Is the increased transmissibility of SARS-CoV-2 variants driven by
within or outside-host processes?
Understanding the factors that increase the transmissibility of the
recently emerging variants of SARS-CoV-2 can aid in mitigating the
COVID-19 pandemic. The enhanced transmissibility could be attributed to
enhanced within-host processes, such as contagiousness (viral shedding
by an infected individual) and infectivity (the probability of a
susceptible individual to get infected), or outside-host processes, such
as viral stability on surfaces and in the air. We utilized a
mathematical model in order to theoretically analyze the specific
mechanisms of virus transfer between an infected and susceptible
individual. This allowed us to examine how the within-host or
outside-host processes affect the overall viral transmission. Our
analysis is based the available data on the Alpha, Epsilon and Delta
variants as well as the currently emerging Omicron variant. We found
that the higher transmissibility of the SARS-CoV-2 variants can be
attributed only to within-host processes. Specifically, enhanced
contagiousness drives the Delta variant transmissibility, while the
Alpha, Epsilon and Omicron are characterized by an enhanced infectivity.
Since outside-host processes have little contribution to the observed
increase in the transmissibility, leading stricter hygienic and
behavioral measures than those that were already applied are not
expected to achieve a pronounced mitigating effect.