An investigational vaccine against respiratory syncytial virus (RSV) in pregnant women has been shown to help protect infants against severe disease, according to the vaccine’s manufacturer.

Pfizer recently announced that in the course of a randomized, double-blind, placebo-controlled phase 3 study, the vaccine RSVpreF had an almost 82% efficacy against severe RSV infection in infants from birth through the first 90 days of life, according to a company press prelease.

The vaccine also had a 69% efficacy against severe disease through the first 6 months of life. A total of 7400 women had received a single dose of 120 μg RSVpreF in the late second or third trimester of their pregnancy. There were no signs of safety issues for the mothers or infants.

Due to the good results, the enrollment in the study was halted on the recommendation of the study’s Data Monitoring Committee after achieving a primary endpoint. The company plans to apply for marketing authorization to the US Food and Drug Administration (FDA) by the end of 2022, and to other regulatory agencies next year.

“The directness of the strategy, to vaccinate expectant mothers during pregnancy so that their newborn is then later protected, is new and a very interesting approach,” commented Prof. Ortwin Adams, MD, head of virologic diagnostics at the Institute for Virology of the University Hospital of Düsseldorf to the Science Media Center (SMC).

In terms of the RSV vaccination strategy presented, “the unborn child has taken center stage from the outset.” Because the vaccination route is the placental transfer of antibodies from mother to child (“passive immunity”), “…the medical points of contact for this vaccination will be the gynecologists, not the pediatricians,” Adams said.

“This concept imitates the natural process, since the mother normally passes immune defenses she acquired through infections to the child via the umbilical cord and her breast milk before and after birth. This procedure is long-proven and practiced worldwide, especially in nonindustrialized countries, for a variety of diseases, including tetanus, whooping cough (pertussis), and viral flu (influenza),” explained Markus Rose, MD, PhD, head of Pediatric Pulmonology at the Olgahospital in Stuttgart, Germany.

The development of an RSV vaccine had ground to a halt for many decades. A tragedy in the 1960s set the whole field of research back: Using the model of the first polio vaccine, scientists had manufactured an experimental vaccine with inactivated viruses.

However, tests showed that the vaccine did not protect the children vaccinated, but it actually infected them with RSV, they then fell ill, and two children died. Today, potential RSV vaccines are first tested on adults and not on children.

Few Treatment Options

RSV causes seasonal epidemics, can lead to bronchiolitis and pneumonia in infants, and is one of the main causes of hospital stays in young children. Monoclonal antibodies are currently the only preventive option, since there is still no vaccine. Usually, 60%-70% of infants and nearly all children younger than 2 years are infected with RSV, but the virus can also trigger pneumonia in adults.

“RSV infections constitute a major public health challenge: It is the most dangerous respiratory virus for young infants, it is also a threat to the chronically ill and immunocompromised of all ages, and is the second most common cause of death worldwide (after malaria) in young children,” stated Rose.

Recently, pandemic-related measures (face masks, more intense disinfection) meant that the “normal” RSV infections in healthy adults, which usually progress like a mild cold, were prevented, and mothers were unable to pass on as much RSV immune defense to their children. “This was presumably responsible in part for the massive wave of RSV infections in fall and winter of 2021/22,” explained Rose.

Thomas Mertens, MD, PhD, chair of the Standing Committee on Vaccination at the Robert Koch Institute (STIKO) and former director of the Institute for Virology at Ulm University Hospital in Germany, also noted: “It would be an important and potentially achievable goal to significantly reduce the incidence rate of hospitalizations. In this respect, RSV poses a significant problem for young children, their parents, and the burden on pediatric clinics.”

Final Evaluation Pending

“I am definitely finding the data interesting, but the original data are needed,” Mertens said. Once the data are published at a conference or published in a peer-reviewed journal, physicians will be able to better judge the data for themselves, he said.

Rose characterized the new vaccine as “novel,” including in terms of its composition. Earlier RSV vaccines used the so-called postfusion F protein as their starting point. But it has become known in the meantime that the key to immunogenicity is the continued prefusion state of the apical epitope: Prefusion F-specific memory B cells in adults naturally infected with RSV produce potent neutralizing antibodies.

The new vaccine is bivalent and protects against both RSV A and RSV B.

To date, RSV vaccination directly in young infants have only had a weak efficacy and were sometimes poorly tolerated. The vaccine presented here is expected to be tested in young adults first, then in school children, and then in young children.

Through successful vaccination of the entire population, the transfer of RS viruses to young children could be prevented. “To what extent this, or any other RSV vaccine still to be developed on the same basis, will also be effective and well tolerated in young infants is still difficult to assess,” said Rose.

Mertens emphasized that all of the study data now needs to be seen as quickly as possible: “This is also a general requirement for transparency from the pharmaceutical companies, which is also rightly criticized.”

This article was originally published in Medscape’s German edition.

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