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Most Important Covid-19 Policy? Pick One and Stick to It

It's one thing Sweden may have gotten right, as other places are discovering.

Most Important Covid-19 Policy? Pick One and Stick to It

It's one thing Sweden may have gotten right, as other places are discovering.

A woman wears a face mask as she waits at a bus stop with an information sign asking people to keep social distance on June 26, 2020 in Stockholm, Sweden.

A woman wears a face mask as she waits at a bus stop with an information sign asking people to keep social distance on June 26, 2020 in Stockholm, Sweden.

Photographer: STINA STJERNKVIST/AFP/Getty Images

A woman wears a face mask as she waits at a bus stop with an information sign asking people to keep social distance on June 26, 2020 in Stockholm, Sweden.

Photographer: STINA STJERNKVIST/AFP/Getty Images

Early last December, when epidemiologist Adam Kucharski of the London School of Hygiene and Tropical Medicine signed off on the final manuscript of “The Rules of Contagion: Why Things Spread — and Why They Stop,” a dangerous new virus was beginning to spread undetected in Wuhan, China. By the time the book was published in the U.K. on February 13, the new plague had just gotten a name — SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2), for the virus itself and Covid-19 for the disease it causes — and begun its rapid spread in the U.S. and Europe. You know what happened after that.

But the fact that you know what happened and Kucharski didn’t (for the U.S. edition, which came out in July, he was at least able to squeeze in a few brief references to Covid-19) doesn’t detract at all from his book. A mathematician by training, Kucharski adeptly explains the basics of how infections spread and fizzle out, and how the insights epidemiologists began to develop in the early 20th century are now being used to study phenomena from financial crises to viral tweets.

Meanwhile, Kucharski has been spending his time researching the Covid-19 pandemic — on which he has co-authored at least 16 papers so far — and frequently weighing in about it on social media and elsewhere. What follows is the abridged and edited version of a conversation we had by Zoom last week. The term “reproduction number” comes up a few times in the discussion. You’re probably familiar with it by now, but just in case: It’s the number of people each person with a disease can be expected to infect. If it falls below one, an epidemic fizzles out.

Justin Fox: It seems like Covid actually has followed the rules of contagion set out in your book, right?

Adam Kucharski: There are many aspects of Covid that fit what you’d expect from this kind of respiratory infection. The big challenge from a response point of view is that the virus lands in that awkward place between something like SARS, which could clearly be contained with these established public health measures, and something like flu that is simply never going to be containable in that kind of way.

JF: The biggest struggle for all of these epidemiological models seems to be figuring out which measures would have which effect.

AK: In some of the early scenarios we looked at in terms of reduction of contacts, we got it pretty close as to what a lockdown would do. But you’re really in uncharted territory as to how long populations can do that. It’s not just an epidemiological picture, it’s a social one, it’s an economic one, it’s a political one. What do you want your society to look like over the next year or two?

JF: I think the first time I encountered you on Twitter was in April in a discussion about Swedish authorities’ assertions that nearly a third of Stockholm residents had already been infected with Covid-19, which turned out to be way off. But the Swedes also made this reasonable argument of, “This will probably be with us for a while, so let’s think about measures that we can put in place and just leave there.”

AK: Some of their early claims about reaching high levels of antibodies just didn’t match up with what we see in the data. The prevalence in Sweden looks a lot like it does in other countries of Europe that have been hit similarly hard. But there is something to be said for learning to live in a society where the reproduction number is just below one. The problem for a lot of countries is they went to enormous effort to get the reproduction number far below one and bring cases right down, and they never really learned to live in that world because obviously it’s not sustainable. Now we’re back in a world where cases are increasing, and you’re almost back to those difficult choices you had in the spring.

JF: Another thing that got a lot of discussion early on was seasonality. It seems like it’s there in the background, but this is so contagious that you don’t see it as clearly as you might with influenza.

AK: During a typical flu season, the reproduction number is quite close to one, so it obviously becomes more sensitive to seasonal effects when you’re near that kind of boundary. But having worked in flu for many years, there’s still enormous debate about why flu is seasonal. Is it behavior, is it elements of humidity and viral transmission? How does that all fit together? Independent of what the virus and the climate are doing, winter drives everyone inside, so I think we’re going to expect more transmission, even if the virus itself survives in exactly the same way.

JF: The dispersion of spread has also gotten attention lately. Flu has a lower reproduction number, but it also spreads pretty evenly. Whereas Covid is more like SARS, where a relatively small percentage of the cases actually transmit, but those often transmit to huge numbers of people.

AK: There are some features of Covid that make it closer to flu than SARS, in particular the ability to transmit before having clear symptoms. There is evidence that SARS and Covid are more clustered in their transmission than flu. In the early stages of an outbreak that gives you the advantage that if you can identify where those clusters are happening you have a lot more success potentially at contact tracing.

But saying that, if you look at flu dynamics globally this year, New Zealand had barely any flu cases, parts of Australia didn’t either. The measures that stopped Covid stopped flu, so I wouldn’t be surprised if some countries reevaluate what they would do if they were in the future faced with something with 1918 flu characteristics. I suspect that the strategy of lock down and close borders would be on the table, whereas it may not have been a few years ago

JF: There seems to be evidence that Covid is getting less severe, the fatality rate is falling. Should that change the policies to deal with it?

AK: If you can find a treatment that reduces the death rate by half, if you have an enormous outbreak that’s still going to be an overwhelming number of deaths. Whereas if you can target transmission, you can potentially stop that big number occurring in the first place. Ultimately improved treatments and vaccines are going to be the solution to this, but I don’t think we’re really there yet in terms of options, where if there’s a large epidemic you’re not going to see large numbers of hospitalizations and deaths.

JF: Your book is mostly not about disease epidemiology, but how its principles can be applied in other areas. One of the really interesting things I learned is that things that go viral online aren’t actually all that viral. Compared to Covid-19 your basic viral internet post has a very low reproduction number

AK: The big difference is the timescale. We’re talking seconds rather than days in terms of transmission, and online it’s a tiny, tiny fraction that are generating the majority of your transmission. That can create the effect where if you get a string of amplification events, something can go from being a complete fringe idea to bang in the middle of the mainstream.

JF: Has anything over the past six months utterly surprised you about the way things have developed?

AK: The big thing that surprised me is just the diversity globally we’ve seen in how countries have approached this. For the most part, scientists have agreed on an evidence base. We’ve been talking to our colleagues around the world since January about this, and all countries have access to the same information. The diversity comes from the political side, from the social side of what the people prioritize. That’s really revealed some quite fundamental differences in terms of how people interpret information and what they see as the best way forward.

Some countries have found something that they can probably sustain for another few months or a year, if the vaccine is available by then. Other countries that perhaps had something that looked effective are now finding that it’s not sustainable.

    This column does not necessarily reflect the opinion of the editorial board or Bloomberg LP and its owners.

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    Justin Fox at

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