AstraZeneca and Oxford University have more work to do to confirm whether their COVID-19 vaccine can be 90 percent effective, peer-reviewed data published in The Lancet showed on Tuesday, potentially slowing its eventual rollout in the fight against the pandemic.

Once seen as the frontrunner in the development of a vaccine against the coronavirus crisis, the British team was overtaken by U.S. drugmaker Pfizer, whose shots — with a success rate of around 95 percent — were administered to the U.K. pensioners on Tuesday in a world-first hailed as V-Day.

Detailed results from the AstraZeneca/Oxford trials have been eagerly awaited after some scientists criticized a lack of information in their initial announcement last month.

However, the Lancet study gave few extra clues about why efficacy was 62 percent for trial participants given two full doses, but 90 percent for a smaller sub-group given a half, then a full dose.

“[This] will require further research as more data becomes available from the trial,” the study said.

Less than six percent of U.K. trial participants were given the lower dose regimen and none of them were over 55, meaning more research will be needed to investigate the vaccine’s efficacy in older people who are particularly susceptible to COVID-19.

Pooling the results, overall efficacy was 70.4 percent, the data on Tuesday showed. That is above the 50 percent minimum set by the U.S. Food and Drug Administration.

Efficacy reflects how well a vaccine works in clinical trials. Effectiveness is how well a vaccine or a product will work in the real world rather than ideal settings.

Vaccine efficacy levels vary

COVID-19 vaccines from Pfizer and German partner BioNTech and from Moderna have reported efficacy levels of more than 90 percent in late-stage trials.

But the AstraZeneca/Oxford vaccine is seen as particularly important to tackling the pandemic in the developing world, as it would be cheaper and easier to distribute.

“The basic message: that the overall efficacy across the trials that are reported here is about 70 percent but with a clear description of its uncertainty,” said Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine.

“The statistical uncertainty is that the efficacy could be as low as 55 percent or as high as 80 percent. The Pfizer/BioNTech and Moderna vaccines both have efficacies above 90 percent and are clearly more efficacious under trial conditions.”

Asked whether the half, then full dose regimen had been a mistake, Andrew Pollard, director of the Oxford Vaccine Group and chief investigator into the trials, told a news conference it had been “unplanned.”

The U.K. rolled out the first coronavirus vaccines to the public on Tuesday, making it the first country to inoculate people with a treatment that went through full testing.

Margaret Keenan, who turns 91 next week, made history as the world’s first person to receive the Pfizer-BioNTech vaccine outside of trial conditions. The vaccine was approved by the U.K. drug regulator last week.

“I feel so privileged,” she said. “It’s the best early birthday present I could wish for because I can finally look forward to spending time with my family and friends in the New Year after being on my own for most of the year.”

An 81-year-old man named William Shakespeare became the second person to get the vaccine. He was inoculated in Coventry. “No, no, not nervous at all,” said Shakespeare, who lives in Warwickshire. “I’m very apprehensive about the side effects … but there’s a small chance of that.”

Now, the vaccine will be given to front-line health workers, nursing home workers, and those ages over 80, before it is given more widely among the U.K. population.

On the eve of the vaccine being rolled out, U.K. Prime Minister Boris Johnson said it was a “huge step forward” in the fight against the pandemic. It will be the country’s biggest vaccination drive ever.

Johnson’s sentiment was echoed by NHS England’s Chief Executive Simon Stevens, who said on Monday that it was a “decisive turning point in the battle against coronavirus.”

Starting Monday, United Airlines is offering passengers COVID-19 tests before they leave to ten select destinations in Latin America and the Caribbean.

Certain countries require people to test negative before entering.

“It took a lot of time for us to look here in Houston, where to get the test,” said passenger Nicole Nogera.

Nogera from Honduras was visiting family and said she hopes to get tested through United Airlines next visit.

Passengers will get a COVID-19 test in the mail, they’ll administer it themselves and send it to a lab 72 hours before the flight.

Customers should expect to receive results within 24 to 48 hours.

Some countries require people to test negative to enter and this will help the passengers.

“We definitely want to be able to provide the customer the peace of mind that they know that these results are the ones required by the foreign government,” said Harry Cabrera with United Airlines IAH.

If someone tests positive, the airlines will work with the passenger to schedule a later flight.

The test costs $119.

The destinations include:

• Aruba (AUA)
• Belize City, Belize (BZE)
• Guatemala City, Guatemala (GUA)
• Lima, Peru (LIM)
• Nassau, Bahamas (NAS)
• Panama City, Panama (PTY)
• Roatan, Honduras (RTB)
• San Pedro Sula, Honduras (SAP)
• San Salvador, El Salvador (SAL)
• Tegucigalpa, Honduras (TGU)