The study could be a significant milestone for Moscow in the global vaccination race, potentially offering President

Vladimir Putin’s

government geopolitical clout in the developing world and the chance to tap into the lucrative global vaccine market. Russia—the world’s fourth worst-hit country with nearly four million cases—has also banked on Sputnik V to avoid new costly lockdowns as authorities plan to vaccinate 60% of the domestic population by the end of the year.

The shot, which was approved by Russian authorities in August before undergoing large-scale clinical trials, has stirred questions in light of its fast-tracked development and lack of published trial data. So far, Sputnik V has been administered to more than two million people world-wide, including in Argentina, Serbia and Algeria, according to Russian authorities.

Tuesday’s results could help clear doubts surrounding the Russian shot.

“The development of the Sputnik V vaccine has been criticized for unseemly haste, corner cutting, and an absence of transparency,” virology professors Ian Jones at the U.K.’s University of Reading and Polly Roy at the London School of Hygiene & Tropical Medicine wrote in the Lancet. “But the outcome reported here is clear and the scientific principle of vaccination demonstrated, which means another vaccine can now join the fight to reduce the incidence of Covid-19.”

Alexander Gintsburg,

the head of the vaccine’s developer, the Moscow-based Gamaleya Institute, said that the data demonstrates Sputnik V’s safety and high efficacy against the virus.

This “is a great success in the global battle against the Covid-19 pandemic,” he said.

Sputnik V’s efficacy rate compares to vaccines developed by

Moderna Inc.

and

Pfizer Inc.

and its German partner

BioNTech SE,

which are around 95% effective.

The Lancet study didn’t address the shot’s usefulness against new variants of the virus, amid some early evidence suggesting strains may prove resistant to current vaccines. Russian officials said on Tuesday that they are continuously testing Sputnik V against new variants and they expect the shot to achieve the same level of efficacy. They also expect it to provide long-term immunity of as long as two years, based on early experimental evidence.

The results published on Tuesday were based on an interim analysis of a Phase 3 trial of nearly 20,000 participants, three-quarters of whom received the vaccine while the rest received a placebo. The analysis was based on a total of 78 confirmed Covid-19 cases, 62 of which were identified in the placebo group and 16 in the vaccine group. The clinical trial, totaling 40,000 volunteers, is ongoing.

Researchers found that the Covid-19 vaccine didn’t produce serious adverse reactions, the Lancet paper said. Most side effects included flulike symptoms, pain at the injection site and headaches.

Among the elderly, the vaccine was well tolerated and demonstrated an efficacy of 91.8%, based on a group of 2,144 volunteers older than 60, the paper said.

Like other Covid-19 vaccines, including ones developed by

Johnson & Johnson

and

AstraZeneca

PLC and Oxford University, Sputnik V uses a so-called viral vector approach. It introduces a genetically altered form of a harmless virus, known as the adenovirus, to serve as a vehicle—or vector—for a fragment of genetic material from the coronavirus.

Each of the vaccine’s two shots is based on a different adenovirus vector, which Russian scientists say achieves a stronger immune response. Sputnik V has simpler logistics requirements compared with some of its peers, with a storage and transportation temperature of between 36 and 46 degrees Fahrenheit. The Pfizer vaccine must be kept at minus 94 degrees Fahrenheit before thawing.

With Sputnik V—a reference to the satellite the Soviet Union launched into orbit ahead of the U.S. in the Cold War space race—Russia could gain clout with some countries, analysts say, as well as participate in a global coronavirus vaccine market estimated by Russian officials at $100 billion annually.

Competing on price, Russia is selling the vaccine at less than $10 a dose, lower than Pfizer and Moderna, and is targeting up to 30% market share among Covid-19 shots in the countries buying Sputnik, according to Russian officials.

AstraZeneca has said that it would test whether a combination of its Covid-19 vaccine, which has shown to be between 62% and 90% effective depending on dosage, and Sputnik V can boost efficacy. Clinical trials for a combined shot are expected to start soon in Azerbaijan, the United Arab Emirates and other countries.

Some 15 countries outside Russia have already authorized Sputnik V, and Moscow has received orders or expressions of interest for 2.4 billion doses, including from Brazil, Mexico and India. In a bid to accelerate the global rollout, Russia will also offer a one-dose vaccine, dubbed Sputnik Light, which Russian authorities say would be between 73% and 85% effective.

To produce its vaccine, Russia relies on a global supply chain, including manufacturing hubs in Brazil, South Korea, India and China. Russia has also mounted an aggressive public-relations campaign abroad, including posting weekly video updates in English and maintaining a Twitter account for Sputnik V.

Sputnik V hasn’t been approved by Western health authorities or received authorization from the World Health Organization, which many developing countries rely on for vetting vaccines. Russia is in talks with the European Medicines Agency about approving the shot in the European Union and has applied for WHO authorization.

In Iran, health-care professionals and lawmakers criticized the government’s announcement that it would import Sputnik V, saying that the vaccine had not been approved by international bodies and that the purchase was politically motivated. Government officials said on Tuesday that Tehran would buy up to 1.5 million doses, with the first batch arriving as early as Saturday.

The domestic rollout has also faced challenges, including production delays and a skeptical populace.

Authorities have recently said that manufacturing is now being ramped up following initial equipment problems. They now expect to produce 11 million doses this month, up from seven million in January.

Around 46% of Russians said they would get a vaccine in a January survey by British polling firm Ipsos MORI, up 5 percentage points compared with December. Still, Russians were among the most reluctant to get inoculated globally, compared with 55% in France, 63% in the U.S. and 86% in the U.K.

Russia doesn’t publish daily vaccination rates, but regional data shows that at least 1.3 million Russians have received a dose so far.

Irina Levashova, a kindergarten teacher in Romodanovo, a small town some 400 miles southeast of Moscow, received her second shot last month along with her husband.

“I have many acquaintances who have been ill or even died from this disease, so I wanted to protect myself and my family,” Ms. Levashova, 58, said, adding that she didn’t experience any major side effects. “As soon as they started talking about vaccinations, I immediately told myself that me and my family would do it.”

—Valentina Ochirova contributed to this article

Write to Georgi Kantchev at georgi.kantchev@wsj.com