by Peter Franklin
Thursday, 11
June 2020

How did Covid come to Britain?

by Peter Franklin
The estimated number of importation events that are attributable to inbound travellers from each of several source countries. Credit: Oliver Pybus and colleagues

Few, if any, European countries have had a worse Covid epidemic than the United Kingdom. But how did the SARS-CoV-2, the virus that causes the disease, reach our shores?

A remarkable new study from researchers at Oxford and Edinburgh Universities provides some important clues. Oliver Pybus, Andrew Rambaut and their colleagues have painstakingly identified 1,356 “independently-introduced transmission lineages” — i.e. separate introductions of the virus into the UK from elsewhere.

Their preliminary analysis comes with a number of careful qualifications. Most importantly, the figure of 1,356 is “likely to be an underestimate of the actual number of times the virus has been introduced to the UK with subsequent onwards transmission.” Furthermore their analysis doesn’t include an assessment as to the size of each of these lineages (though they note that many appear to have gone extinct). Nor does the study “attempt to measure the relative contributions to the UK epidemic of importation versus local transmission.”

Still, it’s fascinating to see how these introductions break down by country of departure:

We estimate that ≈34% of detected UK transmission lineages arrived via inbound travel from Spain, ≈29% from France, ≈14% from Italy, and ≈23% from other countries. The relative contributions of these locations were highly dynamic.
- Oliver Pybus and Andrew Rambaut
The estimated fraction of importation events that are attributable to inbound travellers from each country. Credit: Oliver Pybus and colleagues

Just 0.08% of the introductions came direct from China.

The study also gives us an idea of when each introduction took place. The peak period is mid-March and the overwhelming majority of viral arrivals took place between mid-February and mid-April. At first, Italy was the most important country of departure, but then Spain took over the top spot and finally France.

On Twitter, Oliver Pybus said that the “vast majority” of inbound travellers came by air.

The researchers do not come to policy conclusions. But what are the rest of us to make of the fact that the virus was re-introduced to this country over and over again? It’s unclear just how much difference this makes to the progress of an epidemic in a country, but it does seem reasonable to assume that so many re-introductions makes it harder to suppress the virus (as countries like New Zealand have).

Instead of properly screening and, where necessary, quarantining people arriving at our airports (half of them UK residents) we left ourselves wide open. Repeatedly, we’ve let the virus into our country — and, for that matter, out of it too.

Furthermore, the study indicates that the bulk of these introductions took place when we already knew how serious the disease was — not in the early stages when we were still in the dark.

We need to be prepared for the next pandemic, because there will be one. We can either put the systems in place to disrupt the spread of disease or we can let the spread of disease disrupt us.

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