What you need to know about the numbers behind one of the latest COVID-19 vaccines

HIV-negative South Africans who received two-shots of the Novavax vaccine were about 60% less likely to develop mild to severe COVID-19, University of the Witswatersrand researchers announced late Thursday.

But early study results show that jab offered less protection to people living with HIV.

The Novavax trial in South Africa was carried out among roughly 4 400 people in six provinces. As part of the study, half of the participants were randomly assigned to receive the real vaccine while the other half were given a placebo of saline solution. Importantly, the study took place at a time when the new variant of COVID-19 discovered in South Africa was already accounting for the majority of new infections.

The study found that the Novavax jab reduced the risk of COVID-19 disease in HIV-negative people by 60%. HIV-negative people accounted for 4 160 out of the 4 400 people in the study.

“The trend we’re seeing is that all of the COVID-19 vaccines that have shown efficacy have shown higher efficacy for more severe disease.”

Shabir Madhi

A number’s game: Why the vaccine’s overall reported efficacy dropped

But the vaccine was less effective when tested among a group of 240 people living with HIV.
When the efficacy results of these two groups were combined, results among the HIV-positive group brought the jab’s overall effectiveness down to 49.4%, according to the results released in a press release. This data has not yet been peer-reviewed.

Lead researcher and executive director of the Vaccines and Infectious Diseases Analytics Research Unit (VIDA) at Wits University Shabir Madhi says it’s still unclear why the vaccine performed less well in HIV-positive people. Still, he cautions against reading too much yet into results among people living with HIV.

Because the number of HIV-positive people included in the study was so small, Madhi explained, even a single case of COVID-19 in this group would statistically affect how well statistically the jab would be shown to work.

“I think we should just avoid reading anything into the results among HIV-positive people,” Madhi warned. “[Those results] were among 240 individuals and [in a sample so small] even one case in either direction can [change] that efficacy estimate.”

Why this trend in COVID-19 vaccine research is good news

In the United Kingdom, the Novavax vaccine reduced the risk of mild to serious COVID-19 by 89%. Why were South African results so much lower? Madhi says it’s a testament to how much less sensitive the COVID-19 variant found in South Africa may be to vaccines.

Although further international studies are ongoing to confirm just how well the vaccine works, Madhi expects that the jab could one day reduce the risk of serious illnesses and hospitalisations due to COVID-19 by even more than 60% in people who get the vaccine. If it does, it will continue a trend in which COVID-19 vaccines that have been proven to work continue to provide strong protection against serious COVID-19 disease and hospitalisations.

Madhi explains: “The trend we’re seeing is that all of the vaccines that have… shown efficacy have shown higher efficacy for more severe disease.”