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Home » RSV vaccine candidates are entering phase 3 trials — stopping a dangerous disease

RSV vaccine candidates are entering phase 3 trials — stopping a dangerous disease

A respiratory syncytial virus (RSV) vaccine has been a goal for researchers for a long time, given the serious outcomes of an RSV infection. According to the CDC, an RSV vaccine could dramatically reduce hospital and intensive-care admissions, especially for children and seniors.

A series of clinical trials tested a vaccine made from inactivated RSV in children in the 1960s. Unfortunately, the vaccine candidate worsened the disease in children when they were later naturally infected with RSV.

However, these new vaccines have shown good safety and effectiveness results in clinical trials. This article will review what RSV is and the list of vaccine candidates that are entering or are in phase 3 clinical trials.

woman in blue sweater lying on bed
Photo by Polina Tankilevitch on

Respiratory syncytial virus infections

The respiratory syncytial virus is a common, contagious virus that causes infections of the respiratory tract. It is a single-stranded RNA virus, and its name is derived from the large cells known as syncytia that form when infected cells fuse.

It usually causes mild, cold-like symptoms. But it can cause serious lung infections, especially in infants, older adults, and people with serious medical problems. For seniors, RSV infections are dangerous as immune systems weaken.

RSV spreads from person to person through:

  • The air by coughing and sneezing.
  • Direct contact, such as kissing the face of a child who has RSV.
  • Touching an object or surface with the virus on it, then touching your mouth, nose, or eyes before washing your hands.

(I don’t mean to keep repeating myself, but all the measures you take to reduce your risk for COVID-19 seem to be useful for preventing RSV.)

RSV can sometimes lead to pneumonia (infection of the lungs) and congestive heart failure (when the heart can’t pump blood and oxygen to the body’s tissues).

According to the CDC, each year in the United States, RSV leads to approximately—

  • 2.1 million outpatient visits among children younger than 5 years old
  • 58,000 hospitalizations among children younger than 5 years old
  • 177,000 hospitalizations among adults 65 years and older
  • 14,000 deaths among adults 65 years and older

I know these numbers pale next to COVID-19, but before the pandemic, an RSV vaccine was often at the top of the wish list for a new vaccine. And for those people who had children or parents who had to deal with a serious RSV case, they probably wish there was a vaccine too.

Hopefully, these vaccines will soon become available, as new research is quantifying their need:

  • The JAMA Network published an article on 29 December 2021 that suggests RSV poses a greater risk to infants than influenza, while both are associated with substantial mortality among elderly individuals.
  • The journal PNAS published research on 14 March 2022, which found that administering an RSV vaccine to pregnant mothers reduced antimicrobial prescribing among their infants by 12.9% over the first three months.  

So, speaking of vaccines, let’s take a look at the four candidates for an RSV vaccine.

Vials of medications. Dark blue.

RSV vaccine candidates

Since the four candidates are in phase 3 clinical trials, there’s not a lot to write about their safety and effectiveness. However, if they are in phase 3, it’s probably a good assumption that they have met the standards of safety.

So here is the list of vaccine candidates:

  • Moderna — yes, that Moderna who brought us an mRNA vaccine for COVID-19 has developed a new mRNA vaccine for RSV. Moderna has developed one RSV vaccine candidate for seniors, mRNA-1345, and two for pediatrics, mRNA-1345 and mRNA-1365. At this point, the mRNA-1345 candidate is in phase 1 trials for pediatrics and phase 2/3 trials for seniors. Both trials are recruiting subjects, so you have a chance to join in. The US FDA has granted Fast Track designation for the senior version of the mRNA-1345 — this does not mean that the FDA makes it easy to get approval, it just means that the new drug application goes to the “top of the pile,” so it might get a quicker review.
  • Pfizer — another COVID-19 vaccine manufacturer is developing an RSV vaccine, PF-06928316. However, unlike the COVID-19 vaccine, this is not an mRNA vaccine. It is based on the crystal structure of prefusion F, a key form of the viral fusion protein (F) that RSV uses to attack human cells. There are two forms of the F-protein, and this vaccine targets both. It is undergoing phase 3 trials for seniors and two phase 3 trials for adults. They also have a phase 3 trial that is examining the effectiveness of the vaccines in infants who were born to vaccinated mothers. All four of these trials are still recruiting subjects, so join in if you want.
  • GSK — the GSK vaccine, RSVPreF3 OA, is a single-dose vaccine that contains a recombinant subunit RSV antigen combined with GSK’s proprietary AS01 adjuvant. Four phase 3 clinical trials are currently recruiting subjects — non-pregnant women, pregnant women, pregnant women and infants born to vaccinated women, and booster vaccination after pregnancy.
  • Bavarian Nordic — their MVA-BN RSV vaccine candidate incorporates five different RSV antigens to stimulate a broad immune response against both RSV subtypes (A and B). It theoretically mimics the immune response that is observed following a natural reaction to RSV. They are recruiting for a phase 3 clinical trial in seniors.

Back before COVID-19, I would assume that once a vaccine candidate entered phase 3 clinical trials, we were still four or five years away from getting approval for the vaccine. But today, with the experience some of these companies got developing the COVID-19 vaccines, it could be much faster.

The RSV vaccine is probably one of the most important vaccines, outside of COVID-19, of course, under development. And I wish it were here today.


Michael Simpson

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