The U.K. government has approved AstraZeneca’s COVID-19 vaccine for emergency use, becoming the first government to do so.

The decision by the U.K. drugs regulator will clear the way for tens of millions of Britons to be vaccinated in the coming months. Britain had preordered 100 million doses of the vaccine, more than any other vaccine candidate.

It is the second vaccine to be granted emergency use authorization in the U.K., following approval of Pfizer/BioNTech’s vaccine in early December.

“Today is an important day for millions of people in the U.K. who will get access to this new vaccine,” Pascal Soriot, AstraZeneca’s chief executive officer, said in a statement. “It has been shown to be effective, well-tolerated, simple to administer, and is supplied by AstraZeneca at no profit.”

The British approval may also encourage other governments around the world to also authorize the vaccine. Many public health experts see the AstraZeneca vaccine as crucial to the worldwide fight against the coronavirus because it is less expensive and can be stored at much higher temperatures than several of the other vaccines being developed.

AstraZenca has signed agreements to make and provide more than 2 billion doses of its vaccine. In addition to the doses committed to the U.K., this includes an agreement to provide 300 million doses to the U.S., 400 million for the European Union, and 1.3 billion doses specifically designated for lower-income countries.

Cheaper than rival vaccines

The company has promised not to make any profit on the vaccine during the period of the pandemic. Because of this, and because of the simpler manufacturing methods for this vaccine compared to some of the other candidates, AstraZeneca has priced the vaccine at just $3 to $4 per dose, compared to prices at least five times higher for many competing vaccines.

The vaccine, which the Cambridge, England–based AstraZeneca developed with scientists from the University of Oxford, was found to be safe in large clinical trials conducted in the U.K., Brazil, and South Africa. But data also showed the vaccine as potentially less effective than those from biotechnology company Moderna and Pfizer/BioNTech.

Both of those vaccines were about 95% effective in clinical trials. By contrast, AstraZeneca’s vaccine was just 62.1% effective when given as two standard doses four weeks apart, according to clinical trial results published in the medical journal The Lancet. But, for a smaller cohort, it proved 90% effective when a half-dose was given initially, followed by a full dose. This smaller cohort, however, did not include any people over the age of 55.

Confusion over those results—and how the smaller cohort came to receive the regimen involving the half-dose—sapped AstraZeneca’s stock price, which had reached all-time highs largely on the company’s status as front-runner in vaccine sweepstakes.

The results have clouded the prospects from AstraZeneca’s vaccine, with some speculating that the U.S. Food and Drug Administration will not authorize it for use in the U.S., at least until more clinical trial data is available. AstraZeneca is currently conducting a 30,000-person trial of the vaccine in the U.S.

It is unclear what dosing regimen will be used in the U.K. under the emergency use authorization, but statements from the government and the company implied it would be one in which people are given two standard doses. The government has said its plan is to give as many at-risk people as possible a first dose of the vaccine, providing some protection, with a second dose given within 12 weeks, rather than the four-week period used in the clinical trial.

The hope, the government said, is that this will allow more people to be inoculated and gain at least some protection from the existing stocks of the vaccine.

AstraZeneca’s vaccine works differently than those approved so far from Moderna and Pfizer/BioNTech. Those vaccines use messenger RNA (mRNA) to instruct cells to produce proteins associated with the coronavirus, prompting an immune response from the body. The AstraZeneca vaccine uses a method developed by researchers at Oxford’s Jenner Institute that uses a chimpanzee virus that has been genetically modified to produce the coronavirus spike protein.

The vaccine can be stored at normal refrigerator temperatures, between 2 degrees Celsius and 8 degrees Celsius (32.5 to 46.5 Fahrenheit) as opposed to -70 degrees Celsius (-94 degrees Fahrenheit) for the mRNA vaccines. Those extreme low temperatures have created a logistical problem for distribution of the mRNA vaccines. For instance, in the U.K., where general practice doctors are currently administering the COVID-19 vaccine efforts, many have been unable to participate in the inoculation drive so far because they lack equipment to keep vaccine vials at these ultralow temperatures.