Moderna Inc. has encountered supply challenges for its Covid-19 vaccine, resulting in lower-than-expected sales for the latest quarter and a reduced forecast of dose deliveries and sales for the full year.

The Cambridge, Mass., company cited the complexities of shipping its vaccine doses to more countries, compared with delivering the vaccine during the initial rollout in the U.S. and a handful of other countries.

Moderna...

Moderna Inc. has encountered supply challenges for its Covid-19 vaccine, resulting in lower-than-expected sales for the latest quarter and a reduced forecast of dose deliveries and sales for the full year.

The Cambridge, Mass., company cited the complexities of shipping its vaccine doses to more countries, compared with delivering the vaccine during the initial rollout in the U.S. and a handful of other countries.

Moderna said it is adding workers and taking other steps to improve the supply situation. “We really believe we can fix those short-term supply issues,” Chief Executive Officer Stephane Bancel said on a conference call with analysts Thursday. “Those are what I would call teething problems of scaling up so fast.”

Moderna delivered 510 million doses of its Covid-19 vaccine globally for the first nine months of 2021. For the full-year, Moderna now expects to deliver between 700 million and 800 million doses, down from its previous forecast of 800 million to one billion.

Moderna Inc. lowered its sales outlook for the full year and said that some planned Covid-19 vaccine deliveries will slip into early 2022. The drugmaker now expects product sales of between $15 billion and $18 billion in 2021.

Moderna is reaping a high profit margin from its vaccine sales. The cost of producing and selling its vaccine, including royalty payments, accounts for only about 15% to 16% of total product sales, with most of the rest of its revenue falling to the bottom line. In August, the company said it had signed agreements for sales of $20 billion.

Now, some of those shots will likely be delivered early next year instead, Moderna said, citing longer delivery lead times for international shipments and other factors affecting the company’s production growth. It added that it has been giving priority to deliveries to low-income countries, where the company is charging lower prices.

Moderna’s larger rival, Pfizer Inc., has posted higher numbers for the Covid-19 vaccine it sells with partner BioNTech SE. Pfizer booked $24.3 billion in sales of its Covid-19 vaccine, named Comirnaty, for the first nine months of this year. Pfizer predicted full-year Comirnaty sales would be $36 billion, reflecting expected delivery of 2.3 billion doses for the year.

Moderna has expanded rapidly over the last 18 months as it developed and brought to market one of the first effective vaccines against the pandemic disease. The pharma company said it sold 208 million doses of its Covid-19 vaccine in the latest quarter.

Its quarterly revenue through September of $4.97 billion fueled a $3.33 billion profit, or earnings of $7.70 a share. Those metrics soared above last year’s third-quarter levels, before Moderna’s Covid-19 shot had been approved, when revenue was $157 million and the company recorded a $233 million loss.

Sarasota Hospital workers wait for medical clearance after receiving a dose of the Moderna Covid-19 vaccine in Florida.

Photo: shannon stapleton/Reuters

But the latest results fell short of Wall Street forecasts. Analysts surveyed by FactSet had predicted sales of $6.2 billion and adjusted earnings of $9.09 a share.

Moderna’s stock price dropped 17% to $287.08 in Thursday morning trading. Through Wednesday, Moderna’s stock price had more than tripled in value this year.

In the U.S., more than 70 million people have received Moderna’s two-dose Covid-19 vaccination. That accounts for about 36% of the Americans who are considered fully vaccinated, according to the Centers for Diseases Control and Prevention. Another 6.1 million Moderna shots have been used as boosters in the U.S.

More recently, Moderna’s efforts to roll out vaccines for young people have fallen behind the pace of Pfizer Inc. and BioNTech S.E., the makers of the most-used shot in the U.S. Moderna has asked the Food and Drug Administration to authorize its vaccine for use in 12- to 17-year-olds, but the decision has been delayed as the agency evaluates whether the shots raise the risk of a rare inflammatory heart condition in young men.

Moderna said Thursday that an analysis of its global safety database shows no increased risk of myocarditis among about 1.5 million children under the age of 18 who received its vaccine, primarily in other countries where its vaccine is authorized for that age group. Moderna said the only subgroup showing an increased risk of myocarditis is men ages 18 to 24.

“We continue to believe the benefits of vaccination continue to outweigh the risks,” Moderna President Stephen Hoge said on the earnings call.

Meanwhile, the Pfizer-BioNTech vaccine has already been authorized for use in children as young as 5.

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Moderna’s quarterly sales rose 14% over the second quarter, continuing the company’s ascent in response to the Covid-19 pandemic. A year ago, its Covid-19 shot was still in clinical trials, it hadn’t created any approved vaccines and quarterly revenue had never topped $160 million.

The company forecast that in 2022, sales could range from $17 billion to $22 billion. Moderna said it has already signed advanced-purchase orders worth $17 billion for deliveries next year.

Vaccines remain in high demand in countries that are receiving initial supplies, and for booster shots for people already fully vaccinated. Last month, the FDA approved the use of a Moderna booster shot. Boosters from Pfizer and Johnson & Johnson have also been approved.

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People who were initially vaccinated with a Pfizer-BioNTech or Moderna vaccine can get any booster at least six months after their second dose so long as they are seniors or at high risk because of underlying medical conditions or their workplace.

Moderna said that commercial sales of booster shots for the U.S. next fall could bring in $2 billion of revenue.

Write to Peter Loftus at peter.loftus@wsj.com and Matt Grossman at matt.grossman@wsj.com