In some countries, citizens are grumbling about the inefficient rollout of COVID-19 vaccines. It's unclear exactly when doses will be available. Websites for appointments keep crashing. Lines are long.

And then there are the 130 countries that "are yet to administer a single dose," according to UNICEF. That's 2.5 billion people who so far have been completely shut out of the global vaccine campaign.

Many of these low- and middle-income countries are urgently shopping for vaccines. But with just a handful of pharmaceutical manufacturers having successfully come up with vaccines, doses are incredibly scarce.

Lack of supply isn't the only issue. Now there are questions about whether the vaccines that researchers started working on a year ago will work against new variants of the virus that have emerged in the last few months.

And last week South Africa announced it was going to delay the start of its national COVID vaccination programs. The move came after a small study showed the AstraZeneca shot failed to prevent some mild and moderate infections.

The vast majority of new infections in South Africa — roughly 90% — are being caused by the B.1.351 variant. This strain, which was first identified in southern Africa, has now been found at much lower levels all over the world.

South Africa's decision to put AstraZeneca on hold and switch to Johnson & Johnson has raised concerns about immunization plans globally.

The AstraZeneca vaccine, produced by the U.K.-based pharmaceutical company and Oxford University, is currently the leading candidate to vaccinate people in low-resource countries. It's relatively cheap, doesn't need to be kept at super-low temperatures and currently is being manufactured in plants around the globe. Nations have signed delivery contracts for more than 2 billion doses — nearly twice the volume being promised this year by the next leading manufacturer, Pfizer BioNtech.

Some countries, including Bangladesh and El Salvador, have only been able to secure contracts with AstraZeneca (not counting doses they hope to get from the COVAX program, which is acquiring various vaccines to share equitably among 190 economies worldwide).

In Latin America, governments are banking on hundreds of millions of doses of the AstraZeneca vaccine.

"I think that there is a lot of concern," says Sergio Litewka, a professor of surgery and ethics at the University of Miami, about whether countries in Latin America are going to be able to get enough effective vaccine to inoculate their populations.

"Listen, Latin American countries are purchasing Sinovac, produced by a Chinese pharmaceutical company. Sinovac has a 50% efficacy," he says – far lower than other vaccines. Nonetheless, in the past few weeks nations in the Americas have been rushing to sign contracts in part because they can't get their hands on anything else. The countries "say, well, at least 50% is better than nothing," Litewka notes.

Trials of Sinovac's product in Turkey showed it to be up to 91% effective — but a more recent study out of Brazil found it to prevented infections only half the time.

The World Health Organization's chief scientist Soumya Swaminathan says it's not just AstraZeneca product that is less effective against the new mutant strains.

"Vaccine manufacturers and others are currently evaluating the vaccines to see how effective they are against these new variants," Swaminathan says. "We've also seen from the studies of Novavax and J&J that their efficacy against the 501Y.V2 variant [the one originally found in South Africa] was reduced compared to the previous wild type virus." But those studies have also shown that all these vaccines appear to prevent severe cases of COVID.

"There hasn't been a single [severe] case reported in any of the trials," she says. "Our goal in the first wave of vaccinating people is to protect those at highest risk from severe disease, hospitalization and death."

"For the vast majority of the world the benefit of using a vaccine outweighs the risks," she adds.

But for countries still trying to get their hands on vaccine, the new variants make the process even more complicated. Should countries that have contracts with AstraZeneca start shopping for a different product? Would it be better to wait for one product as opposed to another?

"As of today we don't have anything," says professor Jorge Martin Rodríguez Hernández at the Institute of Public Health at Javeriana University in Bogotá, Colombia.

"The government has created a goal here of [administering] 55 million doses this year," he says. But so far mass immunizations have yet to start, and Colombia hasn't vaccinated anyone outside of clinical trials. "I don't know if the governments will accomplish this plan," Rodríguez says. He thinks it will take until the end of 2022 to get Colombians fully vaccinated rather than the end of 2021 as the government has promised.

Sandra Pitta, a researcher with CONICET, the National Scientific and Technical Research Council in Argentina, says when it comes to getting COVID vaccines, Latin American countries have had to improvise.

"Some improvise less or more than others," she says. At the end of 2020 when Argentina found itself without access to any doses, the government turned to Russia and agreed to use the Sputnik V vaccine.

Pitta says countries are so desperate to get vaccine that they're willing to accept claims of safety and efficacy about the products even without seeing data to back up those claims.

She says Sputnik was given the green light in Argentina before any significant data about the Phase 3 clinical trials for the inoculation had been made public.

The pandemic was so bad in Argentina, Pitta says, that many Argentinians accepted Sputnik out of desperation.

"Doctors and nurses, they are seeing their colleagues get sick and this was like magic — a magic wand. Everybody was so desperate that a lot of people accepted it," she says.

And now that there's concern about how the AstraZeneca vaccine and others work against the new variants. Pitta says Argentina has no information about whether Sputnik is effective or not against the mutant strains. Although the Russian developer of Sputnik says it's developing a vaccine that would provide immunity against "all" coronavirus strains. Pitta is skeptical. "It's magic again," she scoffs, saying that this is just as opaque as Russia's original pitch that Sputnik is more than 90% effective. "Where's the data?"

But faced with a pandemic that's already inflicted so much pain, and the prospect of vaccines being months or even years away, health officials in many countries are willing to go with whatever they can get.

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