A vaccine against RSV has been approved by the FDA. It is enormous.

On Wednesday, the Food and Drug Administration approved the first vaccine that protects against respiratory syncytial virus, otherwise known as RSV.

This is a great moment in the fight against a major scourge. Researchers have been working on creating an RSV vaccine for more than half a century. And it’s no exaggeration to say that RSV vaccines could save hundreds of thousands of lives globally each year.

RSV usually causes cold symptoms, but can also lead to severe lung inflammation or infection in very young and very old people. Each year, it causes up to 160,000 hospitalizations and 10,000 deaths among older Americans, and up to 55,000 adult deaths worldwide. People with heart and lung disease and weakened immune systems are most at risk of severe RSV disease.

RSV season generally begins in the fall and is worst in the winter. The vaccine, known as Arexvy, has been approved for adults 60 and older, will likely be recommended by the CDC in June and available in time for people to receive it this fall, according to The New York Times.

Young babies, especially newborns born prematurely, are also at high risk of serious illness from RSV: each year, the virus causes up to 80,000 hospitalizations and 300 deaths in American children under 5 years of age, and results in 2 million outpatient visits. The virus also causes around 120,000 infant deaths worldwide each year.

For now, this news does not impact pediatric populations. But there is hope on the horizon: Several vaccines that would protect babies and children – and a number of other vaccines for adults – are currently either under review by the FDA or in stages prior development.

RSV was a major contributor to the “triple epidemic” that flooded hospitals last winter. As these vaccines hit the market, they could dramatically change the face of cold and flu season for the better.

For now, here’s what you need to know.

The vaccine is very effective and usually has mild side effects

Arexvy, which is made by drugmaker GSK, is one of two adult RSV vaccines the FDA reviewed at a March 1 meeting. In trials, the vaccine reduced the risk of lower respiratory tract disease in adults over 60 by nearly 83% and the risk of severe disease by 94%.

The vaccine, which is given in a single injection, is constructed using the kind of conventional technology that preceded the mRNA technology used to create many Covid-19 vaccines. It is constructed using a piece of the RSV virus and a few additional molecules. The result is called a recombinant vaccine because it combines fragments of a virus with other products that stimulate the immune system. Other vaccines built with this technology include those used to prevent whooping cough, meningitis and shingles.

Vaccine-related side effects were generally mild and resolved within one to two days. The most common side effect was mild to moderate pain at the injection site, and a smaller number of people reported fatigue after the injection.

FDA will monitor serious side effects

For the most part, the Arexvy vaccine appears to be safe. However, the FDA is watching for a few uncommon side effects that it will watch closely as more people receive the vaccine. Although these have been very rare among people who have participated in vaccine trials, they are serious enough to warrant close monitoring.

In one trial, where each group had 12,500 participants, 10 Arexvy recipients and four people who received a placebo had atrial fibrillation, a rapid, irregular heartbeat that can cause clots to form in the heart. (The condition is extremely common in older people and sometimes reverses; it can be treated with blood thinners when it doesn’t.) One participant in this trial developed Guillain-Barré syndrome, a condition in which the immune system attacks nerve cells, causing weakness. and sometimes paralysis.

In another trial with 2,500 people each in the Arexvy and placebo groups, two people who received the vaccine developed an inflammatory disease affecting the brain and spinal cord, one of whom died.

The FDA requires GSK to monitor the incidence of all of these conditions among people who receive the vaccine in the future.

As always, the question for the agency is whether the risks of side effects from the vaccine outweigh the risks of RSV. Time will help answer this question.

It’s a huge victory!

People started trying to develop an RSV vaccine in the 1960s. Why did it take so long to get it to work?

RSV surface proteins are shape-shifters, taking on different forms depending on whether they have invaded – or fused – with a human cell. To complicate matters, their pre-fusion form is extremely unstable. This meant that for a long time researchers’ only option was to use post-fusion forms of the protein as targets for new vaccines.

As a result, for years, RSV vaccines could only recognize viral particles after they had invaded the cells – too late to make much difference. To make a better vaccine, scientists really needed a clear picture of what these surface proteins looked like. Before cell invasion.

In 2013, structural biologist Jason McLellan, now at the University of Texas at Austin, figured out how to get this picture: he found a way to stabilize a surface protein in its pre-fusion form, then described it in detail. This discovery meant that researchers could now create vaccines targeting an early stage of RSV infection – and they did.

Arexvy is only the first downstream result of this discovery to be approved by the FDA, and more will likely follow soon. Pfizer’s RSV vaccine for adults will likely be approved later this month; Moderna said it expects its adult vaccine to be licensed in the first half of this year; and several companies are working on vaccines to protect babies and toddlers under 2 years old.

Overall, we expect a lot more good news about RSV vaccines in the near future.

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