Scientists have warned that people who have been fully vaccinated against Covid-19 can still pass on the Delta variant to others, dashing hopes of achieving herd immunity.
New research from the University of Oxford found that levels of the virus could be just as high in people who have received both vaccine doses, as those who have not been vaccinated at all.
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Scientists from the Covid-19 Infection Survey have conducted regular PCR tests on more than 70,000 participants since December last year.
Since mid-May, the more infectious Delta variant has become dominant in the UK and has impacted on the effectiveness of vaccines to prevent transmissions.
While both the Pfizer and AstraZeneca vaccines worked well against preventing infection and serious disease, it is still possible for the virus to be transmitted even after receiving two doses.
Dr Koen Pouwels, one of the lead researchers of the study, said: "The vaccines are better at preventing severe disease and are less effective at preventing transmission.
"The fact that you see more viral load (with the Delta variant) hints towards herd immunity being more challenging."
The researchers stressed that real-world data shows the vaccines are still highly effective at reducing hospital admissions and preventing deaths.
Professor Sarah Walker, Chief Investigator for the Survey, added: "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.
"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."
Pfizer protection declines more rapidly
As participants in the study had regular PCR and antibody tests, scientists were able to monitor their Covid-19 immunity levels over time.
Findings showed that the Pfizer jab was 85 per cent effective at preventing PCR-confirmed infection two weeks after the second dose, while the AstraZeneca vaccine was slightly lower at 68 per cent.
However, the protection of the Pfizer vaccine declined more rapidly, falling to 71 per cent effectiveness three months after the second dose.
By comparison, the AstraZeneca jab only reduced in effectiveness a small amount, dropping to 61 per cent over the same period.
Dr Alexander Edwards, Associate Professor in Biomedical Technology at the University of Reading, described the results as "compelling" and stressed the importance of getting double jabbed.
He said: "Overall this study is excellent as it shows that although Delta is better at infecting vaccinated people than previous variants, the vaccines still work remarkably well.
"There are subtle differences between different vaccine types, and some changes over time, but they all work brilliantly.
"It does remain vital to remember that even if double jabbed, you can still get infected and pass the virus on."
This article originally appeared on our sister site, NationalWorld.