Several countries are considering delaying second doses, including Britain, which pursued a plan to separate doses by up to three months. And federal health authorities in the United States have indicated flexibility on expanding the gap between first and second doses to six weeks. But the vaccines in those cases are the same in both doses.

Sputnik, however, developed by the Gamaleya Research Institute, part of Russia’s Ministry of Health, uses a different adenovirus in each of its two doses to deliver bits of the coronavirus’s genetic code. Russia has said it will soon release a one-shot version of its vaccine — essentially using the first dose as the only dose, which it is calling “Sputnik Light.” It is not clear what benefit there would be to delaying a second dose of Sputnik, since there is no recommendation that the second version be administered as a single dose.

Argentina’s decision to delay second doses comes amid increasing concerns of the possibility of a new wave of Covid-19 cases and deaths, fueled by the new variants of the virus that have engulfed several of Argentina’s neighbors, particularly Brazil, but also Chile and Paraguay.

Argentina is canceling all direct flights with Brazil, Chile and Mexico starting Saturday in an effort to block the new variants. It had already blocked flights from Britain and Ireland.

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