That result is a scientific achievement, as are the clinical successes of two other vaccines from Moderna and partners Pfizer and BioNTech that are cleared for use in the U.S. and elsewhere. Pfizer’s shot is the only other cleared for use in the U.K. as of yet.

But a two-dose regimen of Moderna and Pfizer’s vaccines, spaced either three or four weeks apart, each proved roughly 95% effective at preventing COVID-19 in clinical testing. By comparison, the data supporting AstraZeneca and Oxford’s shot, published in The Lancet in November, are less clear. On average, the vaccine was 70.4% effective. But the vaccine’s efficacy differed across two dosing regimens, one of which was given mistakenly before the error was detected.

The vaccine was 62% effective, for instance, in the larger group that received two full doses of the shot, but 90% in a much smaller group given a half dose and then a full dose. Volunteers also had their second shot anywhere from four to 26 weeks after the first, further complicating the findings.

Over the past week, CEO Pascal Soriot told Britain’s Sunday Times said AstraZeneca found a “winning formula” to get the shot on par with the rival vaccines from Moderna and Pfizer. Another published report, citing AstraZeneca’s partner in India, suggested the formula is giving the two shots spaced two to three months apart.

The company hasn’t yet disclosed data supporting that theory, though documents underlying the U.K. authorization show particularly high levels of coronavirus antibodies were seen in a small group of volunteers who got their second dose at least 12 weeks after the first.

The complicated findings, combined with earlier setbacks — testing was temporarily halted worldwide when one U.K. study participant was diagnosed with an unexplained neurological illness — have delayed the program’s progress in various countries. A large trial is underway in the U.S. and currently expected to read out in February, but that study is testing the dosing regimen that appeared less effective. AstraZeneca hasn’t yet formally filed for approval in Europe, according to Reuters.

In the U.K. the MHRA has authorized the two full-dose regimen for adults 18 years and older, but advised providers to administer the second shot anywhere from four to 12 weeks after the first.

And given the urgency of the pandemic in the U.K., where infections are at an all-time high, the country plans to maximize its limited early supply of the shot by giving as many people as possible their first dose. This means the second shot of both the AstraZeneca and Pfizer vaccines will be administered closer to 12 weeks later. “This will maximize the number of people getting vaccine and therefore receiving protection in the next 12 weeks,” the UK’s Joint Committee on Vaccination and Immunization said in a statement.

The most common side effects tied to AstraZeneca’s vaccine were pain and tenderness at the injection site, fatigue, fever and chills and mostly mild to moderate. Side effects were less common after the second dose. The authorization does note “very rare events of neuroinflammatory disorders” reported following vaccination, an apparent reference to the instances that temporarily stopped testing, but added a “causal relationship” hasn’t been established. The MHRA is advising medical personnel not to give the vaccine to people with hypersensitivity to any of the shot’s components.

AstraZeneca and Oxford’s vaccine does have an important advantage that could be crucial in global distribution. It can be stored and transported at normal refrigerator conditions, whereas Moderna’s and Pfizer’s shots must be shipped at much colder temperatures and don’t last as long once thawed. AstraZeneca aims to supply “millions of doses” to the UK in the first quarter as part of a deal to supply the country with up to 100 million total, the company said.

AstraZeneca expects to make about 3 billion doses of its vaccine in 2021, more than either Moderna or the Pfizer and BioNTech team. Some 900 million of those doses have been secured by the U.S., U.K., EU and Japan. The U.S. has pre-ordered at least 300 million doses, more than any other vaccine in development.

The vaccine uses a virus found in chimpanzees to deliver genetic instructions that train the body to neutralize the coronavirus. It’s similar to the shot being developed by Johnson & Johnson, which is in late-stage testing, with results expected next month.

The vaccines from Moderna and Pfizer, by contrast, use messenger RNA encased in a tiny bubble of fat to spur an immune response.