The Covid-19 vaccine by Sinovac Biotech is not able to produce adequate antibodies to neutralise the highly mutated coronavirus strain Omicron, according to new research from the University of Hong Kong.

Both the Sinovac vaccine and another by Pfizer-BioNTech produced "inadequate" antibody responses to the variant, HKU scientists said in a statement on Tuesday night, calling for the use of boosters to potentially enhance protection.

Beijing-based Sinovac on Wednesday responded with a statement saying a third shot of its CoronaVac vaccine could improve its ability to neutralise Omicron, citing its own laboratory studies.

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Sinovac and world's Covid-19 vaccine makers 'ready' to produce Omicron jab

The company did not provide details on the levels of virus-fighting antibodies produced or how long after vaccination the measurements were taken, but said some 94 per cent - or 45 out of 48 - people who had taken a third dose had detectable levels.

Roughly seven out of 20 were able to produce detectable antibodies against Omicron after the typical two-dose regimen, Sinovac said.

While it is difficult to compare studies which may use different methods, the company's findings contrasted with those released on Tuesday in a preprint paper by the HKU researchers.

The university study also examined neutralising antibodies, one arm of immune response that serves as a rough marker for protection against infection.

Of 25 people who received a full two-dose course of CoronaVac, none were found to have detectable levels of neutralising antibodies, according to study author and top infectious disease expert Yuen Kwok-yung and his team.

For those fully vaccinated with Pfizer-BioNTech, five out of 25 people had detectable levels. But those levels were 35 to 40 times lower than against the original coronavirus strain, and saw "significant" reduction as compared to response to previous variants Beta and Delta.

"The Omicron variant virus was able to reduce the effectiveness of two doses of Covid-19 vaccine, particularly against CoronaVac. Therefore, Covid-19 vaccine recipients or even those recovered Covid-19 patients may be at a higher risk of breakthrough or reinfection," the scientists said in a statement on the HKU website.

"The public is advised to get a third dose of the vaccine as soon as possible while awaiting for the next generation of more matched vaccine," they wrote.

The findings, which only looked at one aspect of immune system response, add to other emerging data suggesting that booster doses may be needed to enhance vaccine protection against the strain.

Omicron: what we know so far about symptoms, transmissions, vaccines

Pfizer and BioNTech last week announced that preliminary laboratory studies demonstrated three doses of their vaccine could neutralise the Omicron variant, while two doses showed significantly reduced neutralisation levels.

A number of other laboratory studies in recent days have suggested significant decreases in neutralising activity against Omicron versus previous strains, though experts believe existing vaccines will largely retain protection against severe disease and death.

Neutralising antibodies are used as a marker for vaccine protection against infection, but do not correlate clearly with vaccine effectiveness overall.

Other arms of the immune system are thought to play a role in protecting against severe disease for some vaccines, but are more difficult to measure.

Scientists say it is too early to tell if vaccines will need to be revised in response to Omicron, as they await real-world data on how well people are protected against the strain.

A number of major vaccine makers, including Sinovac and Pfizer-BioNTech, have said they are proactively working on developing versions of their vaccine tailored for Omicron to roll out if needed.

Copyright (c) 2021. South China Morning Post Publishers Ltd. All rights reserved.

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