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Pfizer is racing ahead with plans to manufacture 50 million to 100 million doses of a new omicron-specific version of its coronavirus vaccine, a reflection of rising concerns that current vaccine formulations may need to be tweaked for the new threat.

Pfizer also is testing hybrid combinations of vaccine to target multiple coronavirus forms, as well as larger doses.

The omicron-specific doses will be created "at risk," CEO Albert Bourla said Monday, meaning that if they are not needed, Pfizer will absorb the costs. The company has climbed to the lead in global vaccine production with 3 billion doses in 2021 and is planning to produce up to 4 billion doses in 2022.

If it turns out to be necessary to roll out an omicron-targeting vaccine, Pfizer will be ready, Bourla said.

"In terms of manufacturing, we have so big of a capacity built right now that it won't be an issue to switch immediately," Bourla said.

Bourla disclosed the manufacturing plans at an annual health-care conference sponsored by JPMorgan Chase but did not provide a number for doses involved. A Pfizer spokesman, Steven Danehy, said in an email Tuesday that "we hope to have 50-100 million doses of the omicron specific vaccine available by late March/early April."

Rapid development and manufacturing changes are made possible by the new mRNA vaccine technology deployed by Moderna and by Pfizer, with German partner BioNTech. The companies have said it takes about 90 days from genetic sequencing of a new threat to producing a new lipid nanoparticle vaccine containing the updated mRNA payload, which is an extraordinarily fast time frame for vaccine development.

Pfizer on Monday announced a licensing deal with a San Diego company, Codex DNA, that has a synthetic DNA manufacturing process that should cut the development time even further, to just 60 days, Bourla said.

The coronavirus is proving to be a formidable foe, Bourla added, with immunity from vaccines and natural infection waning quickly. Annual boosters may be required for the next decade, he said.

Moderna is also testing different vaccine formulations, including a hybrid shot that would combine a flu vaccine and a coronavirus vaccine, Moderna CEO Stéphane Bancel said at the conference. Moderna is striving to have an omicron-targeting shot available by the fall, in preparation for a likely coronavirus surge next winter, he said. The company is trying to create a single-dose, annual shot that will address multiple respiratory viruses, he said, "a franchise that will evolve every year."

But as drug manufacturers scramble to keep up with waning immunity and mutations, the demand for boosters and, potentially, variant-targeting vaccines and hybrids is causing alarm among global health authorities and advocates.

Citing inequitable distribution patterns over the past year, they say most of the additional shots will be reserved by wealthy nations and further delay the time Africa and developing countries elsewhere get their populations substantially vaccinated.

Israel has authorized fourth shots for people who are elderly or have compromised immune systems. The Centers for Disease Control and Prevention also has cleared immunocompromised Americans for a fourth shot.

As of this week, the African nation of Cameroon had administered 3,508 doses per 100,000 people, according to tracking data from Johns Hopkins University. The United States had given 157,000 shots per 100,000.

"The rest of the world is getting by on table scraps," said Robbie Silverman, senior manager of private-sector advocacy at Oxfam.

Some Democrats in Congress have called on drug companies to open up their technology to vaccine manufacturers in the developing world to speed up global access. Pfizer and Moderna have resisted those demands, saying open technology sharing is unlikely to work, because of supply chain constraints and the complexity of the mRNA manufacturing process.

The companies have cited their growing efforts to provide greater volumes of vaccine at a discount for distribution in low-income countries. The challenge for industry will be creating boosters and variant-targeting vaccines in the enormous volumes required this year and possibly annually into the future.

Airfinity, a British consulting firm that closely tracks vaccine data, has estimated switching 50% of capacity for all major world vaccine manufacturers, including China, to a variant-specific shot would reduce production in the first six months of 2022 to 5.2 billion from 8.6 billion under the current production rate of existing vaccines.

White House coronavirus response adviser Anthony Fauci said last month that a variant-targeting vaccine was not required, because boosters were proving to be sufficient in preventing hospitalizations and deaths. The White House and the Department of Health and Human Services did not respond to requests for comment on Pfizer's latest moves.

If a switch is required, the Food and Drug Administration will probably accept limited effectiveness testing instead of massive population-scale clinical trials of 30,000 people (which were required to prove efficacy initially in 2020), said John Grabenstein, who is scientific director at the Immunization Action Coalition, a clearinghouse of science-based vaccine knowledge, and is a former global director of medical affairs for Merck vaccines.

Such a streamlined system is already in place for annual updates to flu vaccines, he said. The pivotal question will be whether tweaking the vaccines for omicron is worth disrupting the existing manufacturing pipeline and exacerbating global inequities and debate over vaccine nationalism, he said.

"It's taken over a year now to get to the current production levels globally," Grabenstein said. "They're just now at the hundreds of millions of doses per month, and even at that incredible pace there isn't enough vaccine getting into Africa."