Does Hand Hygiene Reduce Influenza Transmission?

To the Editor—In their editorial accompanying the study performed by Aiello et al [1], Daniels and Talbot [2] assert that hand hygiene is a “critically important” tool to prevent the spread of influenza. However, they fail to cite convincing evidence that this consequential statement is true. Although there is evidence that hand hygiene can reduce respiratory diseases, it has not been determined that it reduces transmission of influenza viruses [3, 4]. Indeed, Aiello et al [1] selected their study design, “[b]ecause the value of hand hygiene has not been established for influenza or ILI prevention during periods of confirmed viral transmission...” The study by Aiello et al [1] did not demonstrate that hand hygiene prevented transmission of influenza. First of all, the authors make clear that most of their influenza- like illness (ILI) cases were likely not associated with influenza infection. Second, their study showed no benefit for hand hygiene. Aiello et al conclude, “ILI incidence between the face mask and hand hygiene group and the face mask-only group were [sic] not substantially different, suggesting that the addition of a hand sanitizer component did not appreciably decrease the rate of ILI in this study population.” They further state, “It is important to note that handwashing habits were the same in both the face mask—only and control groups at baseline and over the study period, which suggests that mask use alone may provide a reduction in respiratory illness regardless of handwashing practices.” Nevertheless, Daniels and Talbot claim, “The importance of hand hygiene, however, cannot be overemphasized.” They state that because—after adjustment for covariates—only the face mask with hand hygiene (FMHH) group showed a significant reduction in ILI over the entire intervention period, “one must question whether the effect noted was due to hand hygiene education and provision of hand sanitizer to the FMHH group.” But Aiello et al [1], in their careful analysis of the limitations of their study, note there was a nearly 10% increase in the number of subjects in the FMHH group who reported wearing their masks more than the average duration (3.5 h/day) during weeks 3–6 of the study, compared with only a 2% increase in the face mask-only group. Daniels and Talbot also discuss the study of Cowling et al [5], which showed a significant reduction in laboratory-confirmed influenza when FMHH was used within 36 h of diagnosis of influenza in an index household contact. But this reduction was for the difference among 3 groups (control group, hand hygiene-only group, and FMHH group). Cowling et al [5] note that the effect was attributable to fewer infections in the FMHH group. The efficacy of hand hygiene in reducing transmission of influenza likely hinges on a fundamental, unresolved issue: Is influenza virus spread primarily by large droplets or by small-particle aerosols? The greater the importance of large droplets, the more likely that hand hygiene will reduce transmission. If small-particle aerosols predominate, hand hygiene may provide little benefit. Evidence exists that 100-fold greater quantities of virus are required for infection by intranasal drops than with aerosols, and that illness after intranasal inoculation is milder than naturally acquired disease, has shorter duration of cough, and lacks the abnormalities in small airway function and transient increase in airway reactivity that characterize naturally acquired disease [6, 7]. One study showed no detectable influenza virus on hands 5 minutes after transfer from environmental surfaces [8]. Another study showed no detectable virus on the hands of 6 of 20 volunteers 2 minutes after inoculation with a high concentration of H1N1 virus [9]. Volunteers with detectable virus at 2 minutes showed a 3–4 log reduction compared with the initial inoculum. In the absence of data demonstrating that hand hygiene reduces transmission of influenza, it has been argued that recommending it is prudent [10]. This is, of course, a highly defensible position. However, the incremental cost of following this recommendation—certainly if alcoholbased hand sanitizer is widely employed— would be considerable when extended to the large at-risk population around the world. Also, if hand hygiene is ineffective and individuals behave inadvisably (eg, venturing into crowds under the false notion that hand hygiene protects them from acquiring or transmitting influenza), increased cases of influenza would result. Although the efficacy of hand hygiene has been established for some respiratory infections, additional studies are needed to determine whether this benefit extends to influenza.