Virus-mediated cell fusion of SARS-CoV-2 variants

SARS-CoV-2 has the ability to form large multi-nucleated cells known as syncytia. Little is known about how syncytia affect the dynamics of the infection or severity of the disease. In this manuscript, we extend a mathematical model of cell–cell fusion assays to estimate both the syncytia formation rate and the average duration of the fusion phase for five strains of SARS-CoV-2. We find that the original Wuhan strain has the slowest rate of syncytia formation ($6.4\times 10^{-4}/h$), but takes only 4.0 h to complete the fusion process, while the Alpha strain has the fastest rate of syncytia formation (0.36 /h), but takes 7.6 h to complete the fusion process. The Beta strain also has a fairly fast syncytia formation rate ($9.7\times 10^{-2}/h$), and takes the longest to complete fusion (8.4 h). The D614G strain has a fairly slow syncytia formation rate ($2.8\times 10^{-3}/h$), but completes fusion in 4.0 h. Finally, the Delta strain is in the middle with a syncytia formation rate of $3.2\times 10^{-2}/h$ and a fusing time of 6.1 h. We note that for these SARS-CoV-2 strains, there appears to be a tradeoff between the ease of forming syncytia and the speed at which they complete the fusion process.