A fourth COVID-19 vaccine has shown that it can protect people against the worst effects of the disease in a large United States-based trial. Two doses of AstraZeneca’s vaccine were 79 percent effective at preventing people from developing symptomatic COVID-19, according to a press release from the company today. No one in the US trial who received the vaccine was hospitalized for COVID-19, the company said.

It’s a good result for a vaccine that’s been dogged by clinical trial issues, poor transparency, and skepticism since last fall. AstraZeneca said the vaccine was up to 90 percent effective in November based on data from the United Kingdom and Brazil, but that number was based on a half-dose of the vaccine given to trial participants by mistake. Additional analyses found that the two-dose vaccine was around 82 percent effective against symptomatic COVID-19 when the doses were given three months apart. It has already been authorized and is being used in other countries around the world.

The company said that participants in the clinical trial did not have any usual or dangerous blood clotting. Over the past few weeks, some European countries halted the use of the AstraZeneca vaccine after at least 13 people who received the vaccine showed signs of an unusual clotting disorder. It’s not clear if the vaccine is linked to the disorder, and the European Medicines Agency said the benefits of the vaccine outweigh that potential rare risk.

Today’s release did not include any information on how well the vaccine performed against variant forms of the coronavirus. South Africa stopped using the AstraZeneca vaccine in February after a small study found that it did not protect against mild and moderate COVID-19 caused by the B.1.351 variant. That variant was first found in South Africa and may dodge antibodies created by vaccines. However, the vaccine appears effective against the more-contagious B.1.1.7 variant, which took over in the United Kingdom and is now spreading in the US.

Like the Johnson & Johnson vaccine, this vaccine was made using a weakened form of an adenovirus, which usually causes mild illnesses like pink eye. The gene for part of the coronavirus called the spike protein is inserted into that virus. When the vaccine is injected, the body makes copies of the spike protein and produces antibodies that can fight the coronavirus. This vaccine can also be stored in the refrigerator and is relatively cheap, especially compared to the Moderna and Pfizer / BioNTech vaccines.

AstraZeneca said it plans to ask the Food and Drug Administration for emergency authorization in the United States “in the coming weeks.” But the US may end up not using much of the vaccine, even if it is authorized. The country already has orders for enough doses of the Pfizer / BioNTech, Moderna, and Johnson & Johnson vaccines to cover every adult by the end of May. The US has tens of millions of doses of the AstraZeneca vaccine on hand, and it announced last week that it plans to send at least 4 million of them to Canada and Mexico.