France is now recommending that people under 55 who've got a first dose of the AstraZeneca vaccine should get their second dose with a so-called mRNA vaccine.
Two of those have been approved so far the ones from Moderna and Pfizer.
The country's top health agency made the recommendation on Friday.
The World Health Organization said the same day, however, that there is no data recommending that countries "mix and match" their vaccines.
Other countries have done it, though, including Germany, which was the first to recommend such a policy for people under 60.
The decision in France will affect a little over half a million people who got their first doses.