Why observational studies shouldn't be used to assess respiratory virus interventions - part 1

Why use poor evidence when better quality evidence is available?

When we first wrote the protocol of the Cochrane review “Physical interventions to interrupt or reduce the spread of respiratory viruses” in 2006, our criteria for the types of studies to be included were as follows: “We will consider individual-level or cluster randomised or quasi-randomised controlled trials. To enhance public health relevance, we will also consider observational studies (cohort and case-control designs), and any other comparative design, provided some attempt has been made to control for confounding.“ 

Published in 2007, the first version included 14 RCTs. This paucity of randomised evidence meant the first three versions included non-randomised observational evidence, provided they compared whatever intervention they were assessed with, either standard practice or another intervention. The lack of a comparator would have meant that no inference could be drawn from the study.

So, for example, in the 2010 version, we included seven case-control studies (all carried ou…