A perverse effect

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The covid-19 pandemic has changed infection patterns for other respiratory diseases such as influenza and the common cold. Non-pharmacological preventive measures – such as isolation, social distancing, hand sanitisers, masks… – implemented against SARS-CoV-2 also reduced the overall circulation of other infectious micro-organisms, especially respiratory viruses. At first glance, a reduced circulation of pathogenic micro-organisms might seem desirable, but it turns out that this is not always the case. In fact, in the summers of 2021 and 2022, coinciding with the cessation of prophylactic measures against covid-19, some of these seasonal respiratory diseases seem to have returned with renewed vigour.

The observed increase in extemporaneous respiratory infections after the peak of the pandemic cannot be explained by the emergence of new, more aggressive or transmissible viral variants. The most plausible explanation is the anomalous increase in the number of people susceptible to infection – i.e., patients who would have become infected and immunised in the absence of preventive measures. And if the absolute number of infections increases significantly, the number of severe cases and hospitalisations also increases proportionally.

One such prominent disease is respiratory syncytial virus (RSV), which in its most benign form often presents as a common cold in adults, but can progress to bronchiolitis or pneumonia in vulnerable groups. This virus belongs to the group of positive-strand RNA viruses. Other members of this family include influenza, rabies, and Ebola. The name syncytial (from the Greek sin, ‘together’ and cyto, ‘box’ or ‘cell’) refers to the fact that infected cells tend to fuse together and appear to be plurinucleated.

Respiratory syncytial virus is particularly harmful to premature babies and the elderly or immunocompromised patients. RSV infection is the leading cause of paediatric hospitalisation for bronchiolitis and a major cause of infant mortality worldwide. As an indicator, in the pre-pandemic year of 2019, respiratory syncytial virus killed more than 100,000 children under the age of five worldwide. About half of these deaths occurred in children under six months of age.

Why does respiratory syncytial virus affect babies? The virus first infects the eyes and nose, then the epithelial cells of the upper airways, then the lower airways, bronchi and bronchioles. Babies’ bronchi and bronchioles are extremely narrow and easily clogged with mucus produced by the infection.

In the absence of vaccination, of course, infection is the only way to acquire some defence; it is, in fact, the ancestral method of immunisation from pre-vaccination times. It is a cheap immunisation, depending on how you look at it, and a risky one, depending on the person. With the added consideration that, in the case of this virus and others we may have in mind, the immunisation produced is temporary and ineffective, and we will be re-infected several times in our lifetime.

Recently, pre-pandemic research has led to the development of a specific antibody-type drug, nirsevimab, which has begun to be used in severe cases. A clinical trial of an RSV vaccine involving several European countries is also showing encouragingly high levels of protection and safety. The invention of tools against viruses is a never-ending process.

© Mètode 2023 - 116. Moments of science - Volume 1 (2023)
Professor of the Department of Genetics at the University of Valencia.