Reply To: SARSCoV2 “outbreak” on isle of Barra in Scotland

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Dave, 19:18 – “So the restrictions stop flu, as covid “cases” soar?”

glenn_uk is right; SARS-CoV-2 is more infectious than the influenza viruses, and there’s residual immunity in the population to ‘flu, whereas SARS-CoV-2 is new, so at the outset, no one’s immune systems had any record of how to make antibodies against it.

There’s a lot of media talk about the R number, but it isn’t very clear. There are actually various R numbers. Each type of virus has its own R-zero, which is how infectious it is. SARS-CoV-2 is more infectious than ‘flu ie. it’s R-zero is higher. The measles virus is even more infectious. We can’t change a virus’s R-zero.

The R that’s in the news media a lot is R-t, which is the infectiousness in a given set of circumstances. We can change a virus’s R-t with our behaviours, eg. “stay away from me, I don’t want you to catch my cold”. R-t is the number of people each infected person is likely to infect, on average. If it’s above 1, infections spread and increase. If we can get it below 1, the number of infections decreases, sometimes even to zero, extinction. The stricter restrictions in Scotland drove two variants to extinction there, while those variants continued to increase in England.

An effective vaccination programme decreases a virus’s R-t; it becomes too unlikely for the virus to find new hosts. That’s how smallpox was wiped out. It nearly worked with polio too, but a conspiracy theory in Africa made people too scared to get vaccinated.