Mexico is one of several countries that have agreements with Russia to supply COVID vaccines, but is still waiting for sufficient quantities to be delivered.

Due to the shortages, Mexico’s assistant health secretary Hugo López-Gatell said Russian scientists are suggesting giving a second booster shot six months after the first dose instead of two Sputnik V shots several weeks apart.

The Russian Direct Investment Fund, which has paid to develop the Sputnik V vaccine and is in charge of marketing abroad, denied the 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 service 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.”

For more reporting from the Associated Press, see below.

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.

López-Gatell, said the Russians have found that the first adenovirus grows much more quickly than the second.

“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.

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% 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% 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.”

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.