At least a dozen countries, most of which are in Europe, have suspended the administration of the COVID-19 vaccine developed by pharmaceutical giant AstraZeneca and Oxford University following a series of reports that some people had developed blood clots, even though scientists have not been able to find concrete evidence that the shot itself was responsible. But these "side effects" have not been the first obstacle the British-Swedish tie-up has had to deal with. The vaccine, code-named AZD1222, has long been battling prejudice and criticism in the race against the COVID-19 pandemic – from doubts about its affordable pricing to demands eclipsing supply and a messy announcement of trial results that sowed seeds of suspicion about its efficacy.

Experts have expressed widespread worry that such unsubstantiated claims and suspensions are only hurting global immunization efforts. The European Medicines Agency (EMA) and the World Health Organization (WHO) say the data available doesn't suggest the vaccine caused the clots and that people should continue to be inoculated. Here's a look at what we know so far and what may be behind this baffling decision to halt.

What has happened so far?

The vaccine first faced suspensions on a batch basis when Austria announced on March 8 that a 49-year-old nurse had died from "severe bleeding disorders" days after receiving the jab. Estonia, Lithuania, Latvia and Luxembourg also suspended the use of doses from the same batch, which had been delivered to 17 countries and included 1 million vaccines.

Italy's medicines regulator AIFA on March 11 also banned the use of a batch as a precaution, triggering a similar decision from Romania.

However, the real alarm was first sounded in Nordic countries when Denmark became the first country to halt the AstraZeneca COVID-19 vaccine completely on March 11 as a precautionary measure over fears of blood clots. One person developed multiple clots and died 10 days after receiving at least one dose. Danish health authorities said the suspension would last for at least two weeks while the cases were investigated, even as they noted that, "It cannot be concluded whether there is a link between the vaccine and the blood clots."

Iceland and Norway followed suit on the same day, temporarily suspending its use. So did Bulgaria, Thailand and Congo a day later.

Bulgaria suspended the vaccine after a woman with several underlying conditions who recently received the jab died. An initial probe had suggested the woman died from heart failure and an autopsy found no link with the vaccination. Thailand's delay was rather abrupt, with Premier Prayut Chan-O-Cha stopped from getting the first jab. Meanwhile, the Democratic Republic of Congo said it would be postponing its mass vaccination campaign citing "precautionary measures."

On Saturday, Norwegian authorities reported that four people under the age of 50 who had gotten the AstraZeneca vaccine had an unusually low number of blood platelets, which could lead to severe bleeding. On March 15, a health worker in Norway died of a brain hemorrhage after receiving the vaccine, though no direct link to the jab has been established, marking the second such fatality within a few days in the country.

Ireland and the Netherlands then announced Sunday that they too were stopping their use of the AstraZeneca vaccine. Hugo de Jonge, the Dutch health minister, said it was just a matter of precaution. Italy's northern Piedmont region also suspended the use of the vaccine after the death of a teacher who had received it the day before, with a national postponement coming a day later.

On Monday, Indonesia and then France, Slovenia, Spain and Latvia followed. After saying last week they would continue with the vaccine, German officials said Monday they would suspend its use for now after fresh reports of new problems. Sweden joined the lot on Tuesday with the Public Health Agency (FHM) saying it was halting the use of the vaccine "as a precautionary measure" following reports of "suspected side effects," which it described as not serious but worth investigation.

However, in a U-turn, Thailand's prime minister on Tuesday received the AstraZeneca vaccine as the Southeast Asian nation lifted the suspension triggered by European nations.

When asked why Turkey did not opt to buy AstraZeneca vaccines, Health Minister Fahrettin Koca had said in December there were doubts around its efficacy and hence they wanted to wait for results. His statement drew ire from many scientists who said trial results spoke enough for its efficacy on top of it being an adenovirus vaccine based on a technology that has been used for decades, most notably in the tuberculosis jab. The country then announced that its domestic vaccines were underway in human trials.

