COVID-19 immunization threshold(s): an analysis

2 As 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 /R 0 )/ ϵ , where R 0 is the basic reproduction number and ϵ is the vaccine efficacy. When a significant proportion of the population is already immune, this becomes (n − 1 /R 0 )/ ϵ , where n is the proportion of non-immune individuals. A similar equation can be obtained for short-term immunization thresholds( q t ), which are dependent on R t . q s for most countries are between 60-75% of the population. Current q t 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.