The COVID-19 vaccine being developed by Oxford University and AstraZeneca led to an immune response in roughly 1,000 patients, early study results show.

The data, published in the medical journal the Lancet, also show that the vaccine caused side effects, including fever, headaches, muscle aches, and injection site reactions, in about 60% of patients. All of the side effects were deemed mild or moderate, and all resolved themselves over the course of the study.

While the Oxford-AstraZeneca vaccine, known as AZD1222, has moved most rapidly into larger-scale studies of any major contender — and AstraZeneca has said that billions of doses could be manufactured — the new data represent the first glimpse researchers have gotten at its efficacy. They show a relatively safe vaccine — though side effects were greater than for a meningitis vaccine, to which it was compared — that engages the immune system to fight the virus. AstraZeneca said that, because of the results, it is likely that future studies will test giving patients two doses of the vaccine.

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