In response to the suspensions of its vaccine, AstraZeneca said it had carefully reviewed the data on 17 million people who received doses across Europe and found there were 37 cases of people who developed blood clots. It said there was "no evidence of an increased risk" of blood clots in any age group or gender in any country.

"This is much lower than would be expected to occur naturally in a general population of this size and is similar across other licensed COVID-19 vaccines," the company said.

Is there any solid proof that the vaccine is responsible for blood clots?

No. The EMA says there is "no indication that the vaccination has caused these conditions." The EU regulator said its investigation was continuing and was conducting a "rigorous analysis" of all data. It said while its review was ongoing, the benefits of the AstraZeneca vaccine outweighed the potential side effects.

The EMA said it was convening its expert safety committee Tuesday and would hold another meeting on Thursday to decide on any necessary actions.

In Britain, where 11 million doses of the AstraZeneca vaccine have been administered – more than any other country – there have been reports of about 11 people who developed blood clots after getting a shot. None were proven to have been caused by the vaccine.

Some doctors pointed out that since vaccination campaigns started by giving doses to the most vulnerable people, those now being immunized are more likely to already have health problems. Experts say that could make it difficult to determine whether a vaccine is responsible.

Blood clots that form in the arms, legs or elsewhere can sometimes break free and travel to the heart, brain or lungs, causing strokes, heart attacks or a deadly blockage of blood flow.

Why did they suspend vaccinations?

Any time vaccines are rolled widely out, scientists expect some serious health issues and deaths to be reported – simply because millions of people are receiving the shots and problems would be expected to occur randomly in a group so large. The vast majority of these end up not being connected to the vaccine, but because COVID-19 vaccines are still experimental and there is no long-term data, scientists must investigate every possibility that the shot could have unforeseen side effects.

WHO’s chief scientist, Dr. Soumya Swaminathan, said officials at the U.N. health agency "don’t want people to panic" amid the reports. She noted that of the 300 million doses of coronavirus vaccines have been given to people globally, "There is no documented death that has been linked to a COVID vaccine."

Is this a concern with the other COVID-19 vaccines?

The EMA is currently examining whether COVID-19 shots made by Pfizer-BioNTech, Moderna Inc. and AstraZeneca might be causing low levels of blood platelets in some patients, a condition that could lead to bruising and bleeding.

Has AstraZeneca run into other trouble?

The vaccine has been approved for use in adults in more than 50 countries and has been proven to be safe and effective in research done in Britain, Brazil and South Africa. But there have been concerns raised about how the vaccine data has been released, and some European leaders, including French President Emmanuel Macron who called the vaccine "quasi-ineffective in the elderly," have questioned the vaccine's effectiveness in older people, citing a lack of information.

Britain first authorized the vaccine based on partial results that suggested the shots were about 70% effective in November 2020. But those results were clouded by a manufacturing mistake that led some participants to get just a half dose in their first shot – an error the researchers didn’t immediately acknowledge.

When it recommended the vaccine be licensed, the EMA estimated the vaccine's efficacy to be about 60%, approving the vaccine one month after the U.K. in January. By then, Pfizer-BioNTech and Moderna had already announced about 90% efficacy for their vaccines.

The data on whether the vaccine protected older adults were also incomplete, leading some European countries to initially withhold the shot from older people.

In the U.S., the Food and Drug Administration (FDA) suspended a study in 30,000 Americans for an unusual six weeks, as frustrated regulators sought information about some possible side effects reported in Britain.

Then in February, South Africa halted a trial including the vaccine because it said it failed to provide full protection against mild disease, all the while another and possibly immune-evading variant of the virus was circulating in the country, raising concerns about vaccine efficacy against new mutations.

