The coronavirus vaccine made by Pfizer Inc. and its German partner BioNTech SE is highly effective in preventing Covid-19 and there are no safety concerns that would prevent it from being granted an emergency-use authorization, according to a report by staff of the U.S. Food and Drug Administration.

The report offers a first look at the U.S. review of the vaccine ahead of a public meeting Thursday of outside advisers to the agency. The FDA could clear the shot shortly after the meeting, with as many as 6.4 million doses immediately available to kick off a vast immunization effort designed to end a pandemic that has killed more than 283,000 Americans.

The findings, posted on the agency’s website Tuesday, echoed previously disclosed data from the companies that showed the vaccine was 95% effective in preventing symptomatic Covid-19. It also said that two months of trial data revealed no significant safety issues.

Pfizer requested an emergency authorization in the U.S. on Nov. 20. Regulators in the U.K. approved the vaccine earlier this month, and on Tuesday, the National Health Service there launched an ambitious vaccination campaign that will begin with tens of thousands of people over the age of 80 in the coming days. The shot is also under review in the European Union.

Pfizer shares gained as much as 3.7% in New York trading on Tuesday. So far this year, the stock has climbed 14%.

The vaccine uses technology called messenger RNA that transforms the body’s cells to fight the virus. Moderna Inc. also makes an experimental Covid-19 vaccine based on messenger RNA, and FDA advisers will meet next week to consider whether it should receive emergency authorization.

In their report, FDA staff wrote that analyses “showed similar efficacy point estimates across age groups, genders, racial and ethnic groups, and participants with medical comorbidities associated with high risk of severe Covid-19.” Common side effects included injection-site reactions, fatigue and headache.

There isn’t enough data on children younger than 16 to determine whether it will work in pediatric populations, FDA staff said. Data is also insufficient to make any determinations about pregnant, lactating or immunocompromised people.

Among participants in the Pfizer clinical trial who didn’t show signs of Covid-19 infection during vaccination, there were eight virus cases within the group who received the shot and 162 in those who got a placebo, according to the report. The vaccine is a two-dose regimen, with the second dose given 21 days after the first.The latest Covid updatesMake sense of the headlines and the outbreak’s global response with the Coronavirus Daily.EmailBloomberg may send me offers and promotions.Sign UpBy submitting my information, I agree to the Privacy Policy and Terms of Service.

The vaccine was about 52% effective after the first dose, though the data don’t indicate whether the protection would extend beyond 21 days when someone would normally get the second dose, the report found. It was 89% effective at preventing severe cases after just one dose, though FDA staff also had caveats for the figure, stating that the number of severe infections in the trial was small so it was difficult to draw conclusions.

Six people died in the clinical trial of almost 44,000 participants, two in the group that received the vaccine and four in the placebo group. The two in the vaccine group were older than 55. One died of cardiac arrest 62 days after vaccination and the other from arteriosclerosis, a condition where blood vessels can harden with age, three days after vaccination.

“All deaths represent events that occur in the general population of the age groups where they occurred, at a similar rate,” the report said.

It’s unclear whether the vaccine is effective at preventing Covid-19’s spread, according to the report. It’s possible the vaccine didn’t work as well at preventing cases where people didn’t have virus symptoms, meaning there will likely be a continued need for basic public-health measures.

“Asymptomatic cases in combination with reduced mask-wearing and social distancing could result in significant continued transmission,” FDA staff wrote.

Side effects were mild and developed more frequently in younger participants in the study, specifically those aged 55 and younger, with no signs of serious safety concerns. After the second injection, fatigue and headache were the most common systemic reactions, occurring in 59% and 52% of the younger participants, compared with 51% and 39% in those over age 56.

If Pfizer’s vaccine is cleared by the FDA, Trump administration officials have said it could begin to be distributed within 24 hours. Between Pfizer and Moderna’s vaccines, 40 million doses are expected to be available by the end of the year, enough for 20 million people to get the two-shot regimen.

The Trump administration is confident that the U.S. will have enough supply to eventually vaccinate everyone, a senior administration official said Monday on a call with reporters, though only the shots made by Pfizer and Moderna are expected to be cleared by the FDA this month. The U.S. also has agreements to get shots from AstraZeneca Plc, Johnson & Johnson and others.

The Centers for Disease Control and Prevention’s immunization committee recommended health-care workers and long-term care residents be first in line, totaling roughly 23 million people. It will be up to the states to determine exactly how to distribute the vaccines. As Pfizer continues its clinical trial to request full approval in the future, the company plans to offer clinical trial participants who are receiving a placebo instead of the vaccine access to a real shot once they become eligible or after six months of follow up, FDA staff said.

— With assistance by Michelle Fay Cortez