AstraZeneca's repeated missteps in reporting vaccine data coupled with a blood clot scare could do lasting damage to the credibility of a shot that is the linchpin in the global strategy to stop the coronavirus pandemic, potentially even undermining vaccine confidence more broadly, experts say.

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The latest stumble for the vaccine came on 23rd March when American officials issued an unusual statement expressing concern that AstraZeneca had included outdated information when it reported encouraging results from a US trial a day earlier. That may have provided an incomplete view of the efficacy data, according to the statement.

AstraZeneca responded that the results, which showed its shot was about 79% effective, included information through February 17 but appeared to be consistent with more up-to-date data. It promised an update within 48 hours. "I doubt it was (US officials') intention to deliberately undermine trust in the AstraZeneca vaccine", said Dr Paul Hunter, a professor of medicine at the University of East Anglia. But this will likely cause more vaccine hesitancy.

Even if the damage is limited to AstraZeneca itself, it would have far-reaching effects since the shot is cheaper and easier to store than many of its rivals' and thus is expected to be widely used in the developing world. International health agencies have repeatedly said the vaccine is safe and effective, but it's not the first time the company has run into problems with public trust. Partial results from its first major trial which the UK used to authorise the vaccine were clouded by a manufacturing mistake that researchers didn't immediately acknowledge. Insufficient data about how well the vaccine protected older people led some countries to initially restrict its use to younger populations before reversing the course. US officials suspended an AstraZeneca study for an unusual six weeks while they sought details about problems reported in Britain before deciding the vaccine wasn't to blame. Meanwhile, the European Union has complained about delays in vaccine deliveries from the company.

Then last week, more than a dozen countries temporarily halted their use of the AstraZeneca shot after reports of rare blood clots in some people who received it. The European Medicines Agency concluded the shot did not increase the overall incidence of clots, but the unwanted attention appears to have left a mark. In Norway, a top official warned on 22nd March, it might not be able to resume its use of the vaccine because so many people were rejecting it.

"People clearly say that they do not want the AstraZeneca vaccine", Marte Kvittum Tangen, who heads a Norwegian doctors' association, told broadcaster NRK. Last week in Bucharest, Romania, vaccination coordinator Valeriu Gheorghita said 33,000 AstraZeneca immunisation appointments had been cancelled in 24 hours and that about a third of the 10,000 people scheduled to receive the vaccine did not show up.  

"This is, unfortunately, more about perception than it is science We have now seen, on several parameters, that the AstraZeneca vaccine provides protection and is safe. But the narrative for the public has not been as clear, said Dr Bharat Pankhania, an infectious diseases specialist at UK's University of Exeter. France is a prime example of the confusion. French President Emmanuel Macron initially suggested the vaccine wasn't effective for older people, before backtracking. Still, France only authorised AstraZeneca's vaccine for use in adults aged 65 and under, citing a lack of data. Then the government changed its mind, based on new data, and said it's fine for all adults. But when there were reports of rare blood clots in some vaccine recipients, the government suspended the use of the shot altogether. When France restarted AstraZeneca, it banned the shot for anyone under 55.

At a European Parliament committee Tuesday in Brussels, Sandra Gallina, head of the European Commission's health directorate, described the situation with AstraZeneca as "a shame". She said, "The struggling vaccination campaigns across Europe were made much more difficult by the bad performance of AstraZeneca. The company has blamed delays in its deliveries on production issues. Even if the drugmaker clears up the latest misunderstanding, it could have a lasting impact".

Julian Tang, a virologist at the University of Leicester, pointed to the decades-old controversy over the measles vaccine as a cautionary tale. "There was absolutely no evidence to prove the (measles, mumps and rubella) vaccine caused autism", he said. But despite the retraction of the paper that made that claim, Tang said some people still worry about the vaccine. The tepid support for the AstraZeneca vaccine in Europe stands in contrast to governments in the developing world that are desperate for supplies. Dr Bruce Aylward, a senior adviser at the World Health Organization, said "The UN agency has a long list of countries very keen to get the shot as soon as possible. We simply cannot get enough of it", he said. But some experts have worried that the scepticism in Europe could eventually cast a pall over the vaccine worldwide. They suggested one measure that could reassure a jittery public: a green light from the US Food and Drug Administration.

AstraZeneca said it would be submitting its data to the FDA within weeks.