Astellas just forged a licensing deal with Affinivax to develop a pneumococcal vaccine. Their target? Pfizer’s Prevnar 13—the world’s bestselling vaccine.

Through the collaboration, the Japanese drugmaker will shell out $10 million up front to develop and commercialize a pneumococcal vaccine based on Affinivax’s Multiple Antigen Presenting System (MAPS), a technology that the biotech has been working on for three years.

Affinivax will be eligible to collect undisclosed development and commercial milestones along the way.

Steve Brugger, Affinivax’s CEO, was not shy about the partnership’s intention. They're going after Pfizer’s blockbuster vaccine Prevnar 13, which reeled in about $6 billion for the New York pharma giant last year.

"That’s pretty much the market we’re after," Brugger said in an interview.

The Cambridge, Massachusetts-based company’s MAPS vaccine platform is "next-generation," the Affinivax CEO said. Pfizer’s vaccine uses a conjugation technology to link polysaccharides—as antigens—with protein antigens that primarily serve as carriers. What’s new about Affinivax’s platform is that it uses biotin and rhizavidin to join the two antigen types, allowing the protein antigen itself to induce an immune response, thus boosting efficacy.

Prevnar 13 protects against 13 strains of pneumococcal bacteria, but Brugger said a MAPS-based vaccine could potentially cover many more strains and elicit stronger immune responses.

The vaccine could also interfere with the bacteria's ability to colonize the nose and throat, Brugger said. Many people, especially children, have the bacteria in their noses, which could trigger illness in people with weakened immune systems.

Affinivax formed in 2014 based on the MAPS technology, which was developed at Boston Children’s Hospital, with a $4 million investment from the Bill & Melinda Gates Foundation. The Gates Foundation poured in an additional $2.5 million last February after the pneumococcal candidate showed more promise. The company has moved its candidate through preclinical proof of concept and is getting closer to an IND submission.

The biotech's new deal with Astellas partially stems from a September 2015 collaboration it inked with Nosocomial Vaccine Corporation, a company set up by ClearPath Development and Astellas. Because Astellas has been trying to expand in the global vaccine market, Brugger said the partnership is "a natural fit."

Even though a "next-generation" vaccine that Brugger boasts about could take years to reach the market, it is still a potential threat to Pfizer’s already-declining Prevnar sales. In fact, Prevnar 13’s $6 billion worldwide sales in 2016 represented a 6% decrease year over year. The company attributed the decline to a double-digit growth spurt in 2015; a "high initial capture rate" among eligible patients reduced the number of people who still need the shot.

EvaluatePharma, in its first-quarter report compiled in January, estimated that sales of Prevnar 13 will be slightly below $5.5 billion in 2022, with a yearly decrease of about 2%.

But there's still a chance that Pfizer could pick up steam from a 2016 FDA approval for expanded use in adults 18 to 49—and, what seems more promising, a new approval in China. That approval covers the vaccine, marketed as Prevenar 13, in children and infants aged 6 weeks to 16 months. Pneumococcal diseases claim the lives of about 30,000 children in that age group in China each year.