PARIS, April 8 (Reuters) - France's top health body will on Friday say that recipients of a first dose of AstraZeneca's traditional COVID-19 vaccine who are under 55 should get a second shot with a new-style messenger-RNA vaccine, two sources aware of the plans said on Thursday.

Reuters had reported on Wednesday that the Haute Autorite de la Sante (HAS), in charge of setting out how vaccines approved by the European Medicines Agency (EMA) should be used in France, was contemplating this possibility.

The HAS has now decided to proceed with the plan, the two sources said. Two mRNA vaccines, one from Pfizer and BioNTech and one from Moderna, are approved for use in France.

Messenger RNA vaccines prompt the human body to make a protein that mimics part of the virus, triggering an immune response, while traditional vaccines such as AstraZeneca's use an inactivated virus to carry a protein from the pathogen - in this case the novel coronavirus - to do the same thing.

An HAS spokeswoman had no comment.

Vaccination programmes have stuttered in Europe and elsewhere in the last month, since a very few mostly young recipients of the AstraZeneca shot were found to have suffered extremely unusual blood clots, leading some countries to suspend its use as a precaution.

Most have resumed using the shot, although some have done so with age restrictions.

In France, the HAS advised on March 19 that only people aged 55 and over should receive the AstraZeneca vaccine, which had already been given to 500,000 people as a first dose.

While the numbers are small compared with the tens of millions being inoculated across the EU, a decision to give a different booster shot would be significant because the approach has not been tested in late-stage human trials.

Germany was the first European country to recommend that people under 60 who have had a first AstraZeneca shot should receive a different product for their second dose.

Some experts say that, because all the vaccines target the same outer "spike" protein of the virus, they could be complementary. But there is no evidence that this approach will be as effective. (Reporting by Matthias Blamont; Editing by Kevin Liffey)