Harvard economics professor Robert Barro has a useful NBER working paper showing that lockdowns during the 1918 flu pandemic succeeded in delaying deaths by flattening the curve, but not in reducing the overall number of deaths. Some would use this paper to argue against the efficacy of the current lockdowns. But there is a key difference between now and 1918: Flattening the curve in 1918 resulted in the end in about the same deaths over the 3 year run of the virus. Flattening the curve now is likely to cut off the deaths in about a year when a vaccine comes on line. Now, a death delayed is more likely to be a life saved.
Non-pharmaceutical public-health interventions (NPIs) were measured by Markel, et al. (2007) for U.S. cities during the second wave of the Great Influenza Pandemic, September 1918-February 1919. The NPIs are in three categories: school closings, prohibitions on public gatherings, and quarantine/isolation. Although an increase in NPIs flattened the curve in the sense of reducing the ratio of peak to average deaths, the estimated effect on overall deaths is small and statistically insignificant. The likely reason that the NPIs were not more successful in curtailing mortality is that the interventions had an average duration of only around one month.
The above abstract is from the following Barro NBER working paper. (The link leads to the American Enterprise Institute version of the paper.):
Barro, Robert J. “Non-Pharmaceutical Interventions and Mortality in U.S. Cities During the Great Influenza Pandemic, 1918-1919.” National Bureau of Economic Research, Inc., Working Paper # 27049 (April 2020).