The impact of vaccination strategies for COVID-19 in the context of emerging variants and increasing social mixing in Bogotá, Colombia: a mathematical modelling study

Abstract

BackgroundMore than 122,000 COVID-19 associated deaths have been reported in Colombia and about 27,000 in the city of Bogotá by the first week of August, with vaccination coverage in the city at 30% for complete schemes and at 37% for partial vaccination. As the incidence of cases currently decreases, questions remain about the potential impact of the delta variant already present in the city.MethodsWe used an agent-based model calibrated to data on age-structured deaths and dominance of variants in Bogotá. We used efficacy data for the portfolio of vaccines available, including known changes for SARS-CoV-2 variants. We modelled scenarios of early and delayed introduction of the delta variant in the city along with changes in mobility and social contact, and vaccine strategies over the next months.FindingsWe estimate that by mid July, vaccination may have already prevented 17,800 (95% CrI: 16,000 - 19,000) deaths in Bogotá. The delta variant could become dominant and lead to a fourth wave later in the year, but its timing will depend on the date of introduction, social mixing patterns, and vaccination strategy. In all scenarios, higher social mixing is associated with a fourth wave of considerable magnitude. If an early delta introduction occurred (dominance by mid July), a new wave may occur in August/September and in such case, age prioritization of vaccination and second dose not postponed are more important. However, if introduction occurred one or two months later (dominance by mid August/September) the age-prioritization is less relevant but maintaining the dose scheme without postponement is more important. In all scenarios we found that increasing the vaccination rate from the current average of 50,000/day to 100,000/day reduces the impact of a fourth wave due to the delta variant.ConclusionsIn Bogotá, the delta variant could still lead to a fourth wave, whose magnitude would depend on its introduction time and the level of social mixing. Its impact can be mitigated by increasing vaccination rates to achieve high coverage quickly, with non-delayed second doses. We found that, at this point, suspending the age prioritization to achieve higher coverage with first doses does not seem to have a major effect on deaths and ICU demand. But, delaying the second dose may not be beneficial and may even increase the incidence of severe outcomes.