"We have seen an explosion — a paper almost every day," said Salim Abdool Karim, an infectious disease researcher who co-chairs the COVID-19 advisory committee for South Africa.

Abdool Karim said it's crucial to test as many of these vaccines as possible against the variant now dominant in his country.

"The South African vaccine strategy calls for a diverse set of candidates," Abdool Karim said.

To judge which vaccines make most sense for his country, Abdool Karim said ideally he wants to know how the vaccine is performing against South Africa's variant in real-world conditions — what's known as clinical evidence.

For instance, a large study of Johnson & Johnson's vaccine found it was about 85% effective at preventing severe disease from the variant dominant in South Africa.

A smaller study of the Pfizer vaccine suggests it prevents as much as 100% of even mild cases from the variant in South Africa.

But on the less hopeful side: A study of the Novavax vaccine suggests that while it's about 89% effective at preventing mild disease from the original strain, it's about 50% as effective against the variant dominant in South Africa.

Worse still, a study of the AstraZeneca vaccine suggests it may have almost no ability to prevent mild disease from the variant in South Africa.

Several such experiments with the Moderna vaccine suggest that because the antibodies it generates are not as effective against the variant in South Africa, it takes eight times as many to knock out that strain as it does to neutralize the original version of the coronavirus.

If that's the case, Abdool Karim said, "the Moderna vaccine produces pretty high levels of antibodies — and so there is enough antibody still to neutralize the virus.".

Indeed, it's worth noting that lab experiments with antibodies from the Pfizer vaccine have found a similar increase in quantity is needed to quash the variant in South Africa.

While there's no hard-and-fast rule, Abdool Karim said when it gets to that point, "I don't know — I'm basically not confident about the vaccine at all.".

Kate O'Brien, director of the World Health Organization's Department of Immunization, Vaccines and Biologicals, said the issue for that variant is not so much what the studies of vaccine efficacy against it have found so far.

Because T cells get involved after an infection is underway, O'Brien said this suggests that even if a particular vaccine is not good at preventing infection by a variant, it may at least still end up substantially reducing the infection's severity

"We know [that type of protection] would be greater than it is for mild moderate disease," O'Brien said