NORWOOD, Mass.— Moderna Inc. is adding two new production lines at the rebuilt former Polaroid plant where it manufactures its Covid-19 vaccine, part of a push to prepare for making booster shots and the future of the pandemic.

At a site brimming with new steel production tanks and heavy equipment, construction workers in neon safety vests are working to get one new line up and running by fall and the other by early 2022.

The additions will help Moderna increase overall production capacity by 50% at its plant in the Boston suburb of Norwood, company officials said.

Moderna and its manufacturing partners also are expanding production capacity outside the U.S., with a goal to roughly triple the annual global output of Covid-19 vaccine doses to about 3 billion in 2022 from as many as 1 billion this year.

“Our plan and our hope is that, as soon as the U.S. has enough doses, we’re allowed to export so we can help as many countries as we can around the world,” Moderna Chief Executive Stéphane Bancel said.

Moderna is expanding production capacity at its main plant in Norwood, Mass., by 50% by early 2022.

Moderna is expanding production capacity at its main plant in Norwood, Mass., by 50% by early 2022.

Analysts expect the company’s Covid-19 vaccine sales will top $17.1 billion this year.

Analysts expect the company’s Covid-19 vaccine sales will top $17.1 billion this year.

The manufacturing expansion shows that Moderna is making a big bet on an enduring Covid-19 vaccine business as health authorities prepare for life after the pandemic lifts but the virus remains.

Wall Street analysts say sales of Moderna’s Covid-19 vaccine could stay strong for a couple of years but then drop off. The mean estimates of analysts surveyed by FactSet are that Moderna’s Covid-19 vaccine sales will top $17.1 billion this year, dip slightly to $16.8 billion in 2022 and come in at $7.4 billion in 2023.

The flurry of activity, at a site with a soundtrack of heavy equipment and walkie-talkies, is also emblematic of Moderna’s dramatic transformation.

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The decade-old startup had no commercial products and was struggling to persuade many investors that it could make messenger RNA technology work before the coronavirus pandemic hit last year.

Now, it is a major biotechnology industry player as a result of the effective Covid-19 vaccine that it quickly developed last year. Moderna has a market capitalization of $81 billion, up from about $7 billion before the pandemic.

The company has taken over its entire 10-story headquarters building about 15 miles north of Norwood, in Cambridge, Mass., which Moderna previously shared with other tenants.

Companies are working on coronavirus booster shots, as some early studies suggest antibody levels against Covid-19 wane with time, making boosters more necessary. We explore what that means for individual consumers. Illustration: Laura Kammermann/The Wall Street Journal The Wall Street Journal Interactive Edition

Also, it has opened offices in other countries including Switzerland, where Moderna works with a contract manufacturer to make doses, plus a back-office support hub in Poland.

“The build has been like nothing I’ve ever experienced, to go as fast as we did,” said Matt Barrows, Moderna’s senior director of manufacturing with more than 20 years of experience in biotech. “The build was the journey and continues to be the journey.”

The expansion hasn’t been without hiccups. Moderna’s initial output was limited, as the company scrambled to build capacity and expertise.

Moderna ripped apart and rebuilt a portion of the Norwood plant and constructed an addition to support large-scale manufacturing of its vaccine.

Moderna ripped apart and rebuilt a portion of the Norwood plant and constructed an addition to support large-scale manufacturing of its vaccine.

The company’s goal is to roughly triple its annual global output of vaccine doses to about 3 billion in 2022.

The company’s goal is to roughly triple its annual global output of vaccine doses to about 3 billion in 2022.

Moderna recently cut supplies to the U.K., Canada and other countries outside the U.S. because a contract manufacturer in Europe underestimated how many workers it needed to hire.

Additional setbacks could keep Moderna from meeting its production goals. Scott Nickerson, head of U.S. manufacturing at Moderna, said the latest challenge is finding enough raw materials used in the production process, especially plastic bags, tubes and filters.

The latest expansion will help Moderna boost its overall supply of Covid-19 vaccine doses to the U.S. and other countries, company officials said.

The company said earlier this week it had agreed to provide the U.S. an additional 200 million doses starting later this year, on top of the 300 million Moderna expects it will have delivered for U.S. use by the end of July.

Outside the U.S., Moderna has supply deals with the European Union and the international Covax initiative to provide doses to low- and middle-income countries.

Mr. Bancel said the company has received inquiries from governments in Asia, Africa and Latin America seeking supplies of Covid-19 vaccines.

The company expects the additional output of doses to include vaccines intended as booster shots that might be needed to prolong immunity and better target new coronavirus variants.

Some of the new supply may also be lower-dose shots intended for young children, if clinical data support their use.

After deciding to pursue a Covid-19 vaccine last year, Moderna quickly made changes to its original manufacturing plant, the former Polaroid plant located in an industrial park south of Boston that the biotech originally opened in 2018.

The company ripped apart and rebuilt a portion of the plant and built an addition to support large-scale manufacturing of the Covid-19 vaccine, according to Mr. Nickerson.

That work yielded the three main production lines that have churned out most of the more than 200 million doses Moderna has delivered for use in the U.S. since regulators authorized its use in December.

For the expansion, Moderna has already brought in some of the steel tanks, white plastic drums, tubes and other equipment needed for production, waiting for finishing touches and validation.

The company currently makes between 40 million to 50 million doses a month in the U.S., most of which is made at the Norwood plant, but some of which is made at a contractor’s plant in New Hampshire. Moderna declined to break down the production or specify the expected monthly dose output after the expansion.

Across a parking lot from the plant, Moderna has converted a former auto-mechanic school into technology-development laboratories where workers in white lab coats and safety goggles hone manufacturing techniques that can be transferred to the production sites.

Down a path through the woods is an office building, formerly used by employee-benefits consultant Mercer, which Moderna is now converting into quality-control labs and a site for manufacturing doses for clinical testing.

Moderna has been on a hiring spree. Last year, the company added nearly 500 employees, boosting its workforce to 1,300. Mr. Nickerson said it is “in the middle of hiring that many more all over again.”

The new hires are a mix of people with experience in biotech manufacturing as well as fresh college graduates with engineering and chemistry degrees from nearby universities including Northeastern University and Worcester Polytechnic Institute, Mr. Barrows said.

Also to support the expansion, Moderna is firming up relationships with material vendors and manufacturing partners, company officials said.

The company has in recent weeks signed new or expanded contracts with manufacturers including Lonza Ltd. and Samsung Biologics Co. , and raw-material suppliers including Aldevron LLC. which supplies the starter genetic material template for vaccine production.

Moderna is converting another building in Norwood into quality-control labs and a site for manufacturing doses for clinical testing.

Moderna is converting another building in Norwood into quality-control labs and a site for manufacturing doses for clinical testing.

Write to Peter Loftus at peter.loftus@wsj.com