The U.S. has purchased 200 million additional Covid-19 vaccine doses from Pfizer Inc. and partner BioNTech SE, at a higher price than previous deals.

The U.S. is paying about $24 a dose, according to Pfizer, up from the $19.50 that the government paid in its earlier deals to supply the country. The deal brings the total purchased from the U.S. to 1 billion doses.

The companies said they expect to deliver 110 million of the additional doses by the end of the year, with the rest by the end of next April. Pfizer said the U.S. has the option to acquire an updated version of the vaccine targeting variants, should they be available and authorized by regulators. Pfizer is testing one targeting the Beta variant, which was first identified in South Africa.

“The price for this order accounts for the additional investment necessary to produce, package and deliver new formulations of the vaccine, as well as the increased cost associated with delivering the vaccine in smaller pack sizes to facilitate delivery at individual provider offices, including pediatricians,” a Pfizer spokesman said in a statement.

White House press secretary Jen Psaki confirmed that the Biden administration’s purchase could be used to inoculate children or serve as booster shots should health regulators deem them necessary.

“We’ve always prepared for every scenario,” Ms. Psaki said, noting that the additional shots will be delivered between the upcoming fall and the spring of 2022.

Officials from the Food and Drug Administration and Centers for Disease Control and Prevention have yet to say that booster shots are necessary. Pfizer said earlier this month it would seek clearance from U.S. regulators to distribute a booster shot of its Covid-19 vaccine.

More than 86 million people in the U.S. ages 12 and up have received the two-dose shot, the most among the three shots cleared by regulators for use, according to federal figures.

While demand has largely stalled in recent weeks, additional demand could open up if the vaccine is cleared for use in younger children. Pfizer is testing the vaccine in children under 12 years old, and has said it hopes to begin having data available before the end of the year.

The new agreement is in addition to the one announced last month for 500 million doses that the U.S. is scheduled to donate to the rest of the world. All of those doses, which were bought at a not-for-profit cost, will be funneled through Covax, the global initiative to help vaccinate developing countries, and they will go to 92 lower-income countries and the African Union.

The U.S. first agreed to buy vaccines from Pfizer about a year ago while the shot was still under evaluation in clinical trials. Authorized for use in December on an emergency basis, the two-dose shot has been widely given in the U.S. In May, the FDA expanded authorization to children as young as 12 years old.

—Andrew Restuccia contributed to this article.