Pandemic: learning from smallpox


In 1980 the WHO declared the world officially smallpox free. The way there was a long and arduous one. In 1664 alone 320,000 people died of the variola virus-induced disease in London. London is also the city that David Earn reported on in his last study in the journal “Plos Biology”. The archives of the English metropolis contain detailed records of deaths from which the epidemiologist and mathematician has now created a chronicle. It shows that the fight against the smallpox epidemic has suffered setbacks again and again, including wars. A risky treatment method brought success in the meantime: the “vaccination” with infectious pathogens.

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Death statistics in London, September 26, 1665 Mr. Earn, what was the aim of your study and what did you find out?

David Earn: I’ve been interested in the patterns that underlie epidemics for a long time. London is an extremely good place to conduct such historical studies, as the causes of death have been recorded there on a weekly basis since the mid-17th century. We have digitized and evaluated these records. Among other things, the patterns of the smallpox epidemic became visible – a horrible disease that has cost an enormous number of human lives over the centuries. What we observed, for example: There were annual outbreaks until the 1840s, after which the periods between outbreaks became longer. The aim of our work was to describe these structural changes and to look for external causes. One of them is the so-called variolation: the targeted infection with a small amount of smallpox virus, which – at least at the time it was hoped – could not trigger a fatal infection and ultimately lead to immunity. And of course we were able to describe the effects of the smallpox vaccination in our work. This discovery was made in 1796. Incidentally, smallpox was the first disease for which a vaccination was developed.

Smallpox deaths in London from 1664 to 1931

Olga Krylova & David J.D. Earn

Stages of the smallpox epidemic in London

It should be obvious that the smallpox vaccination had an impact on the epidemic.

David Earn: If the population is vaccinated, yes. Then the disease disappears, as it eventually did with smallpox. How epidemics change with only moderate vaccination rates, however, is not so obvious. It may be, for example, that only the cycles of the disease are then lengthened. To understand this, we need to develop mathematical models.

As you show in your paper, wars seem to have influenced the smallpox epidemic – interestingly, wars that took place far from the UK too.

David Earn: We are not suggesting that wars had a direct impact. But there is a temporal correspondence with the London statistics: war means that the contact between people changes massively. There are large numbers of young men who spend a lot of time together in close proximity. This can lead to transmission among soldiers and ultimately to the spread of the disease to the civilian population. In 1871, for example, a devastating smallpox epidemic occurred in London – an event that coincided with the Franco-Prussian War.

But Great Britain was neutral in this war and therefore not involved.

David Earn: That’s right, we can’t prove the effect and cause either. It’s a correlation. But it is plausible that the war led to an outbreak and then the disease spread across Europe.

What was the effect of the variolation? Did the method work?

David Earn: People treated by variolation had a lower risk of death compared to natural infection. It is difficult to say whether this was also positive for the course of the epidemic. Because this method also leads to the risk that the transfer rate will increase. In any case, we cannot read any reduction in the number of cases of illness from our data.

As for the variolation, you also relate it to the current coronavirus pandemic. What is it?

David Earn: The argument about variolation is that if you give patients a small amount of infectious material, they will only develop a mild form of the disease. It is clear that no studies were carried out with this method in Covid-19, something like this would not be ethically justifiable. But the use of masks can also be viewed as a kind of variation: if you wear a nose mask and cough, the number of infectious particles expelled will be lower. If you infect someone in the process, it could lead to a milder infection than normal. This was discussed, for example, in October in the New England Journal of Medicine.

If you take a look at the patterns of the Covid-19 pandemic, what could have been done differently or better?

David Earn: In retrospect, of course, you’re always smarter. It is obvious that strict lockdowns are effective. The only question is: how do I balance this against the negative consequences for the economy, for the mental health of the population, and so on? This is not a trivial problem. Incidentally, this question also depends on the level of acceptance among the population for the measures taken. There are big cultural differences – and we can see that in the statistics. Unfortunately, we don’t have a crystal ball to play through different scenarios in one and the same country.

How do you assess the situation in your neighboring country, the USA?

David Earn: There have been contradicting messages in the United States, and I am thinking primarily of what the President said. That harmed the country. Let me put it this way: the state is far from what one would wish for.

Will it be possible to get herd immunity through vaccination?

David Earn: For Canada, I think it is realistic that we will achieve a high vaccination rate. As for the US, that seems less likely to me. The important question, however, is: does the vaccination also prevent infections? This would be necessary to achieve herd immunity. Unfortunately, it is not yet clear whether the vaccinations have this effect.

Your outlook for the year 2021?

David Earn: I hope the vaccinations prove effective and that we will return to a normal or more normal life as the year progresses. I can’t say more – there are still many uncertainties.

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