Respiratory virus circulation during pandemic: Why were some viruses still circulating?

A few months after the beginning of the coronavirus disease 2019 (COVID-19) pandemic in March 2020, several non-pharmacological measures were adopted worldwide, with varying degrees of strictness, to contain the transmission of the virus and mitigate its impacts. These measures, in addition to effectively reducing the circulation of SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus 2), these measures also appeared to impact the circulation of other respiratory viruses. Therefore, this study aims to discuss the most relevant data available regarding the circulation of the major respiratory viruses during the COVID-19 pandemic, exploring the factors that allowed some viruses to continue circulating while others experienced a decline. Several authors report that the detection of influenza, respiratory syncytial virus (RSV), human coronaviruses (hCoVs), human parainfluenza viruses (HPIVs), and human metapneumovirus (hMPV) dropped significantly. However, non-enveloped viruses such as adenovirus, and especially human rhinovirus (HRV), did not seem to be as affected. Hypotheses for this scenario include adopting of non-pharmacological measures to curb the spread of COVID-19, behavioral changes in hygiene habits, intrinsic characteristics of each virus such as transmission mode, the presence or absence of a viral envelope and viral interference. Rhinovirus is particularly intriguing, as it maintained a high prevalence during the years of the pandemic. Further investigation into the possible explanations for this phenomenon may be worthwhile.