AstraZeneca Plc’s coronavirus vaccine fared better than expected in a U.S. clinical trial, providing reassurance about its safety and efficacy.

The shot developed with the University of Oxford was 79 per cent effective in preventing COVID-19, and an independent monitoring board found no safety concerns, the company said Monday. All those immunized were protected from severe disease and death in a study of more than 30,000 volunteers.

The findings should bolster confidence in the vaccine after confusion over its efficacy and the best dosing regimen slowed take-up. The product is at the center of a supply showdown with the European Union just days after concerns about blood clots prompted a dozen member states to suspend immunizations. Even after the European Medicines Agency declared it safe last Thursday, not all EU countries have resumed vaccinations.

“Efficacy is better than we had expected,” said Peter Welford, an analyst at Jefferies in London. “Importantly, after recent largely unfounded safety concerns in Europe, the study confirms the safety profile.”

Astra shares rose as much as 1.4 per cent in London trading. The company said it plans to submit the trial data to U.S. regulators in coming weeks, seeking clearance for emergency-use authorization.

Data Confusion

A shortfall in deliveries to the EU has put Astra at odds with the bloc and led officials to oppose exports of the shot from the region as they seek to hold the company to account.

The new data offer more clarity than the first study results released last November. Those clinical trials, conducted in the U.K. and Brazil, produced a range of readings due to different dosing amounts and regimens used. At least 10 EU countries only approved the shot for under-65s initially as early trials lacked older participants. Many have since reversed course after more data from real-world use showed the vaccine was effective for everyone.

Older adults made up about a fifth of the U.S. trial, which showed efficacy of 80 per cent in that age group. The results “offer confidence that adults of all ages can benefit from protection against the virus,” Ann Falsey, a professor at the University of Rochester’s School of Medicine and one of the trial’s lead investigators, said in a statement.

A number of cases involving rare but serious blood clots also damaged the vaccine’s image. The EMA and U.K.’s regulator said Thursday that no definitive link could be established between the clots and the vaccine, and the benefits of its use continued to outweigh the risks. The U.S. trial review that found no safety concerns looked specifically at blood clots.

Skepticism over the vaccine started last year when Astra and Oxford first reported initial data. At the time, they offered two different efficacy readings of 62 per cent and 90 per cent. Because of a manufacturing error, one group of participants had received a lower first dose, which produced the higher reading, compared with those getting two standard doses. Further analysis suggested it was the greater dosing gap rather than amount that had created the difference.

What Bloomberg Intelligence Says

Why didn’t the efficacy look this good in its U.K. trial with the 4-week dosing interval, and is it likely to do even better with a 12-week gap? Sadly, this trial is too small to definitively settle rare side-effect questions.

-- Sam Fazeli, BI Senior Pharma Analyst

More than 70 countries globally have approved the Astra-Oxford shot for use and the partners plan to produce up to three billion doses of the vaccine this year.

The vaccine is key to the global effort to end the pandemic because it’s easy to store and transport and the company is providing it at no profit during the crisis. Unlike vaccines from Pfizer Inc. and partner BioNTech SE, and Moderna Inc., which have to be kept frozen, the Astra shot can be held at fridge temperature.