MOSCOW (Reuters) - Russia’s first big international shipment of its coronavirus vaccine -- 300,000 doses sent to Argentina last week -- consisted only of the first dose of the two-shot vaccine, which is easier to make than the second dose, sources told Reuters. Unlike most other COVID-19 vaccines, which are given as two shots of the same product, the Russian Sputnik V vaccine relies on two doses delivered using different inactive viruses, known as vectors. The Gamaleya Institute that developed the vaccine says it is more than 91% effective after the two-dose course. But some Russian manufacturers are finding the second dose, which is administered 21 days after the first, to be less stable, two sources said, revealing a new challenge for the country’s ambitious national inoculation programme. The decision to send doses of the vaccine to Argentina caused an outcry at home, where the lifesaving drug is still mostly unavailable to the general public outside the capital Moscow. Russia has not said exactly how many people have received it. The Gamaleya Institute said last week 650,000 doses had been released for Russia’s domestic vaccination programme so far. Argentina is the first foreign country apart from Belarus to approve Sputnik V, a win for Moscow’s drive to secure international blessing for its vaccine. Argentinian officials have said they expect to start administering the vaccine in the days ahead. However, a source close to the manufacturing process, and another in the government, said the shipment was made up only of surplus doses of the first component, which had been produced in greater quantities than the second. The source close to the manufacturing process said vaccine makers were now working to match the production of the two components. “It’s true that technological problems remain... More of the first component is produced per litre in bioreactors than of the second dose,” he said. “But many producers are just putting up more bioreactors to make the second dose. That’s all. If your reactor is producing less per litre, then you need more capacity,” the source said. Generium, one of the private Russian pharmaceutical firms tasked with producing Sputnik V, said it is currently producing both components of the vaccine in equal amounts. Russia’s sovereign wealth fund, the Russian Direct Investment Fund (RDIF), which markets Sputnik V abroad and which organised the delivery to Argentina, declined to comment on challenges with production of the second dose, or whether the booster shots would soon be sent to Argentina. The Russian health ministry, which supervises the Gamaleya Institute, also did not respond. Officials in Argentina could not immediately be reached for comment. SPUTNIK-LIGHT Several vaccine makers globally are considering the option of using just the first component. President Vladimir Putin has referred to a single-component option as a “light-vaccine”, which he said would provide less protection than the two components, but “will still reach 85%”. Gamaleya Institute director Alexander Gintsburg has said that protective immunity after the first shot of Sputnik V lasts around 3-4 months, the TASS news agency reported.