NEW DELHI -- Doctors Without Borders has challenged Pfizer's application for an Indian patent for its pneumonia vaccine so cheaper versions can be available to children in poor countries and to humanitarian organizations.

The medical aid group, also known as Medicins Sans Frontieres, said late Friday that it challenged Pfizer's patent application to allow Indian manufacturers to make affordable versions of the pneumococcal conjugate vaccine.

The group said the vaccine was one of the world's biggest sellers and that New York-based Pfizer had earned $6 billion from its sales in 2015.

Doctors Without Borders said its decision to oppose Pfizer's patent application comes after "years of fruitless negotiations" to get the company to reduce the price of the drug for use in humanitarian projects.