When is systematic review replication useful, and when is it wasteful?

[.]To the Editor,
During the past quarter of a century and following the global explosion in access to scientific literature, systematic reviews have become an excellent way to inform decisions with summaries of the effects of interventions, and to learn about their impact under different conditions or among populations. Systematic reviews play a key role in ensuring that policy decisions are informed by research. Local and national governments as well as international health organizations now regularly commission and request that systematic reviews inform policy decisions. In recent years, there has been a proliferation of systematic reviews –with multiple systematic reviews addressing the same question, often without any justification. While there has been a strong call to control the unnecessary duplication of systematic reviews and the confusion and waste this generates, there is also a growing recognition that there can be valid reasons for replicating systematic reviews [...]