The Delta variant blunts the efficacy of the Pfizer and AstraZeneca COVID-19 vaccines and may remain transmissible by doubly vaccinated individuals, according to preliminary research


Having two doses of a COVID-19 vaccine remains the most effective way of ensuring protection against the Delta variant first identified in India, preliminary research has found.

preprint study by scientists at the University of Oxford found that both the Pfizer/BioNTech vaccine and the AstraZeneca/Oxford vaccine offered good protection against the Delta variant, but that effectiveness was reduced compared with the protection offered against the Alpha variant, which was first identified in the UK.

Vaccines working differently 

Immunity following second doses differed significantly between the two vaccines, the scientists said. There was greater early protection seen in those who had received the Pfizer vaccine, but a faster decline in effectiveness compared with those who received the AstraZeneca jab.

The early results suggested that efficacy of the two vaccines would be similar after 4–5 months.

Another key finding was that Delta infections after two vaccine doses had similar peak levels of virus to those in unvaccinated people.

Sarah Walker, Professor of Medical Statistics and Epidemiology at the University of Oxford, who led the research, 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.

‘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—both in the UK and worldwide.’

The study was based on polymerase chain reaction tests from 384,583 adults during the time when the Alpha variant was predominant, and 358,983 individuals when the Delta variant had taken over.

Scientists did not draw firm conclusions about the Moderna COVID vaccine—the third to be approved for emergency UK use—due to a lack of available evidence. However, they said that there was no reason to suppose that it was inferior in any way to the other two vaccines.

‘Blunted’ effectiveness 

Commenting on the study for the Science Media Centre, Dr Simon Clarke, Associate Professor in Cellular Microbiology at the University of Reading, said: ‘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.

‘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.’

He said that the data ‘certainly supports the case for third “booster” jabs for vulnerable individuals’.

Dr Penny Ward, Visiting Professor in Pharmaceutical Medicine at King’s College London, agreed that ‘a booster dose may be required to sustain effectiveness’.

She also suggested that there was ‘a need for continued focus on test, trace, and isolate activities to reduce the level of spread across the community’.

Deaths prevented 

Also published this week were the latest estimates from Public Health England on the effects of England’s vaccination programme on cases, hospitalisations, and deaths.

It said that more than 82,100 hospitalisations had been prevented in people aged 65 years and over, 95,200 deaths were prevented, as were 23,957,000 COVID-19 infections.

The Health Secretary, Sajid Javid, said: ‘The UK’s phenomenal vaccination programme has made a life-changing difference to tens of millions of people across the country, and we’re quickly closing in on 100,000 lives being saved in England alone’.

This article originally appeared on Medscape, part of the Medscape Professional Network.