Effectiveness
Birschoff et al found N95 respirators protected 4 of 5 participants in their influenza exposure model.(66) Birschoff et al’s second study of live attenuated influenza vaccine strain (LAIV) in subjects wearing N95 respirators in addition to goggles to prevent transocular transmission, was 90% effective (26 of 29 were PCR negative).(69)
A surgical mask overlay has been recommended to provide barrier protection in order to diminish contamination and attrition.(75, 76) This increases the breathing resistance and discomfort.(76, 77) However, the increased CO2 has not been shown to be clinically relevant after a 12-hour shift.(78) Increasing layers of PPE not only increase risk for confusion and contamination, it also increase the complexity of patient care.(79)
Evaluation of the deterioration of the filtration efficacy is difficult. Safety is affected by multiple variables that impact respirator function and contamination over time. Other factors than can potentially influence this include viral aerosol concentration, wearer’s breathing rate, time of patient interaction, effect of humidity, diffusion, and particle retention efficiency of the mask. Research on the physiologic impacts of the long term N95 respirator use has been limited, and most are laboratory based. There is inadequate understanding of the number, size and dispersion of droplets containing live, infectious particles or aerosol.(6)
Respiratory pathogens may remain infectious on respirator surfaces for extended period, with the influenza A and B model surviving 8-12 hours on porous substrates, compared to 24-28 hours on non-porous surfaces.(80-82) Some pathogens transfer well in high relative humidity.(83) However porous surfaces have a lower transfer rates due to entrapment of organisms within their matrix and the greater surface area in the recesses for attachment, hence less accessible to human hands.(83, 84) More than 99% pf pathogens remained trapped in the respirator after handling or following simulated cough or sneeze.(85-87) Respirators may be contaminated with other pathogens that have prolonged environmental survival (eg methicillin-resistant Staphylococcus aureus) resulting in the risk of self-inoculation.(88) The studies on the transfer efficiency of pathogens from mask to skin and other surfaces is limited to the lab setting, which may be different in clinical setting. Nevertheless, this can be mitigated by performing hand hygiene.
MacIntyre CR et al’s randomised control trial (RCT) of HCW in the surgical masks, targeted N95 (intermittent use only in high risk procedure) and N95 arm (continuous use throughout shift) over 4 weeks found less respiratory infection (HR 0.56, 95% CI 0.32 – 0.98), influenza (RR 0.34, 95% CI 0.1 – 1.11) in the N95 arm, followed by the targeted N95 arm.(47, 89) This benefit persisted after adjusting for confounders by influenza vaccination and hand washing.(89) This is a more powerful study compared to Loeb M et al who had only 446 subjects, who found a trend towards increased protection with N95 from SARS thought it was not statistically significant.(90) The influenza rate found in their study (24%) is the same as rates of influenza documented in nosocomial outbreaks in HCW without preventative interventions, and higher than other studies in unprotected HCW.(89-92) This was also found in MacIntyre CR et al’s earlier study, where surgical mask group had double the infection outcomes compared to the N95 group.(93)
However, a recent systemic review and meta-analysis by Smith JD et al reported that there is insufficient data to determine the advantage of N95 over surgical masks.(6) Similarly Long Y et al’s systemic review and meta-analysis found no statistically significant differences in preventing influenza (RR 1.09, 95% CI 0.92 – 1.28), influenza-like illness (RR 0.61, 95% CI 0.33 – 1.14) and viral respiratory infection (RR 0.89, 95% CI 0.7 – 1.11).(7) However, when Loeb M et al’s study was excluded, there was a significant effect on N95 preventing viral respiratory infections.(7) This should be interpreted with caution, as while laboratory studies confirmed it confer superior protection, there is often issues with compliance in real-world practice. This again defers in a pandemic situation.