MUMBAI (Reuters) - The charity Medicins Sans Frontieres has formally opposed U.S. firm Pfizer Inc's application for an Indian patent on a highly effective pneumonia vaccine, saying it could deprive many developing nations of cheaper copies of the drug.

Some of the world's poorest countries and medical charities such as Medecins Sans Frontieres (MSF) depend on India's robust pharmaceutical industry to make cheaper forms of drugs and vaccines developed by big Western pharmaceutical companies such as Pfizer.

If India granted Pfizer a patent on its Prevnar 13 pneumonia vaccine, Indian firms would not be able to produce affordable versions of it for domestic use or exports, MSF said.

"To make sure children everywhere can be protected from deadly pneumonia, other companies need to enter the market to supply this vaccine for a much lower price than what Pfizer charges," Manica Balasegaram, executive director of MSF's access campaign, said in a statement on Friday.

Prevnar 13 is the world's biggest-selling vaccine, and Pfizer earned $6 billion from its sales in 2015, MSF said.

Pfizer spokespersons in New York did not immediately respond to requests for comment on Friday.

Pneumonia kills nearly a million people each year, and is the biggest cause of death among children under the age of five in India.

Pfizer has made the vaccine available at discounted prices under the Global Alliance for Vaccines and Immunisation (GAVI) -- an international public-private partnership to improve access to vaccines in the world's poorest countries. As of November 2015, 58 countries were eligible to procure the vaccine through GAVI, according to the organization's website.

MSF said many other developing countries cannot afford it.

India is eligible to procure the vaccine under the GAVI alliance, but has not bought it.

The vaccine costs about $170 per child in India in the private market.

The GAVI price is $10 per child, but MSF said Indian firm Serum Institute of India had agreed to supply it to MSF and countries in need for $6.

Serum Institute's executives were not available for comment on Friday.

MSF said it had filed a "pre-grant opposition," a filing through which patents can be opposed in India before they are granted. Pfizer first applied for a patent on the vaccine in 2007, according to the Indian patent office's website.

MSF has argued that the process Pfizer has sought a patent on is "too obvious to deserve a patent under Indian law."

The charity said its decision to oppose Pfizer's patent application came after "years of fruitless negotiations with Pfizer to lower the vaccine’s price for use in its projects."

Another Indian firm, Panacea Biotec, also filed an opposition to the Pfizer patent application back in 2010. A Panacea spokesman refused to comment on Friday, as the matter is being considered by the patent office.

(Reporting by Zeba Siddiqui in Mumbai; Editing by Susan Fenton)