The Delta variant of the COVID-19 virus is more transmissible than other variants such as the Alpha form, but to what extent is this the case? Will it displace other variants, and how will it respond to different vaccine efficacies? Working with Chinese mathematicians, NRI’s Professor Robert Cheke has published a new paper in the journal Infectious Disease Modelling addressing these questions.
Assessing and quantifying the transmissibility and competitive advantage of the Delta variant is of major significance for countries around the world so that appropriate measures to mitigate and even eliminate the epidemic can be introduced. The scientists developed a new technique that used Public Health England (PHE) data from 11 April to 17 May 2021, to compare the basic and effective reproduction numbers of the Delta and Alpha variants and they also took account of reduced vaccine efficacies of both first and second doses against the Delta form. The basic reproduction number (Ro), defined as is the average number of secondary infections generated by one case in a population where all individuals are susceptible to infection, of the Delta variant was calculated to be 49% greater than that of the Alpha variant.
However, when the PHE data were taken into account to estimate the effective reproduction numbers, the average number of secondary cases per infectious case in a population made up of both susceptible and non-susceptible hosts at a particular time (Rt), of the two variants, the Delta form was estimated to be 65% higher. So, this competitive advantage of the Delta variant over the Alpha variant accounts for the former’s rapid spread and, given that the vaccines are not 100% effective, illustrates how the pandemic is far from over and how the virus’s ability to evolve poses a continuous and alarming threat.