I heard about a meta-analysis talking about lockdowns and how their affected border closures have had "little to no effect on COVID-19 mortality":
We use “mortality” and “mortality rates” interchangeably to mean COVID-19 deaths per population. ...
Overall, lockdowns and limiting gatherings seem to increase COVID-19 mortality, although the effect is modest (0.6% and 1.6%, respectively) and border closures has little to no effect on COVID-19 mortality, with a precision-weighted average of - 0.1% (removing the imprecise outlier from Guo et al. (2021) changes the precision-weighted average to -0.2%).
Yet if you look at the data of some countries before and after closing their borders you can see significant COVID mortality changes occurring around the same time of the border closures.
As with Taiwan for example (May to July). I'm not sure of any other countries where closing borders may have been as effective, or if the decline in Taiwan COVID mortality may have been for another reason.