Novavax has published data showing promising signs for its booster shot's effectiveness against COVID-19 variants including Delta.

Vaccine manufacturer Novavax has published results showing its COVID-19 booster shot is effective against the Delta variant.

Novavax is yet to be approved in Australia, but the government has ordered 51 million doses primarily to be used as boosters.

The company on Friday published results from an ongoing study in Australia and the US.

It showed a booster shot six months after initial vaccinations increased neutralising antibodies four-fold.

For the Delta variant, there was a six-fold increase in cross-reactive functional antibodies compared to primary vaccines.

Novavax's Gregory Glenn said the research showed the booster could provide broad protection against known and emerging variants.

"The strong results from this study reinforce our confidence in the potential for a booster dose," he said.

The company intends to submit its study for peer review once it is completed.

Another vaccine manufacturer, Moderna, is tweaking its vaccine to better combat Delta.

"I think we're pretty worried now. If you look at the Delta variant, it took a surprising step," Moderna's Stephen Hoge said.

"I don't think any of us, three or four months ago, were going to predict something that was this many times more infectious."

If approved for use in Australia, the company is due to supply 25 million doses from late 2021.

Of these, 10 million would be doses of its current jab and 15 million booster or variant-specific shots.