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The COVID-19 vaccines by Pfizer and AstraZeneca are less effective against the Delta variant of coronavirus compared to the Alpha variant, according to a study led by researchers at the University of Oxford.
The researchers, however, noted that with Delta, Pfizer-BioNTech vaccine and Oxford-AstraZeneca preventive, known as Covishield in India, still offer good protection against new infections.
Two doses of either vaccine still provided at least the same level of protection as having had COVID-19 before through natural infection, they said.
The researchers analysed 2580,021 test results from nose and throat swabs taken from 384,543 participants aged 18 years or older between December 1, 2020 and May 16, 2021.
They also analysed 811,624 test results from 358,983 participants between May 17, 2021 and August 1, 2021.
The yet to be peer-reviewed study found that the people who had been vaccinated after already being infected with COVID-19 had even more protection than vaccinated individuals who had not had COVID-19 before.
However, Delta infections after two vaccine doses had similar peak levels of virus as those in unvaccinated people, the researchers said.
With the Alpha variant, peak virus levels in those infected post- vaccination were much lower, they said.
"We don't yet know how much transmission can happen from people who get COVID-19 after being vaccinated -- for example, they may have high levels of virus for shorter periods of time," said Sarah Walker, a professor at the University of Oxford.
"But the fact that they 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. This means it is essential for as many people as possible to get vaccinated," Walker said.
The researchers, including those from the Office of National Statistics (ONS) and the Department for Health and Social Care (DHSC) in the UK, also found that a single dose of the Moderna vaccine has similar or greater effectiveness against the Delta variant as single doses of the other vaccines.
Two doses of Pfizer-BioNTech have greater initial effectiveness against new COVID-19 infections, but this declines faster compared with two doses of Oxford- AstraZeneca vaccine, they said.
Results suggest that after four to five months effectiveness of these two vaccines would be similar, however, the researchers said long-term effects need to be studied.
According to the study, the time between doses does not affect effectiveness in preventing new infections, but younger people have even more protection from vaccination than older people.
"The fact that we did not see any effect of the interval between first and second doses, and the greater effectiveness of having had two doses, rather than one dose, supports the decision to reduce this to eight weeks now Delta is the main variant of concern in the UK," said Koen Pouwels, senior researcher in Oxford University.
"While vaccinations reduce the chance of getting COVID-19, they do not eliminate it. More importantly, our data shows the potential for vaccinated individuals to still pass COVID-19 onto others, and the importance of testing and self-isolation to reduce transmission risk," Pouwels said.
The study compared protection from infections from COVID-19 vaccines before and after May 17, 2021, when Delta became the main variant in the UK.
It also looked in detail at how effectiveness changes over time as well as other factors like previous infection.
Simon Clarke, Associate Professor at the University of Reading in the UK said the real-world data of how two vaccines are performing show the Delta variant has blunted the effectiveness of both the Pfizer and AstraZeneca jabs.
"Of particular concern, the AstraZeneca vaccine's effectiveness is reduced substantially by Delta and it appears to offer no more protection than what someone would get from having COVID-19 and building some natural immunity," Clarke, who was not involved in the study, said.
"The Pfizer jab provided greater initial protection than the AstraZeneca one, but then after around five months the level of immunity dropped to about the same level seen for both of the vaccines looked at," he added.