# Short, stringent lockdowns halted SARS-CoV-2 transmissions in Danish municipalities

## TL;DR

Short and strict lockdowns in Danish municipalities effectively reduced SARS-CoV-2 infections by around 31%, showing that such measures can halt virus spread.

## Contribution

The study demonstrates the effectiveness of short-term stringent lockdowns using a quasi-experimental design to avoid treatment selection bias.

## Key findings

- Targeted lockdowns led to a significant decrease in citizen mobility and daily infection numbers.
- Infection rates were reduced by an average of 31% in the affected municipalities.
- Synthetic counterfactuals confirmed the lockdowns' impact on curbing infections.

## Abstract

In late 2020, the focus of the global effort against the COVID-19 pandemic centered around the development of a vaccine, when reports of a mutated SARS-CoV-2 virus variant in a population of 17 million farmed mink came from Denmark, threatening to jeopardize this effort. Spillover infections of the new variant between mink and humans were feared to threaten the efficacy of upcoming vaccines. In this study the ensuing short-lived yet stringent lockdowns imposed in 7 of the countries 98 municipalities are analysed for their effectiveness to reduce SARS-CoV-2 infections. Synthetic counterfactuals are created for each of these municipalities using a weighted average combination of the remaining municipalities not targeted by the stringent measures. This allows for a clear overview regarding the development of test-positivity rates, citizen mobility behaviours and lastly daily infection numbers in response to the restrictions. The findings show that these targeted, short-term lockdowns significantly curtailed further infections, demonstrating a marked decrease, first in citizens mobility and then in daily cases when compared to their synthetic counterfactuals. Overall, the estimates indicate average reductions to infection numbers to be around 31%. This study underscores the potential of strict, yet severe lockdowns in breaking ongoing infection dynamics, by utilising a rare quasi-experimental design case that avoids bias introduced through treatment selection.