  The effectiveness of non-pharmaceutical interventions to curb the spread of
SARS-CoV-2 is determined by numerous contextual factors, including adherence.
Conventional wisdom holds that the effectiveness of protective behaviour such
as wearing masks always increases with the number of people adopting it. Here
we show in a simulation study that this is not true in general. We employ a
parsimonious network model based on the well-established empirical facts that
(i) adherence to such interventions wanes over time and (ii) individuals tend
to align their adoption strategies with their close social ties (homophily).
When combining these assumptions, a broad dynamical regime emerges where the
individual-level infection risk reduction for those adopting protective
behaviour increases as the adherence to protective behavior decreases. For
instance, for a protective coverage of 10% we find the infection risk for
adopting individuals can be reduced by close to 30% compared to situations
where the coverage is 60%. Using estimates for the effectiveness of surgical
masks, we find that reductions in relative risk of masking versus non-masking
individuals range between 5% and 15%, i.e., vary by a factor of three. This
small coverage effect originates from system-dynamical network properties that
conspire to increase the chance that an outbreak will be over before the
pathogen is able to invade small but tightly connected groups of individuals
that protect themselves. Our results contradict the popular belief that masking
becomes ineffectual as more people drop their masks and might have far-reaching
implications for the protection of vulnerable population groups under resurgent
infection waves.
