The European Union has given the green light to Johnson & Johnson's COVID-19 vaccine, paving the way for the jab's deployment there.

This is the fourth COVID-19 vaccine to be granted a so-called conditional marketing authorization in the EU, allowing its emergency use. The other three, in order of their approval, are Pfizer-BioNTech, Moderna, and AstraZeneca. However, the J&J vaccine, developed by the company's Janssen Pharmaceuticals unit, is the first of the four to require only a single dose rather than two.

"With this latest positive opinion, authorities across the European Union will have another option to combat the pandemic and protect the lives and health of their citizens," said European Medicines Agency (EMA) executive director Emer Cooke in a March 11 statement.

The European Commission quickly rubber-stamped the EMA's recommendation, meaning the J&J vaccine can now legally start being rolled out.

More safe and effective vaccines are coming to the market.

The Chair of European Commission wrote on Twitter:

We have just authorised the use of Johnson & Johnson's vaccine in the EU, following @EMA_News' positive review.

With the number of doses we ordered, we could vaccinate up to 200 million people in the EU.

— Ursula von der Leyen (@vonderleyen) March 11, 2021

However, it remains uncertain as to when J&J will deliver its vaccines to the EU—a region that is far behind the U.S. in terms of vaccinating its people.

So far, J&J has only promised European vaccines in the second quarter. According to Politico, some EU lawmakers and diplomats expect to see deliveries only in mid- or late-April.

At the time of publication, J&J had not responded to a request for comment on its European delivery schedule.

“The European Medicines Agency’s recommendation is a landmark moment for Johnson & Johnson and for the world,” said J&J chief scientific officer Paul Stoffels in a statement. “We remain confident that the Johnson & Johnson vaccine will prove a critical tool for fighting this pandemic.”

As with the other vaccines, the J&J jab is authorized for use only in over-18s. A clinical trial conducted in the U.S., South Africa, and Latin America found 67% efficacy in cutting symptomatic cases, and complete success in avoiding hospitalizations.

J&J's vaccine uses an adenovirus (in this case a chimp virus that causes cold-like symptoms) that has been modified to contain a crucial gene from the SARS-CoV-2 virus that causes COVID-19. Passing this harmless, modified virus into the recipient's body triggers antibodies and T-cells that target it, with the body remembering its reaction next time it meets SARS-CoV-2.

AstraZeneca's vaccine uses a similar method, while those from Pfizer-BioNTech and Moderna use a newer technique called messenger RNA (mRNA). Like AstraZeneca, J&J has also pledged to make its COVID-19 vaccine available on a not-for-profit basis while the pandemic is underway.

The J&J vaccine gained emergency authorization in the U.S. in late February, and the company has so far delivered around 4 million doses there. This week, U.S. President Joe Biden announced the purchase of another 100 million J&J doses, which will partly be produced by J&J rival Merck, though those will take many months to arrive because of logistical issues.

Other candidate vaccines that the EMA is reviewing include those from Germany's CureVac, the U.S.'s Novavax, and Russia's Sputnik V.