DAKAR, Senegal — She had seen the conspiracy theories on Facebook, the endless anti-vaccine videos. Aminata Gueye shrugged it all off as silly chatter and signed up for an AstraZeneca shot courtesy of a World Health Organization-backed vaccine push called Covax.

Then came some news on the radio in Dakar: Some European countries had suspended use of the vaccine after regulators found apparent links to rare, but potentially fatal, blood clots. Gueye never went to the clinic.

“I was impatient to get vaccinated,” said Gueye, 38. “Now I have doubts.”

Lingering hesitation about the two-dose AstraZeneca coronavirus vaccine may shape the next phase of the effort to vaccinate the world.

The easy-to-store and affordable AstraZeneca shot, developed with the University of Oxford, is central to the WHO-led Covax initiative to distribute doses globally, particularly in developing countries left out of the global vaccine race.

Delays and questions about safety — especially the potential risk of blood clots for younger women — have brought new challenges to Covax just as the rollout was getting underway.

Some countries are now weighing safety information, looking for other vaccine options and trying to reassure weary publics — all at once. Covax may also struggle with the perception that it is relying on a vaccine that Europe and others have flagged for caution.

The U.S. decision to pause the use of Johnson & Johnson’s single-dose vaccine, also over a potential link to clots, could further raise alarm and undermine Covax, which plans to distribute the shot.

“AstraZeneca is the workhorse, the backbone of Covax,” said J. Stephen Morrison, director of the Global Health Policy Center at the Center for Strategic and International Studies. “If people turn away from it, that’s horrible and very dangerous for the global plan.”

“High-income countries,” including the United States, account for just 16 percent of global population but have locked up 53 percent of near-term vaccine supply, according to a team of researchers at Duke University’s Global Health Innovation Center.

The United States has fully vaccinated more than 25 percent of its total population. Covax aims to secure and distribute enough doses to vaccinate 20 percent of the populations of 92 participating countries by the end of the year.

AstraZeneca’s vaccine is expected to make up almost 50 percent of global vaccine supply in lower- to middle-income nations and a third of supply in low-income nations, according to research published last month by the Think Global Health program of the Council on Foreign Relations.

On Thursday, U.S. Secretary of State Antony Blinken called on allies to pledge more money to the Covax program, saying an additional $2 billion could help it reach 30 percent of the targeted countries by the end of the year.

Global caution grows

The WHO, the European Medicines Agency and the African Union stress that the benefits of the AstraZeneca shot outweigh its risks.

But doubts linger.

Denmark announced Wednesday that it will permanently suspend the use of the vaccine over concern about safety risks.

Cameroon withdrew its approval of AstraZeneca’s vaccine in March — days before a shipment was set to arrive — saying it wanted to see more studies of the vaccine in light of the early European probes into blood clot links. Liberia delayed its inoculation campaign by three weeks for the same reason.

Officials in Ivory Coast have blamed a “systematic refusal of the vaccine” for its behind-schedule vaccination campaign and have voiced worry about doses expiring on shelves.

Since March 28, when Lebanon began administering the first 33,600 AstraZeneca doses supplied by Covax, around a third of those given appointments at the government’s largest hospital have not shown up, said Firas Abiad, the hospital’s chief administrator.

When staffers called people to ask why, they cited their mistrust of the AstraZeneca vaccine, he said.

The fact that the vaccine’s staunchest defender, Britain, has decided to offer a different shot to those under 30 is particularly worrying to some Covax participants, experts said. Many lower-income countries tend to have younger populations and few vaccine options.

Some nations that can afford it may turn to Russia’s Sputnik V vaccine. Others may opt to work with China, despite recent news that a top Chinese health official said the efficacy of Chinese vaccines is “not high.”

After hitting pause on AstraZeneca, Cameroon this week received its first doses of China’s Sinopharm vaccine.

“The AstraZeneca vaccine remains an important public health tool against the covid-19 pandemic, and is effective at preventing severe cases, hospitalization and death,” said a spokesperson for the Geneva-based group Gavi, the Vaccine Alliance, which runs Covax alongside the WHO, and the Oslo-based Coalition for Epidemic Preparedness Innovations.

'Benefit of humanity'

Once seen as a top prospect in the vaccine race, the AstraZeneca vaccine’s development has been dogged by missteps: trial blunders, production delays and repeated public communication failures.

The European Union’s medical regulator, the European Medicines Agency, said April 7 that the AstraZeneca vaccine appeared to be causing rare clotting incidents, putting the risk of complications at 1 in every 100,000 doses administered.

The main concern is a type of clot called cerebral venous sinus thrombosis, or CVST, which blocks blood being drained from the brain. But patients also have presented with a low platelet count that makes the clots complicated to treat and increases the bleeding risk.

Out of 62 cases of the brain clots and 24 cases of clots in veins in the abdomen included in the European Medicines Agency’s initial safety review, 18 were fatal.

Andrew Pollard, an Oxford University professor and co-developer of the AstraZeneca vaccine, said the fact that the rare blood clots have been picked up shows safety systems are working.

“Coronavirus continues to be a huge threat to people throughout the world,” he said. “And we continue with our mission to support global vaccination, not for profit, for the benefit of humanity.”

Some countries appear to be weighing their vaccine options.

In Lebanon, the World Bank is funding the purchase of 2.1 million doses of the Pfizer vaccine, which now accounts for the vast majority of doses administered. Lebanese business leaders, meanwhile, are stepping in to import Russian-made Sputnik vaccines.

Beyond that, the country is expecting about 2 million AstraZeneca doses, a fifth of the required total for Lebanon to achieve herd immunity.

Any large-scale snub of the vaccine could have a major impact on the program’s success, said Abiad, the hospital administrator.

“If there’s a lot of hesitancy, we won’t be where we want to be, which is to get as many people as possible immunized,” he said.

'They wouldn't take one'

Many countries must tackle layers of skepticism about vaccines in general.

Lebanon’s economic collapse had already fueled existing mistrust of the government, compounding the impact of outlandish, baseless claims about mind control. A sectarian divide in Lebanon over which kind of vaccine to accept has also emerged.

In West Africa, hesitancy is partially fueled by history. Pfizer, for instance, triggered outrage and an enormous lawsuit after 11 children died in a 1996 drug trial in northern Nigeria — a collective memory that doctors say delayed polio eradication in the region after people rejected Western drugs.

A recent Afrobarometer poll of five West African nations — Benin, Liberia, Senegal, Niger and Togo — found that only 4 in 10 respondents said they would consider getting vaccinated against the coronavirus.

Countries like Cameroon, which suspended approval of AstraZeneca’s vaccine March 19, do not have many options.

“It’s a huge issue,” said Yap Boum, an epidemiologist in the central African country. “We don’t have access to Pfizer or Moderna.”

Still, he’d happily accept the AstraZeneca shot.

“The risk is so low,” Boum said. “But most of the people around me? They wouldn’t take one.”

Rauhala reported from Washington, Morris from Berlin and Sly from Beirut. William Booth in London contributed to this report.