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The Incorrect Assumption Behind Sweden's COVID-19 Strategy

Sweden has chosen a nearly unique path through the COVID-19 pandemic, one that is raising eyebrows around the world. The official story is that our strategy is unique because we are a unique country, with high levels of trust in experts and the government. But unfortunately the truth seems to be that our strategy is based on an incorrect assumption. One that will cost us dearly.

What really happened in Wuhan?

To understand the incorrect assumption we need to first understand what happened in Wuhan.

In December 2019 people in Wuhan City started coming down with pneumonia for no clear reason. Chinese scientists linked the pneumonia to a new strain of coronavirus, subsequently given the designation SARS-CoV-2. Local authorities initially tried to downplay the severity of the outbreak, to avoid a disruption to the economy. This however proved impossible, and a quick succession of draconian lockdown measures ensued.

If you don’t know much about epidemiology, but have a causal scientific understanding of the world, then it’s perhaps obvious that such measures will, if draconian enough, stop the spread of a virus like SARS-CoV-2. Interestingly however, a significant portion of the medical community seems to have taken a much more skeptical position. Gauden Galea, the World Health Organization (WHO) representative in China, told reporters that the move was unprecedented in public health history, that it was certainly not a recommendation of the organization, and that authorities would have to wait and see how effective it was. The WHO has also called it “new to science”.

Science and proven experience

The medical profession, at least in Sweden, walks a tightrope between science and proven experience. Until the science is settled, doctors are taught, the safe bet is to trust in proven experience. For clinical work this is of course a very reasonable attitude. You can’t risk patients’ lives with experimental treatments just because there’s a single case study indicating it could work. But what happens if you apply this type of reasoning to the management of a SARS virus pandemic? Perhaps you think it’s safest to assume the draconian measures in Wuhan had no effect? And if you do make that assumption, then what would you think stopped the outbreak there? Well, the classic epidemiological answer is herd immunity: enough people became infected and immune, thereby preventing the virus from spreading further.

Wuhan City has a population of approximately 11.8 million. About 50 000 people there tested positive for COVID-19 before the outbreak was largely over (Mizumoto 2020). That’s just 0.5% of the population. So how could Wuhan City possibly have reached herd immunity so quickly? Well, it’s well known that there are asymptomatic cases of COVID-19. If large enough a portion of those infected are asymptomatic or have only very mild symptoms, then a majority of the population could possibly have been infected, while only 50 000 needed care. That would make the severity of a COVID-19 epidemic roughly comparable to a severe influenza, and this seems to be exactly what Sweden’s state epidemiologist Anders Tegnell believes. On March 16 he claimed that 90-95% of those infected would hardly notice their illness. On March 26 he made a similar but even more optimistic claim on a Swedish newscast. His predecessor Johan Giesecke has said that he believes COVID-19 is like a severe seasonal flu, and that he believes that Wuhan reached herd immunity. Anders Tegnell’s right hand man, vice state epidemiologist Anders Wallensten, has said that he doesn’t think it’s necessary to take the lockdown measures into account when interpreting the data from Wuhan.

These claims are almost certainly incorrect, and probably based on the incorrect assumption that it was herd immunity that stopped the outbreak in Wuhan City. The incorrect train of thought can unfortunately be traced all the way into the Public Health Authority’s predictions of the intensive care need for COVID-19 in Sweden (March 20, March 27). The latest prognosis (April 3) uses data from the Lombardy region in Italy instead of Wuhan City, but seems to be based on the same incorrect assumption.

Strong resistance to mathematical models

So why does Anders Tegnell and his colleagues at the authority press on with a strategy based on an obviously incorrect assumption? This is a difficult question to answer. There are political considerations of course, but perhaps it can partly also be understood through the lens of the academic turf war known as paradigm shift. Epidemiology has traditionally been a medical science, but has in recent years partly been taken over by mathematicians. Mathematicians publish their assumptions and predictions based on careful calculations. Medical doctors typically make quick decisions based on experience, entirely in their heads.

Anders Björkman, professor of infectious diseases at the Karolinska Institute, has openly defended the latter practice in an opinion piece titled “Don’t combat Corona with mathematical models”. In another opinion piece titled “Disaster prognoses complicate things for decision makers” he repeats the claim that over 90% of those infected with SARS-CoV-2 will have very mild or no symptoms at all. Instead of giving scientific references to support this position he refers to his own judgement. He also claims Sweden’s strategy will result in less than one thousand COVID-19 deaths in total. The official death toll presently stands at 887, with about 60 deaths per day over the last few days.

Swedish radio recently interviewed Lisa Brouwers, the Head of unit for analysis at the Public Health Agency, about their use of mathematical models. According to her the authority wants to avoid making uncertain assumptions, and therefore doesn’t do any mathematical modelling of the spread of the disease and subsequent care need. They also reject the models from Neil Ferguson’s group at Imperial College on the same grounds. Instead they seem to rely on Anders Tegnell’s and his colleagues’ judgement. The problem of course is that neither mathematical models nor human cognition can work without assumptions. You can choose to not acknowledge and disclose them. But you can’t avoid making them.

What will happen in Stockholm?

On April 8 Anders Tegnell claimed that the spread of the disease in Stockholm had plateaued, and that we might even have seen the turning point for the epidemic there. He did not clearly state why he made this prediction. No significant measures have been introduced over the past three weeks. However, at the time of this writing about 4 200 people have sought medical care for COVID-19 in the Stockholm region. If we assume that most of the epidemic is concentrated to the city of Stockholm with a population of roughly 970 000, then that means 0.4% of the population there has now sought care. This fits well with the incorrect assumption that Wuhan City reached herd immunity after about 0.5% of the population there had sought care.

Predictions are difficult, especially about the future. But there seems to be no good reason to believe that the epidemic in Stockholm has plateaued. Instead it is likely to continue growing with a doubling rate of approximately 7 days. The healthcare system is already under significant strain, and the care need can be expected to double next week, and the one after, even if strong measures are introduced in the coming days. Let’s hope that they will be. Otherwise the consequences can be dire.