Delta may not mean change


Germany is in a confusing place with its pandemic developments. Covid cases have been falling as rapidly here as they have been rising in the UK: More than 50% reduction in the past week, dropping the new cases to 6.6 per 100,000 averaged over the week, under 1000 cases per day for the first time since the middle of last summer. At the same time, the Delta variant is rapidly taking over. Last week the Robert Koch Institute reported 8%, this week it’s 15%. Virologist Christian Drosten, speaking on the NDW Coronavirus podcast this week (before the new Delta numbers were available) spoke of the 80% level in England that, he said, marked the watershed between falling and rising case numbers.

I think this is the wrong back-of-the-envelope calculation, because it depends on the overall expansion rate of the virus, and the difference between Delta and Alpha, which is likely particularly large in the UK because of the large number of people who have received just one AstraZeneca dose, which seems to be particularly ineffective against Delta. There’s another simple calculation that we can do specifically for the German situation: In the past week there have been about 810 cases per day, of which 15.1% Delta, so, about 122 Delta cases per day. The previous week there were about 1557 cases per day, of which 7.9% Delta, so also about 123 Delta cases. That suggests that under current conditions (including weather, population immunity, and social distancing) Delta is not expanding. This may mean that current conditions are adequate to keep Delta in check, while Alpha and other variants are falling by more than 50% per week.

This suggests a very optimistic picture: that total case numbers will continue to fall. Within a few weeks Delta will be completely dominant, but the number of cases may not be much more than around 100 per day. And that ignores the increasing immunity: The infections reported this week occurred in the previous week, and the immunity is based on the vaccinations two weeks before that. With about 1% of the population being vaccinated every day, we should have — relative to the approximately 70% non-immune population* 20 days ago — already have about 15% reduced transmission by the first week in July. And at current vaccination rates we can expect, by the end of July that will be 30% reduced, providing some headroom for further relaxation of restrictions without an explosion of Delta cases.

That does raise the question, though, of why the general Covid transmission rate in Germany seems to be lower than in the UK. I don’t see any obvious difference in the level of social-distancing restrictions. Is it just the difference between single-dose AZ versus Biontech? If so, we should see a rapid turnaround in the UK soon.

* I’m very roughly counting each dose as 50% immunity.

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