Table 3.
Synopsis of observational case–control studies evaluating mask and respirator use for SARS
| Author/country (reference) | Study design and participants | Reported results | Comments |
|---|---|---|---|
| Chen/China (12) | 91 SARS IgG positive HCWs compared with 657 SARS IgG negative HCWs who cared for SARS patients in two hospitals. | Double‐layer cotton mask (versus a single‐layer cotton mask) protective against SARS infection in univariate analysis (OR 2·53, 95% CI 1·57–4·07); not significant in multivariate analysis. | Possible recall bias as questionnaire survey conducted 4 months after outbreak; limited data on frequency and type of exposures to SARS patients. |
| Lau/China‐Hong Kong (13) | 72 HCWS with SARS from 5 hospitals compared with 144 matched controls; PPE use examined during (i) direct contact with SARS patient; (ii) general contact with SARS and non‐SARS patients; and (iii) no patient contact. | Almost all HCWs wore N95 respirator or surgical mask in all patient settings. Unadjusted univariate analysis found inconsistent use of masks or respirators not associated with higher risk of SARS in any of the 3 contact settings; multivariate analysis found inconsistent use of >1 type of PPE during direct contact independent risk for SARS. | No serological testing of controls; reporting bias possible. |
| Nishiura/Viet Nam (14) | Period 1: Time from admission of index case to occurrence of secondary cases in one hospital: 25 laboratory‐confirmed SARS cases compared with 90 controls (HCWs and relatives of patients). Period 2: During a nosocomial outbreak in the hospital with strict isolation procedures, quarantine of HCWs and increased use of PPE: 4 laboratory‐confirmed SARS cases compared with 26 controls with only physicians and nurses in both groups. | Period 1: univariate analysis found masks (OR 0·3, 95%CI 0·1–0·7) and gowns (OR 0·2, 95%CI 0·0–0·8) protective; in logistic regression analyses, only masks protective (OR = 0·29, 95% CI 0·11–0·73) Period 2: use of masks (OR < 0·1, 95% CI 0·0–0·3) and gowns (P = 0·010, OR and CI not calculable) associated with non‐infection for doctors and nurses. | Possible recall bias; exposures imprecisely quantified; no serological testing of controls. |
| Nishiyama/Viet Nam (15) | Risk factors for serologically‐ confirmed SARS infection assessed for 85 case and control HCWs who had direct contact with SARS patients. | Multivariate logistic regression analysis found significant risk for SARS amongst HCWs who never wore mask compared with those who always wore a mask (OR 12·6, 95% CI 2·0–80·0, P < 0·01) | Possible reporting bias as interview conducted 7 months after outbreak; nature of exposures to SARS not specified; community exposures not assessed. |
| Seto/China ‐ Hong Kong (16) | 13 SARS‐infected HCWs with no community exposures compared with 241 HCWs without clinical SARS; all reported direct contact with 11 SARS patients in 5 hospitals. | Univariate analysis found HCWs who used surgical masks or N95 respirators, gowns or hand washing less likely to develop SARS; logistic regression analysis found use of any mask significant (OR 13, 95% CI 3–60). | No serological testing of controls; reporting bias possible as interviews conducted a month after cases identified; community exposures not assessed. |
| Teleman/Singapore (17) | Evaluated risk factors for serologically‐confirmed SARS amongst 36 ill case‐HCWs exposed to 3 highly infectious source patients and 50 well control‐HCWs that came within 1 m of serologically‐confirmed SARS patients. | Adjusted logistic regression analyses found that wearing N95 respirator during each patient contact (adj OR 0·1, 95% CI 0·02–0·86, P = 0·04) and hand washing after patient contact (adj OR 0·07, 95% CI 0·008–0·66, P = 0·02) protective. | Small sample size; no serological testing of the controls; limited recall of precise exposure data; no assessment of community/household exposures. |
| Lau/China ‐ Hong Kong (19) | 330 probable SARS cases with ‘undefined’ source of infection compared with 660 controls recruited by random telephone survey matched for age, sex and reference time for behaviours in question. | Matched multivariate analyses found using mask frequently in public places 27·9% of 330 cases versus 58·7% of 660 controls (OR = 0·36, 95% CI 0·25–0·52); washing one’s hands >10 times a day (OR = 0·58, 95% CI 0·38–0·87) and disinfecting living quarters (OR = 0·41, 95% CI 0·29–0·58) protective. | Likely misclassification because no laboratory testing for most cases and no testing of controls; non‐specific questions about exposures and potential protective measures. |
| Wu/China (20) | 94 unlinked, probable clinical SARS cases without reported contact with other SARS cases and 281 community‐based age‐ and sex‐matched controls in Beijing recruited by sequential digit dialling. | Multivariate analysis found ‘sometimes’ and ‘always’ wearing mask when outside home protective (matched OR 0·4, 95% CI 0·2–0·9, P = 0·03 and OR 0·3, 95% CI 0·1–0·6, P = 0·002, respectively). | Likely misclassification because no laboratory testing for most cases and no testing of controls; lack of information about community exposures; recall and self‐selection bias possible. |
SARS, severe acute respiratory syndrome; HCW, healthcare worker.