Hello and welcome to more or less on the BBC World Service, where your weekly guide to

the numbers all around us in the news and in life, and I'm Tim Halford.

Where did the virus that causes Covid-19 come from? Was it made by scientists in a lab,

passed to humans from bats or perhaps from some other animal? Reports on these questions

often focus on the genetic similarity between different viruses which originate in different

species and this similarity is expressed in percentages. This is how the Guardians

today in focus podcast covered it. But a few miles away is she's Englins lab, where

the coronavirus sample taken from that bat cave in 2012 shows a 96% match to Covid-19.

So a theory begins to take hold. Could there be a link between this lab and the outbreak?

Listen Henry Hadlow, who can't be especially loyal since he was listening to someone

else's podcast, got in touch. The Guardians today in focus podcast just covered the Covid-

lab leak theory, citing that Covid-19 is a 96% match with a sample collected by the Wuhan

Institute for Virology in 2012. But is that a lot? EG, what percentage matches Covid-19

with the common cold? Henry ended his tweet with a particularly charming

eye heart more or less sequence of emojis, so we'll forgive him for straining. So what does

it actually mean to say that SARS-CoV-2, the virus that causes Covid-19, is 96% similar

to a strain found in bats? How similar is it to SARS-CoV-1, the virus that caused the

SARS outbreak in 2003? And how similar are different variants of SARS-CoV-2?

Marilyn J. Rousink is a virus ecologist and a professor at Penn State University in the

United States. Well, basically how we determine that is we just determine the nucleotide

sequences of the virus and then we line them up and we just usually a computer does this

for us, but you essentially just count the differences. When you are lining up these viruses and

...