Alex Sigal, a professor at the Africa Health Research Institute, detailed some results from initial experiments on Twitter, revealing there was "a very large drop" in neutralisation of the Omicron variant relative to an earlier Covid strain.
Neutralising antibodies are an indicator of the body's immune response.
There are grave concerns about how well existing vaccines will work against the Omicron strain, as its spike proteins differ dramatically to the original coronavirus variant.
Results better than expected
But the study did offer some hope, showing blood from people who had received two doses of the vaccine and had a prior infection were mostly able to neutralise the variant.
This means booster doses could be key to stopping infections.
"These results are better than I expected. The more antibodies you've got, the more chance you'll be protected from Omicron," Sigal said on Twitter.
According to an official manuscript, which has not yet been peer reviewed, the lab tested blood from 12 people who had been vaccinated with two doses of the Pfizer-BioNTech vaccine.
Blood from five out of six people who had been vaccinated as well as previously infected with Covid-19 still neutralised the Omicron variant.
However, the lab has not tested the variant against blood from people who had received a booster dose as they are not yet available in South Africa.
The lab observed a 41-fold decline in levels of neutralising antibodies against the Omicron variant.
Sigal added the data will likely be adjusted as more experiments are completed.
There is no indication at this stage whether the vaccine is less able to prevent severe illness or death.
Scientists also believe other kinds of cells such as B-cells and T-cells, which are stimulated by the vaccines, could offer protection.
The Omicron variant, first detected in southern Africa last month, has triggered global alarm amid fears of another surge in infections.
The World Health Organisation classified Omicron on November 26 as a "variant of concern", but said there was no evidence to support the need for new vaccines.
There is not significant data yet on how other vaccines from Moderna, Johnson & Johnson and other drugmakers hold up against the new variant.
TGA approves Moderna vaccine as booster
The study comes as Australia’s national medicines regulator approves the Moderna vaccine to be used as a booster shot in adults.
In a statement, the Therapeutic Goods Administration (TGA) confirmed the provisional approval means that individuals aged 18 years and older may receive a third dose at least six months after their initial two jabs.
The Australian Technical Advisory Group on Immunisation (ATAGI) is now considering whether to give Moderna booster shots the final green light.
Pfizer is currently the only booster available under the national program, which launched a month ago.
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