Main Text (Short
Communication)
The 2019-20 seasonal respiratory virus activity in Canada followed a
typical trajectory until March 2020, when a series of stringent public
health measures (PHM) aimed at controlling cases of COVID-19 were
imposed in many jurisdictions across Canada . The PHMs essentially
reduced the effective transmission rate of SARS-CoV-2, but also of other
pathogens transmitted by the respiratory route. Hence, in Canada and
other countries , the prevalence of most seasonal respiratory viruses
(including influenza and respiratory syncytial virus (RSV)) ended
abruptly in mid-March and were virtually suppressed or well below
historical levels throughout the 2020-21 season in Canada (Figure S1). A
notable exception were human rhinovirus/enterovirus (hRV/EV) infections
which persisted through repeated stringent PHMs retaining, to some
extent, their typical seasonal patterns in many Canadian jurisdictions.
The causes of hRV/EV persistence are not fully understood yet. The fact
that hRV/EV are non-enveloped may help sustain transmission thanks to
their prolonged survival on surfaces . Masks may be less effective at
stopping droplets and aerosols transporting hRV/EV . Another possibility
may be that the population groups that drive hRV/EV transmission
(typically young children) did not experience a contact rate reduction
as effective as the groups driving SARS-CoV-2 transmission when social
distancing and other PHMs were in place. Alternatively, hRV/EV
transmissibility to and among children may be similar to or higher than
that of SARS-CoV-2 in the pediatric population . Finally, the baseline
prevalence of hRV/EV may be significantly higher than other respiratory
infections (it also shows typical summertime persistence), allowing
hRV/EV to rebound more easily when PHMs ease.
The persistence of hRV/EV highlights a new and unsuspected role for this
family of viruses as a sentinel for the transmission rate for
respiratory pathogens. Figure 1 suggests that, with the hindsight of 18
months of observations for both hRV/EV and SARS-CoV-2 infections in
Canada, hRV/EV could have been used as a gauge for PHMs effectiveness as
well as early warning for SARS-CoV-2 resurgences. Indeed, in four large
provinces, where the signal is not too hampered by observation noise
(hRV/EV is not reported by the province of Quebec, the second most
populous Canadian province), the positivity rate of hRV/EV approximately
mirrors, and often precedes by several weeks, the positivity rate for
SARS-CoV-2 during the period when the vaccination coverage was null or
low. Although the data to disentangle the intrinsic seasonality of
hRV/EV infections and the effect of COVID-19 PHMs are not available, it
is conceivable that PHMs did impact hRV/EV detection rates, making them
a potential early monitor of the efficacy of PHMs. When COVID-19 vaccine
coverage reached a substantial proportion of the population (starting in
May 2021), a statistical analysis between SARS-CoV-2 and the lagged
hRV/EV positivity rates suggests a decoupling of their incidence
(Supplementary Material); the latter are not vaccine-preventable and
thus followed their typical seasonality and intensity beyond May 2021.
The COVID-19 pandemic has provided a real-life experiment on how
stringent PHMs on social distancing and enhanced personal protective
measures (e.g., hand hygiene, respiratory etiquette, and face masks)
impacted the circulation of seasonal respiratory viruses. Notably, it
highlighted the unique role for hRV/EV incidence to gauge the contact
rate of a population (most likely its younger age groups, where hRV/EV
symptomatic infections may be more prevalent). Direct measurements of
contact rates through population surveys (for example ) are valuable but
not sufficiently timely for surveillance, near-term forecasting and thus
for early warning of resurgence/re-emergence potential for respiratory
infectious diseases. Testing volumes for hRV/EV in Canada during the
2019-20 and 2020-21 seasons were higher than pre-pandemic levels (Figure
S4), suggesting potential different demographics tested during
non-pandemic seasons.
Our observations support the continuation for routine acute respiratory
infections
surveillance that includes hRV/EV, and encourages enhancements (e.g.,
sampling and reporting across all ages and spectrum of illness). The
current COVID-19 pandemic has shown that monitoring the incidence of
hRV/EV may have been undervalued so far: it could be useful for the
subsequent SARS-CoV-2 epidemic activity, and for the longer term,
potentially providing early warnings for other seasonal respiratory
illnesses (notably influenza and RSV) as well as the transmission
intensity at the time of introduction of another emerging respiratory
pathogen in the monitored population.