The WHO reiterated Friday there was "no adequate data" on switching Covid-19 vaccines between doses, after France said under-55s who received an AstraZeneca first jab should get their second from a different vaccine.
The World Health Organization has called for studies on so-called mixing and matching between vaccines, but said there was no comprehensive data so far on which it could make any recommendations.
The UN health agency therefore recommends that the same product should be used for both doses.
In February, the WHO's Strategic Advisory Group of Experts on Immunization (SAGE) issued its recommendations on how the AstraZeneca-Oxford vaccine should be used, WHO spokeswoman Margaret Harris told reporters.
"There is not adequate data to be able to say whether this is something that could be done," she said.
"So they were recommending that at this stage, interchangeability of vaccines was not something that we could give a recommendation on.
"But they did make a research recommendation that this be studied, so that we could understand whether or not it was a strategy that could be used."
Several clinical trials are looking at the efficacy of combining two types of vaccine.
Europe's medicines regulator said this week that the AstraZeneca vaccine could cause very rare blood clots among some recipients, prompting several countries to scale up restrictions on the jab.
France on Friday said under-55s who received a first injection of the AstraZeneca Covid-19 vaccine should be given a jab from a different producer for their second dose, in a ruling affecting 533,000 people.
The Pfizer-BioNTech vaccine was the first Covid-19 jab to receive WHO's green light, on December 31 last year.
The AstraZeneca-Oxford University was the second, on February 15, with SAGE having issued usage recommendations five days before.
That February 10 document said on switching between Covid-19 vaccines: "No data are available on the interchangeability of doses of this vaccine with other Covid-19 vaccines.
"It is currently recommended that the same product should be used for both doses. Recommendations may be updated as further information becomes available on interchangeability."
Going forward, the SAGE document also recommended "interchangeability and 'mix and match' studies within and across Covid-19 vaccine platforms".
On Wednesday, the WHO's vaccine experts said a link between AstraZeneca's Covid-19 jabs and blood clots was plausible but unconfirmed, stressing that reported occurrences were "very rare".
"Based on current information, a causal relationship between the vaccine and the occurrence of blood clots with low platelets is considered plausible but is not confirmed," the experts said.
A WHO spokesman added: "We believe the benefit-risk balance is very much in favour of the vaccine.
"Nothing has changed in our advice -- which is to keep vaccinating."