By Jared S. Hopkins and Matt Grossman

Pfizer Inc. said it expects its Covid-19 vaccine to generate about $15 billion in sales this year, making the shot one of the drugmaker's top-selling products and potentially a steady source of revenue.

"Based on what we've seen so far, we believe it is increasingly likely that a durable Covid-19 vaccine revenue stream, like it happens in flu, is a potential outcome," Pfizer Chief Executive Albert Bourla told analysts Tuesday.

The company said it expects total revenue this year of between $59.4 billion and $61.4 billion.

In the fourth quarter, the Covid-19 vaccine contributed $154 million of the company's $11.7 billion in sales. The U.S. Food and Drug Administration authorized the shot -- the first to be cleared for use in the U.S. -- on Dec. 11. Revenue in the quarter from Pfizer's vaccines overall rose 17% to $2 billion.

Pfizer's sales outlook for its Covid-19 vaccine, which it co-developed with BioNTech SE, opens a window on the business opportunity for drugmakers responding to the coronavirus pandemic. Companies including Gilead Sciences Inc., which sells an antiviral, have reported sales bumps from coronavirus treatments.

It is unclear, however, how long Pfizer, which splits Covid-19 vaccine sales with BioNTech, could sustain the revenue at such a high level, according to analysts.

Other companies' vaccine candidates could be authorized in the coming months, eating into Pfizer's sales. Sales could also be hurt if a variant emerges that evades the company's vaccine.

The Covid-19 vaccine's longer-term revenue prospects, Mr. Bourla said, depend on whether people will need booster shots to stay protected against the virus.

Pfizer and BioNTech are working on a booster shot to provide additional protection against variants.

Pfizer on Tuesday reported a fourth-quarter profit of $594 million, or 10 cents a share, compared with a year-earlier loss of $337 million, or 6 cents a share. Excluding one-time items, Pfizer posted an adjusted profit of 42 cents a share; analysts were expecting 50 cents a share.

For 2021, the company expects an adjusted profit of $3.10 to $3.20 a share, an increase from $3 to $3.10 a share.

Mr. Bourla said in an interview that the company raised its revenue projections for the year amid the rollout of AstraZeneca PLC's vaccine in Europe, study results from Johnson & Johnson's vaccine and insight into manufacturing capabilities for those shots.

The company's shares were down 2.3% in afternoon trading on the New York Stock Exchange.

Pfizer and BioNTech have said recent lab results suggested new coronavirus variants found in the U.K. and in South Africa would have only a small impact on their shot's effectiveness -- though the study did find antibodies generated by the vaccine were slightly less effective against mutations in the variant identified in South Africa.

Pfizer will study whether booster shots given either six months or a year after the second dose could provide additional protection, including against variants, said Pfizer research-and-development chief, Mikael Dolsten. Results could come as early as summer, he said.

A third shot might generate sufficient antibodies to protect against the current variants Dr. Dolsten said.

Pfizer is also preparing, he said, a tweaked vaccine for either the strains found in Brazil or South Africa that could be tested if needed, though Pfizer hasn't decided on whether to start a trial.

Mr. Bourla said the goal would be to test a new vaccine in 100 days.

Other vaccine makers, including Johnson & Johnson and Moderna Inc., have said they are working on a booster shot or modified vaccine after testing found their vaccines weren't as effective against the South Africa variant, in particular.

Still, Pfizer has said it expects global demand for vaccinations against Covid-19 to last at least several years beyond 2022. Pfizer plans to produce two billion doses world-wide this year. Authorities in roughly 50 countries have cleared use of the shots.

Pfizer has agreed to provide the U.S. government 200 million doses by the end of July for nearly $4 billion, or $19.50 per dose. Pfizer now expects to complete delivery by the end of May because regulators agreed the company can count six doses per vial, up from five. Pfizer said Tuesday it has supplied 29 million doses to the U.S. and 65 million doses globally.

Pfizer struck agreements with rivals such as Novartis AG to help manufacture doses during the pandemic. It has also been upgrading manufacturing plants to meet demand, and doubling the number of doses produced in each batch, Chief Financial Officer Frank D'Amelio said in an interview.

"When we started, this equipment didn't exist," Mr. D'Amelio said. "We literally helped design the equipment with the people that make the equipment."

Pfizer's quarterly revenue of $11.7 billion was up 12% from $10.45 billion in the year-earlier quarter. Analysts surveyed by FactSet were expecting revenue of $11.48 billion in the latest period.

Revenue for its cancer drugs grew by nearly one-quarter to more than $3 billion, as sales grew for products such as breast-cancer treatment Ibrance. It also saw sales growth in rare disease drug Vyndaqel and blood thinner Eliquis.

During the quarter, Pfizer completed a deal to spin off its Upjohn unit in a combination with Mylan NV, forming a new generic-drug maker called Viatris Inc.

Write to Jared S. Hopkins at jared.hopkins@wsj.com and Matt Grossman at matt.grossman@wsj.com

(END) Dow Jones Newswires

02-02-21 1521ET