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A British study found that fully vaccinated people who became infected with the Delta variant had the same viral load, which is the amount of virus particles in their bloodstream, as those who had not been immunized.

Graham Hughes/The Canadian Press

A British study has found that fully vaccinated people still have a high potential of spreading the Delta variant of COVID-19 and achieving herd immunity will be difficult.

The study, by researchers at the University of Oxford and the Office for National Statistics, is the largest examination yet into the effectiveness of the three main vaccines – Pfizer-BioNtech, Oxford-AstraZeneca and Moderna – against the Delta variant.

Researchers analyzed 3.3 million test results from more than 740,000 volunteers between Apr. 26, 2020, and Aug. 1, 2021. The tests were part of a weekly infection survey conducted by the ONS, and the researchers compared results before and after May 17, 2021, when the Delta variant overtook Alpha as the main version of the disease in the U.K.

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The study found that fully vaccinated people who became infected with the Delta variant had the same viral load, which is the amount of virus particles in their bloodstream, as those who had not been immunized. By contrast, vaccinated people who contracted the Alpha variant had a much lower viral load than unvaccinated people. A higher load makes transmission far more likely.

“You still are less likely to get infected if you’ve had two doses, but if you do get infected you will have similar levels of virus to someone who hasn’t been vaccinated at all,” said Sarah Walker, a professor of medical statistics and epidemiology at the University of Oxford.

“We can’t be confident that that definitely means they transmit as much, but they definitely have the potential to transmit. Anyone who thinks that if they get infected and having been vaccinated they can’t transmit, I’m afraid that isn’t likely to be true,” she said.

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Dr. Walker added that the study’s findings mean that reaching herd immunity won’t be as easy as health officials expected. “The hope was that unvaccinated people could be protected by vaccinating lots of people,” she told a media briefing on Wednesday. The higher levels of virus in vaccinated people “are consistent with the fact that unvaccinated people are just going to be at higher risk I’m afraid. And that proportionately, the level of deaths and hospitalizations will be more for those people.”

Koen Pouwels, a senior researcher in population health at Oxford who participated in the research, said other studies have shown that fully vaccinated people clear the virus quickly, which indicates they might not be infectious for a long time. However, he said that remained unclear.

“I think the fact that we do see more viral load hints that, indeed, herd immunity might become more challenging because it is a hint that the vaccines are probably best at preventing severe disease and slightly less [effective] at preventing transmission,” Dr. Pouwels told the briefing.

The researchers stressed that all three vaccines offer strong protection against the Delta variant, especially for severe cases that often lead to hospitalization. The team compared the effectiveness of the Pfizer and AstraZeneca jabs in more detail because Moderna’s vaccine has not been widely used in Britain.

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The comparison showed that while two doses of the Pfizer vaccine initially offered more protection than AstraZeneca’s shots, the effectiveness wore off faster. The Pfizer vaccine was 92 per cent effective against infections with a high viral burden two weeks after the second dose. That protection fell to 78 per cent three months later, according to the study.

AstraZeneca’s effectiveness over the same period fell from 69 per cent to 61 per cent. The study suggested that both would provide similar protection at a lower level after five months.

Dr. Walker was asked if the difference could explain why Israel, which relied primarily on the Pfizer vaccine, has seen a big increase in infections recently, even though the country has one of the highest vaccination rates in the world. Israel recently began a booster campaign for elderly people to slow the spread of the virus.

“What our data suggests and supports in terms of Israel is that the effects of Pfizer may change in a different way from AstraZeneca even though for both of them two doses is still effective,” she said. She added that the vaccines are based on different technologies.

The Pfizer vaccine uses genetically engineered messenger RNA, which instructs the body’s cells to make proteins found on the surface of the COVID-19 virus. That prompts a response from the immune system. The cells also break down the instructions and get rid of them, which Dr. Walker said could explain why the effectiveness fades.

The AstraZeneca vaccine uses genetic material from the COVID-19 virus and puts it in a harmless virus. Once injected, the viral vector teaches the body’s cells how to make the COVID-19 proteins, which trigger the immune system.

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“These vaccines are just doing very different things,” she said. “They may be having the same protection overall, but how they are getting to that protection is quite different and we need to think about that long term.”

She added that further research is needed to assess whether booster shots should be a mix of vaccines. “We definitely need more studies looking into whether actually you can get even better protection by offering the immune system different types of vaccination,” she said. “There’s a long way still to go with this story and really lots of positives.”

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