Federico Martinón-Torres, Francisco-Javier González-Barcala

Every year, respiratory syncytial virus (RSV) makes its expected and dreaded seasonal appearance at a more or less predictable time, depending on multiple factors that still remain unclear but depend basically on a combination of latitude, relative humidity, and temperature. There may be some incertitude, but we can be sure that the season will occur more or less within a certain timeframe, so we can prepare and plan the selective prophylaxis campaign with palivizumab and organize resources in hospitals, at least in pediatric units, where the impact is better characterized and anticipated. However, the advent of the COVID-19 pandemic has disrupted RSV seasonality, and the annual epidemic has all but disappeared, presumably due to the implementation of general anti-SARS-CoV-2 measures, although we cannot rule out other mechanisms such as viral competition. This lull in the storm leaves us in an uncertain and worrying position regarding what to expect and when to expect it, particularly in countries like Spain that do not have a specific epidemiological surveillance system for RSV – instead, we base our observations on the simple accumulation of cases or shared perceptions among clinicians, and events as they unfold in other countries.

It is intriguing to wonder where RSV, a virus whose only known reservoir is human, hides – a enigma that makes it even harder to understand and anticipate its seasonality. The strongest proposal is that RSV infects monocyte-derived dendritic cells, where it can remain dormant for long periods thanks to the endogenous production of nitric oxide. More recently, RSV has been reported to replicate in Hofbauer cells, macrophages that play a central role in other viruses with marked vertical transmission, such as the Zika virus. This is worrying in a virus like RSV that is capable of producing viremia and extrapulmonary dissemination. The recent demonstration of high concentrations of viable RSV in extracellular amoebal vesicles of a respirable size is another cause for concern, and one that forces us to rethink the currently accepted mechanisms for the spread and transmission of this virus, and even to reevaluate the usual non-pharmacological control measures.

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