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Novavax's protein-based COVID-19 vaccine candidate demonstrated nearly 90% efficacy against COVID-19, in a cohort where half the cases were due to the new U.K. variant, the company said on Thursday.

However, the news was less positive from a trial in South Africa, where overall vaccine efficacy was under 50% against cases largely due to the trickier South African variant.

Moreover, findings from that study suggested prior infection with the wild-type strain may not fully protect against new infection from the variant strain.

The vaccine is a recombinant version of the coronavirus spike protein, produced in insect cells, and thus represents a relatively conventional technology that also differs from the COVID-19 vaccines now in distribution.

In an interim analysis of a phase III trial conducted in Great Britain, 89.3% efficacy (95% CI 75.2%-95.4%) was achieved against PCR-confirmed symptomatic COVID-19. This was based on 62 COVID-19 cases among some 15,000 participants, with 56 observed in the placebo group and six in those receiving the active vaccine. Of these 62 cases, only one was classified as severe.

And Novavax touted a preliminary analysis indicating that 32 of the 62 cases were the so-called U.K. coronavirus variant. Based on PCR testing, vaccine efficacy was 95.6% against the original COVID-19 strain and 85.6% against the U.K. variant strain in the post-hoc analysis.

The phase III study enrolled adults ages 18-84, including 27% age 65 and older. The primary endpoint was PCR-confirmed symptomatic COVID-19 at least 7 days after the second of the two-dose regimen in serologically negative participants.

Preliminary safety data indicated that adverse events were infrequent and mostly comparable between groups. The manufacturer said it plans to share full results of the trial via pre-publication servers and submit them to a peer-reviewed journal.

South African Variant More Elusive

Novavax also provided topline results from a phase IIb trial in South Africa of the vaccine in which about 6% of participants were HIV-positive. Here, the vaccine showed 60% efficacy (95% CI 19.9%-80.1%) in the HIV-negative population; it was much less effective in the HIV-infected participants, leading to overall vaccine efficacy of 49.4% (95% CI 6.1%-72.8%) for the entire study population.

There were 29 COVID-19 cases in the placebo group (one severe) and 15 in the vaccine group (none severe).

Most of these cases involved the so-called South African variant that includes multiple mutations in the coronavirus spike protein. Preliminary sequencing data indicated that the variant was responsible for 25 of 27 cases analyzed.

The trial enrolled over 4,400 patients starting in August, with data from September through mid-January. However, about a third of patients enrolled were seropositive for COVID-19 infection at baseline. Based on when the trial started, pre-trial infections were thought to be caused by the original non-variant strain, while infections occurring during the trial were variant virus.

"These data suggest that prior infection with COVID-19 may not completely protect against subsequent infection by the South Africa escape variant," the manufacturer said.

The release noted that plans are underway to develop a booster dose and/or combination bivalent vaccine for the new strains, and to test these new vaccines in the second quarter of 2021.

Novavax uses nanoparticle technology and a proprietary adjuvant with the recombinant protein. The vaccine can be stored at 2º to 8º C and shipped in a ready-to-use liquid formulation.

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