However, later that month, a trial in Scotland found that even though the vaccine may not be as protective against milder forms of COVID-19 as the Pfizer-BioNTech one, it significantly reduced the risk of hospitalization. The risk fell 84% four weeks after receiving the first dose in a data-based vindication. This data has spurred countries to rethink the suspension. Germany and Belgium are currently reviewing data while France has said those aged 65-74 would continue to receive the AstraZeneca shot.

"All the data we have seen about the AstraZeneca vaccine suggests it's very safe and is saving people from dying of COVID," said Dr. Paul Hunter, a professor of medicine at the University of East Anglia. "But this may be more of a perception problem because every time there is a vaccine issue, we hear the name ‘AstraZeneca’ soon after."

Pricing and vaccine politics

AstraZeneca's shot, which is among the cheapest available at $2.15 a dose for the European Union, was billed as the vaccine of choice for poorer nations and the clot reports have had an impact beyond Europe.

In January this year, AstraZeneca came under fire after South Africa revealed that it had paid $5.25 – twice the EU's fee – for each dose of the vaccine that it would use to vaccinate 1.5 million health workers.

WHO Director-General Tedros Gebreyesus said that the greatest threat that most countries face now is a lack of access to vaccines. A New York Times analysis found that if all vaccine orders were delivered, the European Union could immunize its residents twice, the U.S. and the U.K. four times and Canada six times over.

"Almost every day, I receive calls from senior political leaders around the world, asking when their country will receive their vaccines through COVAX," said the WHO chief, referring to the WHO-led process through its partners to ensure an equitable distribution of jabs globally.

"Some of them are frustrated, and I understand why. They see some of the world's richest countries buying enough vaccines to immunize their populations several times over, while their own countries have nothing."

Meanwhile, the director-general of Italy's medicines authority AIFA branded the suspensions as a political decision, not a scientific one.

"We got to the point of a suspension because several European countries, including Germany and France, preferred to interrupt vaccinations ... to put them on hold in order to carry out checks. The choice is a political one," Nicola Magrini told daily la Repubblica in an interview.

Magrini said that the AstraZeneca vaccine was safe and that the benefit to risk ratio of the jab is "widely positive." There have been eight deaths and four cases of serious side effects following vaccinations in Italy, he added.

AIFA will take two to three days to collect all required data and once "doubts are cleared we can carry on at a faster speed than before," Magrini said.

What are experts recommending people do?

The WHO and the EMA – as well as regulators in several countries – say people should continue to be immunized and that the small risks of getting vaccinated far outweigh any potential harm.

"The safety of the public will always come first," said Britain's drug regulator. "People should still go and get their COVID-19 vaccine when asked to do so."

"Several more countries have suspended the use of AstraZeneca vaccines as a precautionary measure, after reports of blood clots in people who had received the vaccine from two batches produced in Europe," Tedros said.

"This does not necessarily mean these events are linked to vaccination, but it's routine practice to investigate them, and it shows that the surveillance system works and that effective controls are in place," he said, speaking at a twice-weekly WHO webinar.

WHO's chief scientist Soumya Swaminathan said: "We would, for the time being, recommend that countries continue vaccinating with AstraZeneca."

She said the vaccines have been in use for a few months now, and even though they are so rapidly scaled up, 300 million people already have received at least one dose.

Swaminathan noted that 2.6 million people have died of COVID-19, but after all those vaccination doses had been given to people across the world using different vaccines, "there is no documented death that's been linked to a COVID vaccine."

Meanwhile, a professor of experimental medicine at London's Imperial College said the decision to temporarily stop the rollout of the Oxford-AstraZeneca jab could be a "disaster."

Professor Peter Openshaw assured those who were waiting for an AstraZeneca-Oxford vaccine not to be worried, adding it was clear the benefits of being jabbed outweigh "the possible concern over this rather rare type of blood clot."

"It really is a completely one-sided argument statistically that we need to be vaccinating," he told the BBC on Tuesday. "I think it is a disaster for the vaccination uptake in Europe, which is already on slightly unsteady ground in some countries."