CHICAGO (Reuters) – Novavax Inc’s COVID-19 vaccine had efficacy of 43% against infections caused by the South African variant in a group that included people with and without HIV, and 51% in people who were HIV negative, according to a new analysis published on Wednesday.

The post-hoc analysis was published in the New England Journal of Medicine along with full data from the company’s trial in South Africa, which included nearly 2,700 volunteers who had not been previously infected with the coronavirus.

Results announced in January showed efficacy of 49.4% against symptomatic COVID-19 in the South African trial looking at a mixture of the original virus and the South African variant, and 60.1% among those who were HIV-negative.

The study also showed that prior infection with an earlier version of the virus did not reduce the risk of COVID-19 caused by the South African variant among people who got placebo shots.

The average age of trial volunteers was 32. Most cases were mild-to-moderate.

The study did not provide data on efficacy of the Novavax vaccine in preventing severe disease or hospitalization, “one of the most important factors in determining the usefulness of a vaccine,” said Dr Peter English, a retired consultant in communicable disease control and former chair of the British Medical Association’s Public Health Medicine Committee.

“Most vaccines are less effective at preventing mild disease than they are at preventing severe disease; so this vaccine could turn out to be much more effective at preventing hospital admissions and deaths – we simply don’t know, yet,” said English, who was not involved in the study.

The trial’s main goals were to test how the vaccine performed in people who were HIV negative, as well as those who were HIV positive and medically stable. The study excluded other volunteers taking drugs to suppress the immune system or those with auto-immune diseases, which could reduce vaccine efficacy. Among those evaluated, 94% were HIV negative and 6% were HIV positive.

Novavax has said it is making new versions of its vaccineto protect against emerging virus variants. The company is expected to report data from its ongoing late-stage study in the United States and Mexico in the second quarter.

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