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WORLD | 19-08-2021 09:35

Covid vaccines are less effective against Delta variant, large study finds

Large study finds Pfizer-BioNTech efficacy ebbed over time; Data shows vaccines didn’t diminish viral load of delta cases.

Covid-19 vaccines are less effective against the Delta variant, according to results in the United Kingdom from one of the largest real-world studies into the efficacy of the shots.

Pfizer Inc and BioNTech SE’s messenger RNA vaccine lost effectiveness in the first 90 days after full vaccination, though that shot and the one made by AstraZeneca Plc still staved off a majority of Covid infections. When vaccinated people did get infected with Delta, they were shown to have similar levels of virus in their bodies as those who hadn’t had shots. This suggests that vaccinating large portions of a population might not protect those who don’t get inoculated, casting doubt on the idea of achieving herd immunity.  

There isn’t yet data to show how much the vaccines continue to protect against hospitalisations and severe cases of Covid over time.  

These latest results are likely to fuel calls to give booster shots to the fully vaccinated even as countries around the world still lack enough supply for first immunizations. The United States plans to start offering booster shots on September 20 to all vaccinated US adults. UK authorities are still deciding how broadly boosters should be given. In Israel, which started giving third doses of Pfizer-BioNTech this month, initial results show they have been 86 percent effective for people over the age of 60. 

The UK survey, run by the University of Oxford and the Office for National Statistics and published Thursday in a preprint, analysed more than three million PCR tests from a random sample of people for a detailed picture of infection patterns as delta became the dominant variant this year.

“We’re seeing here the real-world data of how two vaccines are performing, rather than clinical trial data, and the data sets all show how the delta variant has blunted the effectiveness of both the Pfizer and AstraZeneca jabs,” said Simon Clarke, an associate professor in cellular microbiology at the University of Reading. 

Days since second dose Effectiveness against high viral burden infections
  Pfizer-BioNTech shot AstraZeneca shot
14 92% 69%
30 90% 67%
60 85% 65%
90 78% 61%


By roughly four and a half months after the second dose, Pfizer’s shot will probably be about on par with Astra’s at preventing infections with a high viral burden, said Koen Pouwels, an Oxford senior researcher who helped lead the study. There wasn’t a statistically significant difference in the Astra shot’s effectiveness over time.

The study results may bring further unease as infections start to creep up in Britain, along with hospitalisations and deaths. The latest UK Department of Health data show that 33,904 people tested positive for Covid-19 on August 18, taking the weekly number to 211,238. That’s an increase of almost eight percent from the previous seven days. 

There were 655 deaths within 28 days of a positive test recorded in the latest week, also up about eight percent. The number of patients admitted to the hospital rose 4.3 percent to 5,623. Still, hospitalisations and fatalities in Britain remain low when compared with previous waves. 

The United States is also facing a new wave of infections, primarily among the unvaccinated. US President Joe Biden’s plan for boosters is stoking criticism that the U.S. is hoarding doses while poorer nations languish.

The evidence that delta increases transmissibility more than other variants even in vaccinated people backs up a recent assessment by the US Centers for Disease Control and Prevention. The UK results cast further doubt on the possibility of achieving herd immunity via vaccination, said Sarah Walker, a professor of medical statistics and epidemiology at Oxford, who helped lead the study. 

“The hope was that unvaccinated people could be protected by vaccinating lots of people,” Walker said. “The higher levels of virus that we’re seeing in these infections in vaccinated people are consistent with the fact that unvaccinated people are just going to be at higher risk, I’m afraid.” 

One important piece of the puzzle that’s still missing is the data relating to hospitalisations and severe cases of Covid, said Penny Ward, a visiting professor in pharmaceutical medicine at King’s College London, who wasn’t involved with the study. The findings may also support giving a booster dose of mRNA vaccine to people who got the Astra shot, which uses a different technology, Ward said in a statement. They also drive home the need for better Covid treatments, she said.

“No vaccine is completely protective against infection with the delta variant,” Ward said. “The low incidence of hospitalisation seen to date suggests that in this respect, at least, the vaccines are protecting individuals from developing severe Covid.” 

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by Naomi Kresge, Bloomberg


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