AstraZeneca Plc reported a slightly lower efficacy rate for its COVID-19 vaccine after the results of an American clinical trial were criticized as outdated, raising further questions over the embattled shot and potentially delaying its approval in the U.S.

The vaccine was 76 per cent effective at protecting volunteers against the coronavirus, the U.K. drugmaker said in a statement on Thursday. That compares with an earlier estimate of 79%, which was based on data gathered through Feb. 17.

The latest twist created another layer of uncertainty for a product already facing dwindling support in Europe following months of confusion and missteps. Although the vaccine is still expected to play a crucial role in curbing the global pandemic as governments around the world have ordered millions of doses, the repeated blunders risk further eroding public confidence in the immunization.

“The difference between 76 per cent and 79 per cent is a rounding error, probably just a handful of cases,” said Paula Cannon, a professor of molecular microbiology and immunology at the University of Southern California’s Keck School of Medicine, where she leads a research team that studies viruses. “But it’s so important for us to be completely transparent and accurate because we are building public trust.”

The findings were based on 190 symptomatic cases that developed among the 32,449 volunteers who participated in the trial, which includes 49 cases that weren’t counted as part of the initial analysis. The company didn’t disclose how many of those infections occurred in the group that received the vaccine and how many in the one administered a placebo.

There are another 14 people with probable or possible infections, so the total number of cases may still change -- which may cause the overall efficacy rate to adjust again.

Too Rushed?

Astra shares were little changed in London trading. The Cambridge, England-based company emphasized that the marginal changes in protection rate did not detract from the vaccine’s overall efficacy.

“The primary analysis is consistent with our previously released interim analysis, and confirms that our COVID-19 vaccine is highly effective in adults,” said Mene Pangalos, the company’s executive vice president of BioPharmaceuticals research and development.

“We look forward to filing our regulatory submission for Emergency Use Authorization in the U.S. and preparing for the roll-out of millions of doses across America,” he said.

Some of Astra’s challenges reflect the unprecedented urgency of COVID-19 vaccine development, in which years-long processes have been compressed into several months.

“The problem with AstraZeneca is that they have had a couple of black eyes,” said Cannon. “I don’t think it’s malicious, it just reflects trying to get a vaccine out in the middle of a pandemic.”

Older Patients

In an unusual move earlier this week, a group of experts working with the company on the safety of its U.S. clinical trial contacted government agencies late on Monday to express concern about data that AstraZeneca made public just hours earlier. The company responded by saying it would soon release fresh estimates.

The vaccine developed with the University of Oxford was 100% effective at preventing severe disease and hospitalizations.

Older volunteers got the most benefit from the shot, with 85% of those aged 65 and over protected against symptomatic COVID. That finding is particularly important as it shows the vaccine benefits those most at risk from the virus, said Paul Griffin, director of infectious diseases at Mater Health Services and associate professor of medicine at the University of Queensland in Brisbane.

The U.S. trial data was seen as important because earlier research conducted in the U.K. and Brazil sowed confusion by producing two different efficacy readings. Plus, those tests failed to include enough elderly people to establish efficacy for that crucial patient group.

“Getting that message out to the community has been challenging, when other vaccines have had a much more simple message,” Griffin said. “Having a lot more complex one -- with lots of different facets being released at different times -- also has confused people. It has made some people apprehensive.”

--With assistance from Dong Lyu.