Abstract: We included 29 trials (5718 participants). All studies except one were at an unclear or high risk of bias. Studies were small, reported low numbers of SSI events and were often not clearly reported. There were 16 trials that included people with wounds resulting from surgical procedures with a 'clean' classification, five trials that included people undergoing what was considered 'clean/contaminated' surgery, with the remaining studies including people undergoing a variety of surgical procedures with different contamination classifications. Four trials compared wound dressings with no wound dressing (wound exposure); the remaining 25 studies compared alternative dressing types, with the majority comparing a basic wound contact dressing with film dressings, silver dressings or hydrocolloid dressings. The review contains 11 comparisons in total. Primary outcome: SSI It is uncertain whether wound exposure or any dressing reduces or increases the risk of SSI compared with alternative options investigated: we assessed the certainty of evidence as very low for most comparisons (and low for others), with downgrading (according to GRADE criteria) largely due to risk of bias and imprecision. We summarise the results of comparisons with meta-analysed data below: - film dressings compared with basic wound contact dressings following clean surgery (RR 1.34, 95% CI 0.70 to 2.55), very low certainty evidence downgraded once for risk of bias and twice for imprecision. - hydrocolloid dressings compared with basic wound contact dressings following clean surgery (RR 0.91, 95% CI 0.30 to 2.78), very low certainty evidence downgraded once for risk of bias and twice for imprecision. - hydrocolloid dressings compared with basic wound contact dressings following potentially contaminated surgery (RR 0.57, 95% CI 0.22 to 1.51), very low certainty evidence downgraded twice for risk of bias and twice for imprecision. - silver-containing dressings compared with basic wound contact dressings following clean surgery (RR 1.11, 95% CI 0.47 to 2.62), very low certainty evidence downgraded once for risk of bias and twice for imprecision. - silver-containing dressings compared with basic wound contact dressings following potentially contaminated surgery (RR 0.83, 95% CI 0.51 to 1.37), very low certainty evidence downgraded twice for risk of bias and twice for imprecision. Secondary outcomes There was limited and low or very low certainty evidence on secondary outcomes such as scarring, acceptability of dressing and ease of removal, and uncertainty whether wound dressings influenced these outcomes. It is uncertain whether covering surgical wounds healing by primary intention with wound dressings reduces the risk of SSI, or whether any particular wound dressing is more effective than others in reducing the risk of SSI, improving scarring, reducing pain, improving acceptability to patients, or is easier to remove. Most studies in this review were small and at a high or unclear risk of bias. Based on the current evidence, decision makers may wish to base decisions about how to dress a wound following surgery on dressing costs as well as patient preference.

Plain-language summary: We conducted a review of all available, relevant evidence about the impact of dressings on the prevention of surgical site infections in surgical wounds healing by primary intention. This review examined data from 29 randomised controlled trials (which provide the most reliable evidence). These investigated the use of dressings in surgery that had a low risk of surgical site infection (clean surgery) and surgery with a higher risk (potentially contaminated surgery). We found no clear evidence to suggest that one dressing type was better than any other at reducing the risk of surgical site infection, nor that covering wounds with any dressing at all reduced the risk of surgical site infection. Additionally, there was no clear evidence that any dressing type improves scarring, pain control, patient acceptability or ease of removal. Currently decision makers may opt to make decisions about whether and how to dress a wound based on patient and clinician preferences and dressing costs. It is important to note that many trials in this review were small and the evidence was of low or very low certainty meaning that current information is uncertain. Assessed as up to date September 2016.