The cleanest way of comparing the pure biological spreading power of viruses is to look at their R0 (pronounced R-naught). It’s the average number of people each infected person passes a virus on to if nobody were immune and nobody took extra precautions to avoid getting infected.
That number was around 2.5 when the pandemic started in Wuhan and could be as high as 8.0 for the Delta variant, according to disease modellers at Imperial.
“This virus has surprised us a lot. It is beyond anything we feared,” said [virologist Aris Katzourakis.] “The fact it has happened twice in 18 months, two lineages (Alpha and then Delta) each 50% more transmissible is a phenomenal amount of change.”
It’s “foolish”, he thinks, to attempt to put a number on how high it could go, but he can easily see further jumps in transmission over the next couple of years. Other viruses have far higher R0s and the record holder, measles, can cause explosive outbreaks.
“There is still space for it to move higher,” said [virologist Wendy] Barclay. “Measles is between 14 and 30 depending on who you ask, I don’t know how it’s going to play out.”
