Novavax Inc. slashed its full-year revenue outlook in half on Monday, citing a lack of demand for its Covid-19 vaccine from an international initiative to vaccinate lower-income countries and delays in winning expanded authorizations in the U.S.

Novavax said Monday it now expects 2022 sales between $2 billion and $2.3 billion, down from its previous forecast of $4 billion to $5 billion. 

The new guidance reflects an unexpected shortfall in demand for Covid-19 vaccines, Novavax Chief Executive Stanley Erck said in an interview. Covax, the international vaccine initiative, has backed away from an order of 350 million doses of the Novavax vaccine because of a glut of other vaccines it has received from other companies, he said.

Gavi, the Vaccine Alliance, a nonprofit organization that helps run Covax, didn’t immediately respond to request for comment.

Moderna Inc. said earlier this month that its second-quarter profits were hurt by write-offs and expenses related to lower demand from Covax.

Gavi said at the time: “Global supply is currently greater than global demand. Covax is working with all of its suppliers to ensure that the size and frequency of shipments reflects country demand, while also working to support countries with delivery.”

Demand is also uncertain in the U.S., where the federal government has the right to purchase 110 million doses, Mr. Erck said.

Novavax has already shipped an initial 3.2 million doses. Talks are continuing with the U.S. government about further purchases, but the talks are “not mature enough for me to be able to predict where they’re going to go, to tell you the truth,” Mr. Erck said.

The less-than-expected demand is the latest setback for Novavax, of Gaithersburg, Md. The company faced multiple delays in advancing its Covid-19 vaccine.

The company has expressed hope the shot would appeal to people reluctant to take other vaccines, such as those made by Moderna Inc. and by Pfizer Inc. and partner BioNTech SE, that use the newer messenger RNA technology.

Novavax’s shots, by contrast, uses an engineered version of the spike protein that sits on top of the coronavirus to trigger an immune response and build up a defense if the person is later exposed to the virus.

In the second quarter, Novavax reported sales of $185.9 million, well short of the average analyst forecast of $1.02 billion, according to FactSet. The company reported a net loss of $510.5 million, compared with a net loss of $352.3 million in the year-earlier quarter.

The second-quarter sales shortfall was driven in part by the timing of when Novavax recorded sales for the shipment of vaccines to Europe and elsewhere, Mr. Erck said. The company had shipped vaccine doses worth $600 to $700 million into Europe, but the process for having the vaccines released and booked as revenue took longer than expected, he said.

Novavax booked $400 million in sales from the international shipments in July that the company expected to book in the second-quarter, Mr. Erck said.

Novavax won U.S. authorization for its Covid-19 vaccine in July in adults 18 and older, and recommended by the Centers for Disease Control and Prevention for use in people who haven’t been previously vaccinated.

Novavax is awaiting expanded authorizations that would widen the population eligible to take its vaccine to adolescents and as a booster shot after a person has received their primary vaccination of two doses, Mr. Erck said.

“While maybe we have not seized the moment in ‘22, it is a foundation laying opportunity for the company and I think we’re looking towards 2023 and beyond,” Novavax Chief Commercial Officer John Trizzino said in an interview.

Mr. Erck said that it has become clear in recent weeks during discussions with Gavi that the initiative wouldn’t be ordering any doses this year.

Write to Joseph Walker at joseph.walker@wsj.com