158 6 Chapter 6 studies were ongoing at the time of this review (Ali 2020; Bourne 2017; Ding 2019; Drennan 2014; Foot 2017; Hajizadeh 2020a; Hansen 2017; NCT04136418; NCT04416295; NCT04533412; Steed 2017) . One study had nished data collection, but as the results were not yet published, study authors wished to withhold results until after publication (Bourne 2017). A further seven provided insu cient detail to allow a decision on eligibility. We were unable to establish contact with the study authors, so some studies are still awaiting classi cation (Baumann 2012; Borji, 2018; Carcereny, 2016; Mao 2020; NCT04256070; Reguera 2017; Xu 2010). Thus, 26 new studies (57 citations) were added to this review, in addition to the 26 studies already included in the previous version of the review.
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