Background Throughout the COVID-19 pandemic, many countries have relied on population-wide ‘lockdown’ measures to cut chains of transmission and prevent health services from being overwhelmed. Because of their substantial social and economic costs, these policies have typically only been implemented when it becomes clear that less stringent measures are insufficient to control virus spread. Nevertheless, the consequences of delaying lockdowns can be even more costly. In this study, we sought to quantify the extent to which delaying implementation of lockdown measures increases total case numbers and ultimately prolongs the length of lockdown required, by considering the first wave of cases across Europe.

Methods Data pertaining to COVID-19 cases and containment/closure policies across Europe were obtained from online repositories maintained by Johns Hopkins and Oxford Universities, respectively. We identified key policy dates in each country, including the first date for which any restrictions were recommended or required, the date of lockdown, and the date lockdown was eased. We estimated the parameters governing the growth of COVID-19 during the first wave within each country, using linear splines fit to incident ~ cumulative cases. To allow for standardised comparisons across countries, we considered three distinct periods of growth: initial uncontrolled growth, growth under recommended restrictions, and growth under lockdown. Using stochastic simulations, we then estimated: the natural (i.e. observed) growth of cases, and the counterfactual (i.e. hypothetical) growth of cases that would have been observed in each country had lockdown measures been implemented 1, 3, 5, or 7 days earlier. Under each condition, we estimated the percentage change in total cases, and the percentage reduction in the required length of lockdown.

Results 39 European countries had both cases and policy data available, of which 33 entered lockdown and had estimable growth parameters. When lockdown was implemented 3 days earlier, total first wave cases were reduced by 16.93% (Q1-Q3: 10.40–19.49) and the median required length of lockdown was reduced by 6.52% (Q1-Q3: 1.92, 10.53). When lockdown was implemented 7 days earlier, total first wave cases were reduced by 31.46% (Q1-Q3: 23.53–37.86) and the median required length of lockdown was reduced by 14.69% (Q1-Q3: 5.48, 24.40).

Conclusion Delaying implementation of national lockdown policies is likely to result in substantial case increases that ultimately prolong the length of lockdown required. Our results highlight the importance of acting swiftly to minimise the spread of COVID-19 when case numbers are increasing exponentially.