COVID-19 immunization threshold(s): an analysis

Balbas, Luis Alfredo Bautista; Conesa, Mario Gil; Balbas, Blanca Bautista; Jimenez, Ainhoa Alcaide; Caravaca, Gil Rodriguez

2As COVID-19 vaccine research efforts seem to be yielding the first tangible results, the proportion of individuals needed to reap the benefits of herd immunity is a key element from a Public Health programs perspective. This magnitude, termed the critical immunization threshold (q), can be obtained from the classical SIR model equilibrium equation, equaling (1 - 1/R0)/ {epsilon}, where R0 is the basic reproduction number and{epsilon} is the vaccine efficacy. When a significant proportion of the population is already immune, this becomes (n - 1/R0)/ {epsilon}, where n is the proportion of non-immune individuals. A similar equation can be obtained for short-term immunization thresholds(qt), which are dependent on Rt. qs for most countries are between 60-75% of the population. Current qt for most countries are between 20-40%. Therefore, the combination of gradual vaccination and other non-pharmaceutical interventions will mark the transition to the herd immunity, providing that the later turns out to be a feasible objective. Nevertheless, immunization through vaccination is a complex issue and many challenges might appear.