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.