Coronavirus Vaccines

Vaccine may not stop virus transmission, says Moderna chief scientist

SINGAPORE • Moderna's vaccine might prevent most people from getting sick with Covid-19, but it may not stop transmission, said the firm's chief scientist.

"They (clinical trial results) do not show that they prevent you from potentially carrying this virus transiently and infecting others," said Moderna's chief medical officer Tal Zaks in a recent interview on documentary news website Axios.

However he does "believe" the vaccine can reduce transmission of the virus from one person to another. It just has not been tested in its trials, he said.

"Do I believe that it reduces transmission? Absolutely yes, and I say this because of the science," he said.

"But absent proof, I think it's important that we don't change behaviours solely on the basis of vaccination," said Dr Zaks, suggesting that people continue to use non-medical methods like wearing masks and social distancing to stem the spread of the coronavirus.

The American biotechnology company had earlier announced that its vaccine was 94.5 per cent effective at protecting people from Covid-19.

Dr Zaks urged the public not to "over-interpret" the vaccine's promising trial results and assume life could go back to normal after adults are vaccinated.

Earlier this month, Dr Ugur Sahin, chief executive of Germany's BioNTech, which is behind the vaccine developed with Pfizer, said he was confident its candidate could prevent transmission "maybe not 90 per cent but maybe 50 per cent", reported Agence France-Presse.

Like the Pfizer-BioNTech vaccine, the Moderna vaccine uses a synthetic version of coronavirus genetic material, called messenger RNA or mRNA, to program a person's cells to churn out many copies of a fragment of the virus. That fragment sets off alarms in the immune system and stimulates it to attack, should the real virus try to invade.

Although a number of vaccines using this technology are in development for other infections and cancers, none has yet been approved or marketed.

Bloomberg reported that various public opinion surveys have found that a significant proportion of people are sceptical of vaccines, or at least have some misgivings.

But if a vaccine does not meaningfully reduce viral transmission, it leaves the un-immunised relatively vulnerable. And if the coronavirus continues to be passed from person to person, it has a better chance to keep mutating and potentially evade human defences.

According to Science Magazine, Pfizer and Moderna tested only trial members who developed potential Covid-19 symptoms.

Without knowing if others members of the trials might have been asymptomatic spreaders, there was no way to say for sure whether the vaccine prevented them from infecting others.

"When we start the deployment of this vaccine, we will not have sufficient concrete data to prove that this vaccine reduces transmission," Dr Zaks said.

It is not yet possible to find out whether vaccines are able to produce a sterilising immunity based on current clinical trials as they have not been set up to provide that information, said Bloomberg.

Earlier this week, it was reported that the Covid-19 vaccine developed by Oxford University and AstraZeneca shows an average 70 per cent effectiveness in preventing those vaccinated from falling ill with the virus.

While this is lower than the figures for the Pfizer-BioNTech and Moderna vaccines, developers of the Oxford-AstraZeneca vaccine said there are early indications it might help stop transmission of the disease.

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A version of this article appeared in the print edition of The Straits Times on November 27, 2020, with the headline Vaccine may not stop virus transmission, says Moderna chief scientist. Subscribe