How COVID spread through travel in 2020

It’s the question on many people’s minds: with the pandemic still ongoing, can I go away on holiday this year? Scientists across the world have been tracking the spread of coronavirus through different countries by looking at the genetic sequences of the virus and building up a sort of virus family tree. Now, scientists doing this detective work at the University of Bern in Switzerland have discovered that, last summer, a coronavirus variant called EU1 spread broadly throughout Western Europe by hopping onto holidaymakers who took it and spread it back home. Eva Higginbotham spoke to lead author Emma Hodcroft about the role travel played in the spread last year, and what that can tell us about how we might think about holidays abroad this year…

Emma - Last summer we were looking at SARS-CoV-2 sequences in Switzerland, and we were particularly interested in seeing if we could tell something about how cases might have been spreading within Switzerland. But when I started to look into this more closely, I found that we had clusters of sequences that were closely related, and not only were spreading in Switzerland but also in Spain and in the UK. And as time went on, I started to see them popping up across Europe. When we first started looking into this, we were really worried that this might be the more transmissible variant that we were all very concerned about at the end of 2020, but actually the more we looked at it, we realised that it seemed to be that travel was the key. And so what we showed in our paper is that this variant, which is called EU1, spread from Spain soon after borders opened last summer after the lockdowns finished, and it was able to basically hop aboard the holidaymakers and travel across Europe. And it became the most prevalent variant circulating in Western Europe by the end of 2020 last year.

Eva - How can you tell that this was spread by travel and not by it being more transmissible? What's the difference there?

Emma - So we did do some lab experiments that allowed us to look at the mutations themselves and see if they had an effect, and we couldn't see that they seem to make any difference in the lab dish. But perhaps even more importantly, what we see in the patterns last summer is that the variant really spread most when travel was the highest, and that instead of continuing to spread in the autumn and kind of taking over - for example like the variant from the UK, the alpha variant, did - it actually just started to plateau after that. And this suggests that its advantage was transient; it didn't last forever. And that matches really well with this idea that it took advantage of travel, but then after the summer holidays and the travel slowed down, it just couldn't really take off anymore.

Eva - So how big a role, would you say, did travel play in the spread of this variant last year?

Emma - So we really think that travel was the key here. We think this variant started to spread in Spain - possibly through some agricultural workers, and as people started to visit family and friends across Spain as the lockdown eased - and then we really start to see it being detected in other countries in Europe only after the borders reopened in Europe. And we can actually look at, for example, the number of people who travelled to Spain and when they were there, and we found that this correlates pretty well with how much EU1 ended up traveling back to that country. So that's a strong indicator that it really was travel that played the real role here.

Eva - And do we know if this sort of travel induced spread of this variant led to worse outcomes in the countries that it got into?

Emma - So we don't think that EU1 necessarily, for example, led to the rise in cases that we saw in September; we think that this was probably more likely to be seasonal. However, it does matter what number of cases you start with. We've learned this in the epidemic. So when you're growing exponentially, you'll get up to higher numbers much faster if you start out with a higher number. And so we do think that the number of cases people brought back from EU1 might have meant that that country's case number started to go up perhaps earlier, or went up a little bit faster. One way of thinking about it is thinking of a lot of sparks. They won't all start a fire, but the more sparks you import, the chances are that fire could take off a little bit sooner.

Eva - So what might this mean for this year, then? Should people not be going on holiday abroad?

Emma - I think one of the things that EU1 can teach us is what mistakes we made last year, and then of course we can think about how we can not make those again. Last summer, we let people continue traveling to Spain even when case numbers were going up. We didn't have really any testing associated with travel. And unfortunately it looks like the test and trace systems didn't catch those EU1 cases when they came back, so they were able to really get a good foothold in the countries where they spread. The situation is different this year. So for one, thankfully the vaccine rollout is going well in most countries, and so the proportion of people who are much less likely to get the virus is going down. And we've also seen that testing is playing a much bigger role in travel this year, with most countries requiring a negative test before you arrive. Those two things I do think will make a big difference in how likely it is that people will bring home SARS-CoV-2. But I think it's also really important for us to remember that those may not always be perfect, and we should always be keeping an eye out on what impact travel might be making on the number of introductions.

Eva - And what's the take home message?

Emma - One of the biggest take home messages for me about EU1 is that it spread so effectively across Europe, despite the fact that it's not more transmissible. So we always want to keep in mind what human behaviours could be impacting the spread of a variant, and what might we be able to do to help cut those transmission chains.

Eva - And one of those things might be not going on holiday abroad just yet?

Emma - One of those things might be not going on holiday, or at least waiting to go on holiday until you're fully vaccinated. And I think another important thing to keep in mind is to just be flexible this holiday season. Maybe think about going somewhere that's less crowded, or spending a little more time on the beach and less time in crowded indoor spaces, and not being afraid to decide if you go somewhere and it seems a little bit more busy, or people aren't adhering to guidelines as much as you'd like, you can always come back another time. It's all small things, but it can make a difference as far as transmission.