MEXICO CITY — Russian authorities have been having so many problems producing second doses of their Sputnik V coronavirus vaccine that Russia probably will be unable to supply enough to people who already got the first dose, Mexican officials said Monday.

It’s the latest account of production problems for Sputnik V, which the Russian government has been promising to other countries but has not been able to supply in sufficient quantities.

Sputnik is unusual among coronavirus vaccines in that the two doses are different and not interchangeable.

The Russian vaccine uses a modified version of the common cold-causing adenovirus to carry genes for the spike protein in the coronavirus as a way to prime the body to react if COVID-19 comes along.

That is a similar technology to the vaccine developed by AstraZeneca and Oxford University. But unlike AstraZeneca’s two-dose vaccine, the Russian approach uses a slightly different adenovirus for the second booster shot.

Mexico’s assistant health secretary, Hugo López-Gatell, said the Russians have found that the first adenovirus grows much more quickly than the second.

López-Gatell said Russian scientists are now suggesting abandoning the idea of giving the two separate Sputnik V shots several weeks apart and instead giving a second booster shot six months later.

“Over the course of months, the quantity of first doses they managed to produce got out of alignment with the quantity of second doses they were able to produce,” López-Gatell said.

The Russian Direct Investment Fund, Russia’s sovereign wealth fund which has bankrolled Sputnik V and is in charge of marketing it abroad, denied the Mexican reports.

“The comments made by Mexican authorities regarding the production of the second dose of the Sputnik V vaccine are not true,” the Fund’s press office said in a statement. “The manufacturing of both components of Sputnik V is being upscaled in Russia and abroad to fully meet the rising demand for the vaccine internationally.”

Mexico has so far received only 1.9 million Sputnik V doses, out of a total of 24 million it has signed a contract for. It has been forced to rely more on the Pfizer vaccine, of which it has received 10.6 million doses, as well as about 10 million doses of two Chinese vaccines. It also received about 4.6 million doses from AstraZeneca.

Mexico has recorded over 219,000 test-confirmed COVID-19 deaths, but the country does so little testing that the government’s own estimates of probable deaths now amounts to around 347,000.

It is the latest in a series of setbacks for the Russian vaccine.

In April, regulatory authorities in Slovakia said they have not received enough information about the Sputnik vaccine from its producer to be able to assess its benefits and risks.

Slovakia’s State Institute for Drug Control said about 80 percent of the requested data had not been provided. It said the vaccine delivered to Slovakia was different from the original, two-dose Sputnik V vaccine that is considered 91 percent effective, according to a study in the medical journal Lancet.

The official Twitter account of the Sputnik V vaccine lashed out, claiming that Slovakia’s drug regulator “has launched a disinformation campaign against Sputnik V.”

However, after the vaccine was successfully tested in a lab in Hungary, Slovakia’s government appeared set to discuss the possible use of the vaccine again. Health Minister Vladimir Lengvarsky said last week he will talk with his country’s experts and “the Russian side about further developments on this issue.”

The RDIF welcomed the results of the Hungarian tests and said it asked the Slovak drug regulator to apologize “for spreading incorrect information about Sputnik V.”

On Monday, Deputy Prime Minister Igor Matovic promised to roll out Sputnik V “in the coming days.”

Slovakia announced Tuesday it is halting the use of the AstraZeneca vaccine after the country’s drug control institute concluded last week that the death of a 47-year-old woman who received the AstraZeneca jab was “likely” linked to the vaccine.

Also in April, Brazil’s health regulator cited safety concerns while rejecting several states’ requests to import almost 30 million doses of Sputnik V.

Russia has been actively marketing Sputnik V abroad despite the comparatively slow rollout at home and limited production capacities. Dozens of countries have approved the use of Sputnik V, and many signed deals with the Russian Direct Investment Fund to get shipments of the shot.