Heterologous vaccination as a strategy to minimize inequity in COVID-19 vaccine access: A modeling study in Thailand

BackgroundVaccinations are the best hope to control the COVID-19 pandemic and save lives. Due to the high demand and failure to share vaccines equitably, there were not enough vaccine supplies to cover the majority of people in low- and middle-income countries during the early stage of vaccination. To cope with this problem, Thailand, an upper-middle-income country, decided to employ a heterologous vaccination strategy as the primary COVID-19 vaccination regimen in the country. The CoronaVac (CV) vaccine was administered as the first dose, followed by the ChAdOx1 nCoV-19 (AZ) vaccine as the second dose. However, there is no study to assess the effectiveness of the heterologous vaccination employed in Thailand compared to the standard homologous vaccination. MethodsWe delineated the course and timeline of COVID-19 vaccination in Thailand. An age-structured compartmental model for COVID-19 transmission and vaccination was constructed and employed to assess the effectiveness of the heterologous vaccination strategy. The impact of the vaccine prioritization strategies on COVID-19 mortality and infections was also investigated. ResultsWe found that the CV+AZ heterologous vaccination strategy outperforms the separate CV and AZ homologous vaccinations in reducing cumulative cases and deaths when combined with other non-pharmaceutical interventions. Furthermore, the results suggested that prioritizing vaccines for the elderly could be optimal in reducing COVID-19 mortality for a wide range of vaccination rates and disease transmission dynamics. ConclusionsOur modeling results suggested that to minimize the impacts of inequity in early COVID-19 vaccine access in low- and middle-income countries, those countries may use early accessible but maybe lower-efficacy vaccines as the first dose of heterologous vaccination in combination with higher-efficacy vaccines as the second dose when they are available.