The @SwissScience_TF proposes to halve confirmed #SARSCoV2 cases every two weeks to reach less than 500 cases in January. What does this mean regarding the reproduction number? A thread about doubling times and half-lifes of #SARSCoV. (1/n)

The figure (in German) shows the relationship between the reproduction number and the time in which the number of new #SARSCoV infections double (red) and reduce by 50% (green). (2/n)

In early March 2020, the virus spread uncontrolled in Switzerland with a reproduction number of 2.8 which corresponds to a doubling time of 3 days. (3/n)

During the 'lockdown' from March to May 2020, the reproduction number dropped to around 0.7 and the number of new infections were reduced by 50% every 9 days. (4/n)

This means that for one week of uncontrolled spread in early March, three weeks of 'lockdown' were necessary to reduce the number of new infections to the same level again. (5/n) https://www.medrxiv.org/content/10.1101/2020.07.21.20158014v1

During summer, the reproduction number was around 1.1 and the number of new infections doubled roughly every 4 weeks. (6/n)

With the beginning of autumn and the collapse of contact tracing, the reproduction number increased to 1.6 and the number of new infections doubled every 7 days. (7/n)

Switzerland introduced nationwide measures with strict upper limits on gatherings on 28 October 2020. This resulted in a drop of the reproduction number below the critical threshold of 1. Per 2 November 2020, the reproduction number was around 0.9. (8/n) https://ibz-shiny.ethz.ch/covid-19-re/ 

But just as a reproduction number of 1.1 led to a slow growth of the epidemic during summer, a reproduction number of 0.9 is not sufficient to rapidly reduce the number of new infections. (9/n)

Hence, it is critically important to reduce the reproduction number to 0.8 or less which would allow to reduce the number of new infections by 50% every two weeks. An adaptation in behavior and of current measures should allow to reach this goal. (10/n)

This would allow Switzerland to continue with a reproduction number around 1 throughout the rest of the winter after January, without the massive costs to health, the economy and the society. (11/n)

But still: There are close to 100 #SARSCoV2-related deaths per day and Switzerland has to expect at least 3,000 deaths for this 'second wave' by the end of the year. (12/n)

Conclusion: If the measures of 28 October had been introduced a few weeks earlier, the number of deaths would be lower, the situation around Christmas more stable, and the required duration of strict measures shorter, which would not least have benefited the economy. (13/n)