Some of the world's biggest drugmakers are joining forces with rivals to help produce Covid-19 vaccines, forging unusual alliances that promise to substantially increase supplies by this summer.

Normally big pharmaceutical companies compete to sell cancer, arthritis and other drugs. The desperate need for Covid-19 vaccines, however, is turning fierce industry competitors into fast pandemic friends.

Sanofi SA recently agreed to help make a vaccine from Pfizer Inc. and its partner BioNTech SE after Sanofi's experimental Covid-19 shot suffered a five-month setback, freeing up a production line in Frankfurt.

"We were looking to contribute," said Thomas Triomphe, executive vice president for vaccines for Sanofi, which will start in June performing a crucial final step to make 125 million doses.

Novartis AG also agreed to help Pfizer and BioNTech produce more doses, while Baxter International Inc. and Endo International PLC have agreed to help Novavax Inc. produce its shot.

"This is a time when the pharma companies are saying, 'We'll go back to fighting when this is over. We'll take you to the cleaners and maybe drive you to bankruptcy, but right now we need to be working together,'" said James Bruno, who consults for drug companies.

The collaborations, along with the authorization of newer vaccines and fine-tuning by the vaccine makers themselves, could help significantly boost global output, as health authorities scramble to vaccinate people amid the threat of emerging variants that may increase spread of the virus.

Early supplies have been limited, as vaccine makers needed time to increase production and overcome early hiccups and problems getting raw materials.

The companies are supplying enough doses in the U.S. in February to vaccinate 20 million people, but output is projected to increase to 65 million people in June, according to analysts from Evercore ISI.

Overcoming rivalries may be the easiest part of the tie-ups, as partners must quickly learn the complex vaccine-making process and install the necessary equipment.

Transferring the know-how to make mRNA vaccines, like the ones from Pfizer-BioNTech and Moderna Inc., is especially challenging because the gene-based technology behind the shots is new, manufacturing experts say. Especially difficult is increasing production of the fatty envelopes that help protect the molecules carrying mRNA shots to their cellular targets.

"It's all being built from scratch, and it's taking awhile to get that up and validated," said Jim Robinson, a manufacturing consultant who previously worked at Sanofi and Merck & Co.

So far, only shots from Pfizer-BioNTech and Moderna are authorized in the U.S., though a vaccine from Johnson & Johnson could get greenlighted as soon as this week.

The production alliances are the latest example of industry rivals coming together to fight the pandemic, starting with research tie-ups.

They build upon a year-long effort by the drug industry and partners to crank up capabilities to make everything from the tiny vials that hold the shots to raw ingredients that make them.

The partnerships are bringing together companies that before the pandemic filed patents, ran competing advertisements and deployed sales representatives to secure sales for their medicines.

Pfizer, which has cancer drugs that compete with products from Sanofi and Novartis, and BioNTech brought on partners to help nearly double its 2021 production target to 2 billion doses.

"We're continuously looking at everything to try to create more than the 2 billion doses," Pfizer Chief Financial Officer Frank D'Amelio said in an interview.

Production can't start overnight. Vaccine manufacturing is a complex process that often requires training staff, upgrading facilities and buying new equipment, such as pumps, tubes and stainless steel bioreactors. Regulators must sign off before production starts.

Sanofi will perform fill-finish, a familiar step in the vaccine process, whereby vials are filled with vaccines, capped and readied for shipment, Mr. Triomphe said. He added that the company has six months before giving priority to its own Covid-19 vaccine.

Sanofi still had to buy machines to accommodate Pfizer's vials because Sanofi uses different ones for its own vaccines as well as freezers to store at subzero temperatures.

"All these elements are very different when you switch from manufacturing one product to another," said Mr. Triomphe. Sanofi said Monday it would also help with fill-finish for Johnson & Johnson's vaccine.

Novartis also signed up to do fill-finishing work for Pfizer and BioNTech, and Novartis's Swiss plant is scheduled to deliver doses as early as July, said Steffen Lang, who oversees the company's manufacturing and supply operations.

Mr. Lang said the company is making tweaks to the plant's production line, hiring more workers and shifting production of other products to make room.

Novartis is speaking with companies about helping with earlier steps in mRNA vaccine production that are tougher to scale, Mr. Lang said. He declined to specify the companies.

Both AstraZeneca PLC, which has a vaccine authorized in the U.K. and several other countries, and Moderna have signed deals with contractors to make their shots.

AstraZeneca, which says it will double output of its Covid-19 vaccine in April despite some recent manufacturing problems, continues to look for ways to expand production. The company recently said it would increase supply through an existing partnership with Germany's IDT Biologika GmbH.

Novavax, which has a vaccine in the late stages of development, doesn't own a manufacturing plant and has to lean on other companies to produce the shot.

Baxter said last month one of its plants in Germany will help fill and finish Novavax's vaccine.

Likewise, End agreed to do similar work, upon the urging of the U.S. government, said Robert Polke, who oversees manufacturing at the company.

Endo only had to make minimal upgrades to its Rochester, Minn., plant, which makes sterile-injectable products and had extra space.

"It's like I have the car, but I may need to put some new tires in the back and put a new air filter in the engine," Mr. Polke said.

More companies may follow. Merck, a pioneer in vaccines, is in discussions with companies about lending its expertise after scrapping two Covid-19 vaccine programs with lackluster results. Teva Pharmaceutical Industries Ltd. is holding similar discussions.

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