. 

Source: bmcinfectdis.biomedcentral.com/articles/10.1186/s12879-024-10342-y