The first attempt to mass-produce the experimental vaccine that has played a critical role in halting the coronavirus pandemic in the United States was a complete failure.

Operators at a Pfizer plant outside Kalamazoo hoped the trial run would provide quick validation of the company’s bet on novel mRNA technology. It was also an early test of Pfizer’s strategy of refusing government assistance in order to develop and rapidly ramp up commercial scale production of its vaccine.

However, as soon as pressure began to build in the plant’s array of pumps, tubes, and tanks on Sept. 11, it became clear that something was wrong, Pfizer team leaders told The Washington Post in the most detailed public accounting of the company’s efforts to produce mass quantities of vaccine.

When operators checked a vat at the end of the production run, they discovered that the majority of the key ingredient — the fat molecule encasing the messenger RNA — was missing. Pfizer had evidence that the mRNA vaccine could prevent the virus from spreading. The vaccine is made up of a lipid nanoparticle, which is a sphere of fat molecules encasing a strand of messenger RNA that instructs human cells to make proteins that activate antibodies and prime the immune system against future viral invasions.

However, it would be ineffective unless Pfizer could quickly transition the new nanoparticle technology from the lab to mass production — a feat never before accomplished — and make immunizations available in the United States and around the world. The company and its vaccine partner BioNTech would eventually master the task of mass-producing mRNA vaccines, making it the clear winner among drug companies to emerge from the pandemic. The company is producing more vaccine than any other company and has gained an advantage in the race to use next-generation mRNA technology for the treatment of other diseases.

However, the failure of the September test demonstrates that success was far from certain. Pfizer’s ambitious production goal eventually forced the company to accept government assistance in procuring critical supplies, thwarting its earlier efforts to avoid a closer partnership with federal health authorities.

The company has also been chastised for selling the majority of its early supply to the wealthiest nations, who paid the highest prices, contributing to severe global inequities. Pfizer was able to fulfill President Biden’s order for 500 million doses to be distributed to low-income countries this month, but advocates point out that the pledge is still only a fraction of global need.

Pfizer’s spigot opened wide in March of this year, halting the death toll in the United States and allowing the economy to begin a strong rebound. The breakthroughs are causing an increase in investment in new technology. According to the company, it expects to produce enough vaccine for 3 billion shots in 2021, which is twice as much as initial projections, and enough for 1.5 billion people to receive the two-dose immunization. It has stated that it will sell $26 billion in vaccines in 2021, making it the best-selling medicine in history.

According to an analysis by the investment advisory firm Berenberg Capital Markets, Pfizer and other companies are developing mRNA vaccine pipelines for influenza, HIV, tuberculosis, rabies, rotavirus, malaria, and Zika. BioNTech and Moderna are conducting extensive research into the use of mRNA in the treatment of cancer.

Pfizer had announced a collaboration with the German biotech company BioNTech three days earlier to develop and manufacture an experimental coronavirus vaccine. BioNTech had already determined how it would create an mRNA vaccine to combat coronavirus, but it required a large production partner with engineering and distribution expertise to produce vaccine on a global scale.

Pfizer acquired UpJohn’s 1,300-acre manufacturing campus south of town, past the county airport, in 2003 through mergers. It employs 2,800 people and primarily manufactures generic drugs and bulk pharmaceutical ingredients ranging from birth control injections to coral snake antivenom. It is one of the largest manufacturers of steroids in the world. The building where most of the coronavirus vaccine production happens is a quarter-mile long.