Sweden, Denmark, and Norway: Who's Mitigation Strategy is Better?
There is a common perception that Sweden is paying a high price for their soft approach to locking down, while other Scandinavian countries, Denmark and Norway, have much fewer deaths because of their more aggressive mitigation strategies. This simple analysis shows that Sweden and Norway have had essentially the same effectiveness and Denmark is slightly better. All three have flattened the curve. A good discussion on the differences in timing and mitigation policy can be found here: The Local: After Lockdown Differences in Scandinavian Countries. Norway initially was the most aggressive. Norway issued lockdown orders on 3/12/20, Denmark and Sweden on 3/13/20.
[05/28/20 Update: Norway health chief: lockdown was not needed to tame Covid]
A simple analysis is presented here using the average time between infection and death, death rate, and per capita population to compare the three nations.
We can calculate the timing of infection if we know the time of death and the delay between death and infection:
Tinfection (time of infection) = Tdeath (time of death) - Tdelay (infection to death).
Normalizing deaths and infections to 1 million population is useful to compare results between nations. The death rate is used to scale the magnitude of infections, but does not affect the conclusions relative to timing. Current antibody testing and ongoing studies of asymptomatic infections indicate that the death rate is likely in the range of 0.1% to 0.5%.
The analysis below shows that Sweden and Denmark mitigation measures had almost identical results with significant flattening of the curve, while Norway was able to flatten the curve even more effectively. But the difference is not as dramatic as presented in general reporting. The analysis also indicates that Sweden started to flatten the curve significantly before the other two countries even locked down.
For this analysis we assume the following parameters:
- Tdelay = average time between infection and death = 20 days (CDC Report Suggesting 4.2 + 16.1 Days Infection to Death)
- Death Rate = Actual death rate is 0.5%
- Sweden Population: 10.23 Million
- Norway Population: 5.38 Million
- Denmark Population: 5.8 Million
- Sweden's "lockdown" date of 3/13/20 is the marker in the graphs below.
Figure SND2 shows the daily infection rate per million scaled by the death rate and projected back in time by the average time between infection and death. As you can see from this graph, Sweden has a significantly higher infection rate per 1 million prior to lock-down in any of these countries: 3X that of Denmark and 6X that of Norway. This infection rate prior to the lockdown is likely the result of outbreaks in Sweden due to a larger proportional influx of infected individuals from southern Europe in mid to late February. After the lockdown, you can see that Denmark did a little better than Sweden and Norway at flattening the curve, but all follow the same general trend.
In figure SND3 we can see a logarithmic scale of cumulative reported deaths from data and in figure SDN4 we have the logarithmic scale for cumulative infections. Again it is appears from examining these curves that Sweden and Norway are essentially the same relative to flattening the curve, and Denmark has done slightly better. All three were on an identical trajectory prior to the lockdown.
An examination of the multiplication rate to compare Sweden, Norway, and Denmark is shown in figure SND5. The multiplication rate is calculated by comparing the ratio of cumulative deaths on a given day to the number nine days prior. Nine days is useful because that is the average time an individual is infectious. Here we can see Norway had already driven down the rate in early April, whereas Denmark had driven the infection rate down the most by mid April, and Sweden is close but not as effective early on, but tracks very closely with Norway after the full effect of the lockdown was in place. The significant reduction in infection trends for Sweden prior to official direction indicates that citizens of Sweden were already practicing social distancing as they became aware of the outbreak in Italy. The curves also show that Norway reversed a growing infection trend with their lockdown on March 12th one day prior to Denmark's on March 13th.