Vaccine manufacturers are struggling to secure supplies of giant plastic bags used in bioreactors that mix pharmaceutical ingredients, creating a bottleneck that threatens the rollout of Covid-19 shots around the world.

Some vaccine makers have been days away from stalling production because of the shortage of the bags, which can hold up to 2,000 litres of material, according to three people familiar with the matter.

Covid-19 jabs developed by companies including BioNTech/Pfizer, Moderna and Novavax are made in the bags — which are used as sterile liners in the tanks where the vaccines are produced — although they use differing sizes.

Stanley Erck, chief executive of Novavax, called on companies not to hoard supplies of the bags, adding that it had been “really complicated” to work around the shortages.

“We just had a minor breakthrough where we thought we were going to be out of bags in one particular facility. And we just got enough to get through February, March and April,” he told the Financial Times. “Otherwise, [the factory] would have shut down.”

MilliporeSigma, a division of Germany’s Merck, is a main supplier of the bags worldwide, which are designed to fit into the company’s proprietary Mobius mixing machines.

One person familiar with the matter said Millipore was warned last summer that it needed to increase supplies significantly.

MilliporeSigma said there had been “unprecedented demand” for Covid-19 related products, and that it had been working around the clock to expand facilities, prioritise customers working on pandemic products and find additional raw material suppliers.

“We have significantly expanded our production capabilities in Danvers, Massachusetts . . . for global distribution of this product used in vaccine manufacturing,” the company said.

A person familiar with the company said it had done its best to scale-up complex manufacturing while maintaining quality standards. They added that MilliporeSigma was also reliant on a web of smaller suppliers who were scaling-up at speed.

Thermo Fisher, which also makes the single use liners, said it had increased production capacity by 50 per cent in 2020 and anticipated expanding it by another 50 per cent in 2021.

Pfizer and Moderna did not respond to a request for comment.

Any small hiccups in the supply chain can have significant repercussions, given that vaccine manufacturers are working round the clock to produce as many shots as possible. Vaccines have never been manufactured on such a large scale in such a short period.

Mixing can be done using stainless steel containers, rather than bags, one industry insider told the Financial Times, but altering production methods would cause a delay to deliveries because regulatory authorities must approve such changes.

The shortage of the bags follows other supply chain problems, such as a challenge obtaining enough lipid nanoparticles — which the mRNA vaccine makers need to deliver genetic code into the body — and securing the right kind of syringes to extract as many doses as possible from a vial.

Germany’s Merck, which is also one of the few lipid producers, recently expanded production to meet demand from Mainz-based BioNTech.

Jesse Goodman, former chief scientist at the US Food and Drug Administration and a professor at Georgetown University, said governments and producers had to work together to optimise supply chains, warning that any problems could also affect other vaccines and therapeutics.

“There is concern that this could impact scale-up and ability of manufacturers, including to make doses for global use, for example through Covax,” he said, referring to the global initiative to supply vaccines to developing countries. “It merits immediate attention.”