The respiratory syncytial virus season is months away, but competition between the only two approved vaccines (so far) for the pathogen is already heating up. GSK is suing Pfizer, claiming that company’s vaccine infringes on four of its patents.

The GSK vaccine for RSV, Arexvy, and Pfizer’s vaccine, Abrysvo are both new products, having won their FDA approvals in May for preventing the lower respiratory tract disease caused by RSV in adults 60 and older, an age group particularly vulnerable to the pathogen. GSK’s approval was first. In the complaint filed on Wednesday in federal court for the Delaware District (case number 23-cv-831), GSK now claims it was also sooner to begin RSV vaccine research.

Other research predated GSK’s work. The complaint notes 1960s efforts that failed with inactivated virus, live-attenuated virus, and vaccines that target specific proteins in the pathogen. Recent RSV vaccine research has focused on RSV F, a protein that the virus uses to fuse to and enter cells, causing infection. In the early 2000s, scientists at the National Institute for Allergy and Infectious Diseases, part of the National Institutes of Health (NIH), identified two versions of the protein, a prefusion form and a postfusion form. The prefusion form was shown to be better at eliciting antibodies that confer protection, so this is the version that became the basis for much RSV vaccine research.

Pfizer’s Abrysvo has two versions of a prefusion form of the F protein. As Pfizer reported clinical trial progress for that vaccine and provided updates on the regulatory process, the pharmaceutical giant said its vaccine builds on the NIH research. Other companies, such as Moderna and Icosavax, are also developing RSV vaccines that address this conformation of the F protein.

GSK’s complaint makes no reference to the NIH research but claims its own RSV work dates to the early 2000s. The British pharma giant says Pfizer’s research started in 2013—at least seven years after GSK. The company goes on to say that its scientists focused on RSVPreF, the prefusion form of the protein.

“GSK scientists realized that previous vaccines using the RSVPostF conformation induced an immune response only to the RSVPostF conformation, which induced an immune response only to the RSVPostF conformation, which led to limited or no efficacy,” GSK said in the complaint. “Beginning in the mid-2000s and continuing into this decade, GSK scientists worked to develop an RSVPreF antigen that targets the RSVPreF conformation present on the virus before it attaches to the cell surface.”

GSK said it patented compositions of RSV vaccines and methods for preparing those compositions. According to GSK, Pfizer knew of at least one of them, a patent for recombinant RSV antigens. In 2019, Pfizer filed an opposition with the European Patent Office claiming the European counterpart of that patent is invalid, the complaint said. Pfizer also claimed that patent and another one were invalid in 2022 actions filed in London.

In an emailed statement, Pfizer confirmed that GSK initiated litigation in federal court asserting patent infringement related to RSV vaccines.

“Pfizer is confident in its intellectual property position and will strongly defend its right to bring its innovative RSV vaccine Abrysvo to patients,” the company said.

In addition to clinical research in older adults, Abrysvo was tested as a maternal vaccine that protects an infant through vaccination of the mother during pregnancy. Antibodies produced by the mother’s immune system pass through the placenta and confer protection to the fetus. In May, an FDA advisory committee voted unanimously that clinical data support the vaccine’s effectiveness in infants. An FDA decision for Abrysvo as a maternal vaccine is expected later this month.

GSK is seeking an unspecified amount in damages that include lost profits and royalties from Pfizer’s alleged patent infringement. That sum could be increased to triple the amount found, plus interest, according to the complaint. GSK is also asking for an order that permanently stops Pfizer from making or selling its RSV vaccine in the U.S. for adults over 60. The company stopped short of seeking to restrain Abrysvo’s use for preventing RSV disease in infants. But GSK still sees this use as an infringement and it “will seek appropriate damages or other monetary relief in lieu of an injunction for such infringing use,” the complaint said.

Public domain image by the CDC