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Study: Fully vaccinated people with "breakthrough" COVID Delta infections carry as much virus as the unvaccinated

Breakthrough COVID deaths extremely rare
Breakthrough COVID deaths extremely rare 05:14

A study by University of Oxford scientists has found that people who contract the Delta variant of COVID-19 after being fully vaccinated carry a similar amount of the coronavirus as those who catch the disease and have not been inoculated. The researchers stressed that vaccination still offers good protection against catching the disease in the first place, and protects against getting seriously ill with it.

The survey of real-world U.K. data indicates, however, that vaccinated people with "breakthrough" infections could still pose a significant infection risk to those who have not been vaccinated.

"With Delta, infections occurring following two vaccinations had similar peak viral burden to those in unvaccinated individuals," the study, which has not yet been peer reviewed, concludes. Viral "burden" or viral load refers to how much coronavirus-infected people carry and thus "shed," or release into the environment around them, where it can potentially infect others.

The survey compared U.K. government data on more than 380,000 people who tested positive for the coronavirus between December and May of this year, when the first-discovered Alpha variant accounted for most of the cases in Britain, with figures for more than 350,000 people infected over the following four months, when Delta was dominant.

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Oxford's lead researcher, Dr. Sarah Walker, told The Telegraph that the study shows two doses of the Pfizer/BioNTech, Moderna or AstraZeneca vaccines "are still protective. You are still less likely to get infected - but if you do, you will have similar levels of virus as someone who hasn't been vaccinated at all."

The data used for the study do not show how likely it is that a fully vaccinated person with the Delta variant can pass on the infection to another individual, compared to an unvaccinated individual with the virus. But the high viral loads found in the study are a strong indicator that the risks of transmission from both vaccinated and unvaccinated people with the Delta variant could be similar.

The findings could have implications for policy makers who've banked for months on hopes that by vaccinating a large proportion of any given population, they will also protect people who cannot or will not get inoculated themselves by reducing transmissions overall.

"The fact that they [fully vaccinated people] can have high levels of virus suggests that people who aren't yet vaccinated may not be as protected from the Delta variant as we hoped," Walker told the British newspaper. "It comes back to this concept of herd immunity, and the hope that the unvaccinated could be protected if we could vaccinate enough people. But I suspect the higher levels of the virus in vaccinated people are consistent with the fact that unvaccinated people are still going to be at high risk."

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The message from Walker and her team at Oxford was clear: Vaccination remains the best way to protect against infection, and certainly against serious illness or hospitalization with COVID-19, including the Delta variant.

None of the coronavirus vaccines approved for use in the U.S. or U.K. thus far eliminate the risk of infection, but they all reduce that risk by between about 70% and 90% — and they've proven much more potent at preventing hospitalizations and deaths.

"There are lots of reasons why the vaccines may be very good at reducing the consequences of having the virus," Walker told The Telegraph. "You may well still have a milder infection and might not end up getting hospitalized."

She said that while the results of the ongoing vaccine effectiveness study were important, "they aren't everything, and it is really important to remember the vaccines are super-effective at preventing hospitalizations."

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