For months, Russian President Vladimir Putin has trumpeted his country’s Sputnik V COVID-19 vaccine as an answer to the world’s severe shortfall of doses, especially in the European Union, whose 27 nations have lagged badly in rolling out vaccines. And on Monday, the Russian leader received a major boost for the vaccine, when India approved it for use for its 1.4 billion people.

But that hardly means Russia has a smooth ride to acceptance for its vaccine—the very first to be released into the world, way back in September. Sputnik V’s developers claim a 91% efficacy rate, a figure it pushes heavily on its Twitter, Facebook, and Instagram sites. And yet, a mix of fraught political tensions between the West and Russia, as well as deep suspicions over the medical claims, are unlikely to vanish anytime soon.

India’s approval of Sputnik V came just as the world hit a major landmark: 1 billion COVID-19 vaccine doses have been produced globally, according to Airfinity, a London-based science information and analytics company.

However, while its backers are promoting Sputnik V as crucial to the deployment of future billions of doses, they still have to demonstrate that they can overcome both political and logistical barriers.

Political chaos

Verbal sniping over Sputnik V has intensified in recent weeks.

Last month, the EU’s internal market commissioner Thierry Breton, who oversees the bloc’s vaccination program, bluntly told Russia that the bloc had “absolutely no need” for its vaccine—a strangely confident assessment, given the EU’s snail’s pace in inoculating its 450 million people. The Russian Direct Investment Fund, or RDIF, which developed the vaccine with the country’s Gamaleya Research Institute of Epidemiology and Microbiology, accused Breton of being “clearly biased,” while Putin said Western politicians were spreading “sometimes outright fables.”

“More and more countries around the world are showing interest in our vaccines,” Putin said in March.

As of Monday, about 60 countries have approved Sputnik V, according to the developers. And on Tuesday, RDIF head Kirill Dmitriev told reporters on a Zoom call that they would transfer the technology for Sputnik V to manufacturers and countries elsewhere, “for free.”

That is a marked difference from Pfizer-BioNTech and Moderna: India and South Africa have appealed to the World Trade Organization to have those companies share their COVID-19 vaccine patents with the world, allowing drug manufacturers to begin producing them in labs anywhere. Dmitriev said India could begin producing about 50 million Sputnik V doses every month this summer, and that Iran and Serbia—both political allies of Moscow—would also soon begin production. Kazakhstan and Belarus are already making Sputnik V locally.

“If it wasn’t for Sputnik, those production capacities would not be used very well,” said Dmitriev. “Russia and India will be the largest production hubs for the Sputnik V vaccine in the world, followed by South Korea and China.”

The Swiss-Italian manufacturer Adienne has also signed a deal with the RDIF that should see production commence near Milan in July. That would mark the first manufacturing of the Russian vaccine on EU soil.

The European Medicines Agency (EMA) has yet to recommend Sputnik V’s deployment in the EU, as is the case with the Novavax vaccine; the drug regulator is still combing through trial data. Per the EU’s collaborative approach to bulk-buying vaccines, the EMA’s recommendation would be an essential precursor to any potential order being placed with the RDIF. However, two EU members have already taken delivery of Sputnik V doses outside of this framework: Hungary, which began putting them in arms two months ago, and Slovakia.

Slovakia’s experience with the Russian vaccine has been politically explosive. Former Prime Minister Igor Matovič ordered 2 million doses without consulting his coalition partners and was last month ejected from office as a result.

Then, last week, the Slovakian drug regulator ŠÚKL announced that the 200,000 doses already delivered to the country were not the same as those given to the EMA for evaluation or delivered to other customers. The RDIF angrily denied the allegation and demanded the doses’ return, claiming Slovakia had made “multiple contract violations.” A Hungarian lab is now evaluating the Slovakian doses, and Matovič—now the Slovakian finance minister—is still trying to salvage the deal.

Meanwhile, in the neighboring Czech Republic, the Russian jab has also been causing political ructions, or at least contributing to them. Foreign Minister Tomáš Petříček and Health Minister Jan Blatný have in recent days both been fired from their positions; both had resisted calls for the country to buy Sputnik V.

Complicated manufacturing

Politics aside, the RDIF’s Dmitriev—like EU commissioner Breton—might be in danger of overpromising and underdelivering.

Technology transfers are only one issue. In interviews, experts caution that producing COVID-19 vaccines in vast quantities across the world is far more difficult than is often assumed. “The patent is not the issue,” Gian Gandhi, Unicef’s Covax coordinator for the organization’s supply division, told Fortune in an interview in February. “It is knowing how to produce the vaccine, not who owns the patents.” Gandhi says the organization supports technology transfers of COVID-19 vaccines, but for them to have full effect, they needed to include plenty of expert knowledge and help from the original developers.

Others agree that the rush for pharmaceutical companies to begin producing COVID-19 vaccines appears too optimistic.

“The manufacturing process is extremely complicated for vaccines,” Caroline Casey, Airfinity’s lead analyst and COVID-19 vaccine expert, told Fortune on Tuesday. “It is not as easy as releasing the recipe of the vaccine and having another site produce it to the same quality.” She says there have been several examples of slower manufacturing at pharmaceutical factories producing other companies’ COVID-19 vaccines.

“Some are performing very well; others are behind schedule,” she says. “Even if you are able to do the technology transfer, you need to make sure there is a site and a team of experts able to do it. The technology transfers we have seen from other candidates have not necessarily gone well.”

Adding to those concerns, the Russian vaccine has its own peculiarities. Unlike Pfizer-BioNTech and Moderna, both of which are built on messenger RNA (mRNA) technology, Sputnik V is a so-called adenovirus or viral vector, which uses a modified virus to instruct the body how to fight off COVID-19. That is like the J&J vaccine. But Sputnik V requires two doses, each of which is a different viral vector. “Russia has been struggling to produce the second vector,” Casey says.

Indeed, Russia itself has found it hugely complicated to manufacture Sputnik V—perhaps one reason why, by mid-March, the country had vaccinated only about 3.5 million people out of a population of 144 million.

On Monday—just as India was approving Sputnik V for use—the New Yorker’s Moscow correspondent Joshua Yaffa reported that dozens of vaccination points outside Moscow and St. Petersburg had been closed for lack of vaccines. Manufacturing had proved difficult, according to those interviewed. “Every production facility and every bioreactor comes with its own surprises,” Ilya Yasny, head of scientific research at Moscow investment fund Inbio Ventures, told the New Yorker. “Human cells can be very capricious. They might grow or not grow. They are living organisms and require constant adjustments and corrections.”

As Greek Prime Minister Kyriakos Mitsotakis put it last week, when fending off opposition calls for a Sputnik V order outside of the EU’s collaborative-purchasing framework: “Even if we wanted vaccines from Russia and took the risk to bypass the European process, there are not enough.”