Respiratory syncytial virus bronchiolitis: what is the situation?

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Since October 2022, cases of bronchiolitis in young children have been increasing rapidly globally, causing social alarm and concern. The Centers for Disease Control and Prevention in the United States, Europe, and other regions are already watching this Pandemic with some trepidation.

This type of bronchiolitis is mainly caused by the respiratory syncytial virus (RSV). This is a common respiratory virus that usually causes mild cold-like symptoms. Most adults recover within a few days, but it relatively often causes serious infections in babies.

In these infections, the small airways (bronchioles) become inflamed and clogged with mucus, making it difficult to breathe. This gives rise to a multitude of pediatric visits in primary care and, in the most extreme cases, to hospitalizations.

The VRS is behaving abnormally

RSV Pandemics have been very regular before covid-19, starting every year in late fall or winter and dying down during the spring/summer months. However, the measures that have helped keep us safe from covid-19 over the last two and a half years (closures, social distancing, wearing masks, handwashing) have also limited the spread of other viruses, including RSV.

For that reason, during the 2020 and 2021 seasons there was a sudden drop in RSV cases and hospitalizations. However, this year, the season has started earlier than usual. And although the increase in cases has been more evident since the end of October, RSV began to be detected even during the spring.

In addition, the virus is spreading faster and causing more hospitalizations than usual, which is straining the healthcare system in several countries. In other countries, such as Spain, on the other hand, an increase in hospitalizations has not yet been observed compared to seasons prior to the pandemic, although it is too soon to draw conclusions.

This is not entirely unexpected, as as children return to daycare or school, where pandemic preventive measures are no longer followed, viruses, including RSV, return again.

The immune deficiency theory

But why is RSV spreading faster and causing more serious infections this year? Both children and mothers develop defenses against viruses when they come into contact with them. But during the covid-19 pandemic, the general population had little opportunity to become infected with RSV or any other virus, so they did not acquire immunity against them.

This immune deficit could explain why RSV returns with more force when preventive measures aimed at containing covid-19 have disappeared. It is especially noticeable in newborns, since mothers transfer their defenses to the fetus through the placenta and, after birth, through milk. Thus, newborns are protected against RSV during the first months of life as long as their mothers are.

The solution to RSV bronchiolitis is getting closer

There is now a drug, approved at the turn of the last century, that can help protect some babies who are at high risk of getting seriously ill from RSV. It is an antibody called palivizumab that is given as monthly injections to premature babies and young children with certain heart and lung conditions. But this drug is expensive and has limited efficacy, so it is administered only in very specific cases.

In recent years, thanks to the work of many laboratories around the world, the development of new treatments and vaccines against RSV has experienced great advances. Currently, there are around 39 approaches in different phases of clinical trials.

One of the most promising is the antibody nirsevimab, recently approved by the European Medicines Agency. This medication prevents the virus from entering our cells. The main advantage of nirsevimab over palivizumab is that a single intramuscular injection protects neonates and infants during their first RSV season.

vaccinate mothers

However, the ideal way to prevent RSV bronchiolitis would be to have a vaccine. Young children, however, do not have well-developed immune systems and respond poorly to vaccination. For this reason, for children under 6 months, what is recommended is passive immunization, that is, the administration of antibodies.

An interesting alternative is the vaccination of mothers. In this case, the newborns, as explained above, would benefit from the transfer of antibodies from the mother through the placenta and/or through lactation.

Vaccines based on subunits (parts of the virus, mainly the so-called F protein) are the most advanced for use in pregnant women. Pfizer has recently reported high efficacy and safety of its vaccine in pregnant mothers to prevent severe respiratory tract disease due to RSV.

In the case of children older than 6 months, one of the options considered is their vaccination by intranasal administration of complete attenuated viruses, that is, they do not cause disease. These vaccines are in the last phases of clinical trials.

In short, we are getting closer to having the right tools to prevent the dreaded bronchiolitis caused by RSV.

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