OBJECTIVES: Differences in responses to the COVID-19 pandemic among Northwestern European countries have generated extensive discussion. We explore how the impact of the first pandemic wave might have differed, had Belgium, Denmark, Germany, the Netherlands, Sweden and the United Kingdom adopted responses from the other countries, or had it delayed its own response.

METHODS: The time-varying reproduction number R_t for each country was estimated using time-series of laboratory-confirmed COVID-19 deaths. Counterfactual assessment of the impact of responses was conducted by interchanging the reduction in reproduction number by calendar date between countries from March 13^th to July 1^st, 2020. The impact of a delayed response was evaluated by lagging the time-series of the reproduction number with one day or three days.

RESULTS: The cumulative number of COVID-19 deaths for any of the six countries would have differed substantially, had the response of another country been adopted on the respective calendar date. The order, from the lowest to the highest expected mortality rate, was obtained with the responses of the Netherlands, Belgium, Denmark, the United Kingdom, Germany, and Sweden, with a seven- to twelve-fold difference between the lowest and highest outcome. For the Netherlands, delaying its response by three days resulted in a doubling of the cumulative COVID-19 mortality rate.

CONCLUSIONS: During the fast-growing first COVID-19 wave, small differences in initial epidemiological situations between countries, together with small differences in the timing and effectiveness of adopting COVID-19 response from neighboring countries, result in large variations in mortality rates.