Introduction
Coronavirus disease 2019 (COVID-19), caused by the severe acute
respiratory syndrome coronavirus 2 (SARS-CoV-2), remains a global
pandemic and public health concern 1.
Despite the rollout of vaccination
programs and implementation of non-pharmaceutical interventions (NPIs),
the number of infected cases kept
rising rapidly particularly after the emergence of the delta variant and
recently, the emergence of the omicron variant. Globally, as of March
17, 2022, there have been over 460
million COVID-19 cases and over 6 million deaths 2.
Both influenza and respiratory syncytial virus (RSV) are the most common
respiratory viruses mainly affecting young children and older adults,
especially in low and middle income countries (LMICs)3–7. Globally, it is estimated that in the year of
2016, influenza and RSV were associated with 39 and 25 million acute
lower respiratory infection episodes, and 58,000 and 77,000 deaths,
respectively 8. In most temperate regions, influenza
and RSV normally circulates in autumn and winter months9. During the
COVID-19 pandemic, the activity of both viruses was low early on due to
the large-scale implementation of NPIs but then the resurgence of RSV
epidemic was observed in a number of countries globally since late 202010, followed by the resurgence of influenza epidemic
in the winter of 2021 11. While there is no doubt that
the comeback of influenza and RSV will pose greater pressures on
health-care providers who are already over-stretched in response to the
COVID-19 pandemic, it is not yet
known for individual COVID-19 patients, whether co-infection of
influenza or RSV could further increase their clinical severity.
In the present study, we aimed to
systematically and critically review the existing evidence on the impact
of co-infection with influenza or RSV on disease
severity in COVID-19 patients.