Authorities in the US and the EU have already started planning for such an eventuality.
It comes a day after Denmark became the first country in Europe to abandon the jab over links with very rare blood clots.
The grim milestone is expected to be passed on Thursday.
The decision was taken due to the vaccine's link with very rare cases of blood clots, the country's health agency said.
The aim of the study is to assess how well people's immune systems respond when their second "boost" dose is a different type of vaccine to their first "prime" dose.
Johnson & Johnson will delay the rollout of their COVID-19 vaccine in Europe following reports of blood clots in six people after receiving jab.
Chinese vaccines "don’t have very high protection rates," said the director of the Chinese Center for Disease Control, Gao Fu, at a conference Saturday.
AstraZeneca's deliveries to the European Union have been beset by delays.
It comes after the regulator said there was a possible link between very rare cases of blood clots and the AstraZeneca vaccine.
The European Medicines Agency said the Oxford/AstraZeneca vaccine could potentially cause rare blood clots and Commission President von der Leyen got sidelined in Ankara. Here's a look at two key stories making the news in Brussels this week.
France's health authority has recommended that adults under 55 who received a first dose of AstraZeneca's COVID-19 vaccine should receive a different vaccine for their second dose.
"All Sputnik V batches are of the same quality and undergo rigorous quality control at the Gamaleya Institute," said the official Twitter account of the Sputnik V vaccine.
The study by Imperial College London also found that the relationship between infections and deaths is diverging, "suggesting that infections may have resulted in fewer hospitalisations and deaths since the start of widespread vaccination".
It's been claimed the vaccines given to Serbia's neighbouring citizens were about to expire. Speaking to Euronews, the country's prime minister, Ana Brnabić, did not deny the allegation.
Only the US and Peru have had daily death tolls higher than 4,000. Brazil's pandemic toll is quickly approaching 340,000, the second-highest in the world.