"This vaccine provides us with another tool to overcome the current emergency", said EMA Executive Director Emer Cooke.
"The antibody decay generated by the vaccine in humans goes down very slowly (.) We believe there will be protection potentially for a couple of years", he added.
EU countries started inoculating people with the Pfizer-BioNTech vaccine two weeks ago, after the European Medicines Agency (EMA) recommended the shot's conditional approval under the brand name Comirnaty with an authorisation that allows five doses to be extracted from each vial.
As of 1403 GMT, shares of Moderna were up by 2.76% at $119.5 in the U.S. premarket.
While studies have shown a single shot of Moderna's vaccine can promote resistance to COVID-19, scientists warn that limiting Americans to a single dose could come with risks.
Minster for Health Stephen Donnelly said on Wednesday that Ireland has ordered 875,000 doses of the Moderna vaccine and that "the logistics are being finalised".
Alejandro Cravioto, chairman of WHO's Strategic Advisory Group of Experts on Immunisation (SAGE), told media persons during a news briefing that two doses are needed to make the vaccine more effective. One user offered supposed Moderna Inc vaccines for $180, and claimed the vaccine from Pfizer Inc and BioNTech SE could be had for $150 and AstraZeneca's for $110 per vial.
"Patients should never try to secure a vaccine online - no legitimate vaccine is sold online - and only get vaccinated at certified vaccination centers or by certified healthcare providers", a Pfizer spokesman said in a statement. "Our work will always be guided by the scientific evidence and our commitment to safeguard the health of European Union citizens". Israel authorized the vaccine on Monday.
The European Union authorised the coronavirus vaccine developed by USA drug company Moderna on Wednesday, EU chief Ursula von der Leyen said, just hours after medical regulators gave it the green light.
Both Moderna's and Pfizer-BioNTech's shots are mRNA vaccines, made with a groundbreaking new technology. Instead, they use a piece of genetic code that trains the immune system to recognize the spike protein on the surface of the virus, ready to attack if the real thing comes along.
Austrian Chancellor Sebastian Kurz tweeted that approval of the Moderna vaccine "is another important step in the fight against the pandemic".