AN new role launched this week, it suggested that a vaccine developed by AstraZeneca and the University of Oxford not only protected clinical trial participants from developing the disease, but could also significantly reduce transmission of the virus that causes the disease.
In the recent explosion of data on Covid-19 vaccines, this suggestion stood out. The question of whether Covid-19 vaccines reduce transmission has been critical and unanswered, creating uncertainty about whether people who have been vaccinated can still be infected and transmit SARS-Cov-2, the virus that causes Covid, to those that have not yet been vaccinated.
Media reports have seized a reference in the Oxford researchers’ article that a single dose of the vaccine reduced positive test results by 67%, pointing to it as the first evidence that a vaccine could prevent transmission of the virus. But the article, which has not yet been peer-reviewed, does not prove or state this – although it points to the possibility.
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Instead, it showed that fewer people carried the virus as a result of vaccination. Fewer people with viruses, the researchers argued, would amount to a reduction in the amount of viruses circulating in a community.
“These data indicate that [the AstraZeneca vaccine], used in authorized schemes, can have a substantial impact on transmission, reducing the number of infected individuals in the population ”, wrote the authors.
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If a person’s test is negative, Andrew Pollard, one of the study’s authors and a professor of pediatric infection and immunity at Oxford University, told STAT by email, “then it is a reasonable assumption that they cannot transmit”.
But it is a big and unjustified leap, external experts agree, from this suggestion to prove the decrease in the transmission of vaccinated people.
“The study showed a decrease in [viral] spillage, not ‘transmission’, ”said Carlos del Rio, professor of infectious diseases at Emory University School of Medicine. “The end result is, no, you can’t draw a conclusion or a straight line.”
Kathleen Neuzil, director of the Center for Vaccine Development at the University of Maryland School of Medicine, agreed.
“There are many, in my opinion, moving variables to make much of a single result here,” she said. “The trend, however, is consistent and is in the right direction.”
Researchers studying the Oxford-AstraZeneca vaccine are not looking to see whether people who received the vaccine are less likely to transmit the SARS-2 virus. However, they asked all volunteers in one of their studies, conducted in the UK, to spin a cotton swab into the nostrils weekly to test for infection, using the polymerase chain reaction or PCR. None of the other large vaccine studies have taken this step as part of their protocols.
What they found was that, while a single dose of the two-dose vaccine reduced positive test results by 67% – the number that many media reports have seized – two doses reduced positive tests by a smaller amount, 49.5% .
This decline is difficult to explain, and the Oxford article does not attempt to do so.
“Biologically, how do we explain this? Does this signal that these numbers cannot be directly compared? ”Asked Natalie Dean, a vaccine-focused biostatistics at the University of Florida, who noted that booster doses often improve the performance of a vaccine, not corrode it. “You only have certain expectations and this is difficult to understand. How would the effectiveness decrease after receiving reinforcement? “
More research is needed on the AstraZeneca vaccine, said del Rio, adding that current studies, first reported in in December, do not answer many important questions. Diverging perspectives on the study – a compilation of four differently designed studies – have divided the world, as the UK, much of Europe and India have approved the vaccine, while the US and Switzerland await more information.
A large US study is being conducted with the US government, similar to those carried out by Moderna and Johnson & Johnson. The results may come as early as the end of February.
“I just keep telling my volunteers that the US study is the one that will be the definitive study to tell us how this vaccine works against severe, hospitalized and symptomatic Covid,” said Anna Durbin, vaccine researcher at Johns Hopkins Bloomberg School of Public Health who is involved in the American study of the AstraZeneca vaccine.
Unfortunately, in a pandemic, there is nothing more difficult than waiting.