At a plant nestled along a highway 20 miles north of Boston, hundreds of Pfizer workers are gearing up to produce millions of doses of a new vaccine that looks more and more like the next phase of fighting Covid-19.

       Work on the project started the day after Thanksgiving at the 70-acre facility in Andover, Massachusetts, just as the World Health Organization designated a new coronavirus strain, Omicron, a variant of concern. The goal of the effort: make a booster shot customised against the highly mutated virus in less than 100 days.

       Researchers are alarmed by some 30 mutations in Omicron’s spike, the protein that facilitates coronavirus’s entry to cells. That’s prompted Pfizer, its partner BioNTech SE and their messenger RNA rival Moderna Inc. to start crash efforts to target it directly.

       “It was the list of mutations we never wanted to see,” said Moderna President Stephen Hoge, who heads the company’s scientific operations. The vaccine maker, whose mRNA factory stands just 40 miles from Pfizer’s, started working on Omicron the Tuesday before Thanksgiving, and meetings ran straight through the holiday. Many employees “had their Thanksgivings ruined” by omicron, Hoge said.

       While the companies are tight-lipped about the details of exactly where they stand, both are bent on a fast response. When news of the variant emerged from South Africa, Pfizer CEO Albert Bourla decided almost immediately to begin large-scale manufacturing of an Omicron-specific shot. The work ramped up so quickly that Pfizer hasn’t tallied its costs.

       “I couldn’t give you the number right now; I’m not even sure we talked about it,” said Mike McDermott, Pfizer’s global supply chain head, who gives Bourla regular updates on progress.