The European Medicines Agency (EMA) on Friday approved the Oxford AstraZeneca vaccine for people over the age of 18 - the third vaccine given the green light in the EU.
The vaccine showed a 60 percent efficacy in the clinical trials, but this is well below the level of protection shown by authorised Pfizer BioNTech and Moderna vaccines - which proved 95 percent effective.
But it's cheaper and easier to store than the two already approved.
The AstraZeneca vaccine requires two shots - the second dose to be taken between 4 and 12 weeks after the first.
Side effects were usually mild or moderate - including pain and tenderness on the injection, headache, tiredness, muscle pain, chills and fever.
EMA executive director Emer Cooke welcomed the decision.
"With this third positive opinion, we have further expanded the arsenal of vaccines available to EU and EEA member states to combat the pandemic and protect their citizens," Cooke said.
"As previous cases, the human medicines committee (CHMP) has rigorously evaluated this vaccine, and the scientific basis of our work underpins our firm commitment to safeguard the health of EU citizens."
The study involved around 24,000 participants. Half received the vaccine, half were given a control injection - either a dummy injection or another non-COVID vaccine.
Of the participants who were given the vaccine, 64 out of 5258 got COVID-19 with symptoms, compared with 154 of 5210 who were given the control injection.
Most of the participants in the studies were between 18 and 55 years old so there were concerns raised that there was a lack of data about how effective it is with older participants (over 55 years).
Antonella Viola, professor of immunology at Italy's University of Padua told Reuters he's not certain.
"Sixty percent efficacy with a double dose and with doubts for the age group over 55 years... herd immunity will never be achieved with this vaccine, it's mathematical."
"I would only use it for younger people at most."
But Cooke backs the decision.
"None of them is a magic wand on its own," he said.
"But together they provide tools and options to prevent different aspects of the disease."
