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expert reaction to new data from PHE on the B.1.617.2 Delta variant

Public Health England (PHE) has released more data on the Delta variant first detected in India (B.1.617.2).


Dr Yuliya Kyrychko, Reader in Mathematics, University of Sussex, said:

“The latest estimates of R number1 suggest that new infections have started to grow in all regions of England, indicating that outbreaks observed in the North West of England some weeks ago, are no longer geographically contained in that area.

“This is further supported by the latest report from the Public Health England, which shows that almost 90% of new infections are now associated with the Delta variant, which is estimated to be just over 60% more transmissible than the Alpha (Kent) variant.  On a positive note, the same report has slightly lowered estimates for the increased risk of hospitalisation and emergency care/hospitalisation.  Moreover, latest evidence suggests that two weeks after the second dose of vaccine, the effectiveness of protection against symptomatic disease with Delta variant stands at 80%, providing strong support for the current strategy of implementing second doses as soon as practically possible in relation to the first dose.”



Dr Konstantin Blyuss, Reader in Mathematics, University of Sussex, said:

“Over the last several weeks, there has been a growth in confirmed cases of Covid-19, and the latest data indicate that the R number1, as estimated from data over this period of time, has also increased around the country.

“In the context of latest sequencing data, which shows that around 90% of new infections are due to Delta variant, the report released by Public Health England earlier today shows that over 90% of new infections are associated with household transmission, rather than with cases travelling into the country, which explains the reasons for the recent growth in cases around the country.

“Importantly, testing times for Delta variant have now significantly decreased, which allows for new infections to be discovered sooner, thus substantially improving monitoring and introducing surge testing wherever it is needed.

“Despite substantial observed growth in cases, data from the same report indicate that two doses of vaccine provide a significant protection in terms of reducing risks of hospitalisations and deaths.  This is illustrated by the fact that out of the total of 383 people hospitalised with Delta variant, only 42 (11%) were fully vaccinated, and out of 42 people who have died with confirmed Delta variant, only 12 (29%) were fully vaccinated.”



Prof Francois Balloux, Professor of Computational Systems Biology and Director, UCL Genetics Institute, UCL, said:

“The latest PHE report largely confirmed and strengthened earlier evidence about the behaviour of the B.1.617.2 (delta) lineage.  There have been over 33,000 cases of the delta variant reported in the UK over the last week, and the variant now represents over 90% of all cases of COVID-19 in the UK.  Transmissibility of the delta variant in the community is estimated to be ~41% higher than for B.1.1.7 (alpha), with estimates for within-household transmission possibly even higher.

“People infected with the delta variant are more likely to be hospitalised.  Though, hospitalisations and deaths remain low considering case numbers.  The case fatality rate is currently down to 0.1% thanks the high vaccine coverage among the most vulnerable.  Risk of infection by those vaccinated with one dose looks increased for the delta variant, but protection provided by two doses of vaccine seems largely unaffected.  Despite this, there are some breakthrough cases and there have been some deaths in doubly vaccinated people.

“Cases have been going up over the last weeks.  Though they might be slowing.  The week on week increase was lower in today’s ONS weekly report.  Cases reported on Thursday were also roughly similar to the day before.  This may either indicate a plateauing, suggest a future reduction in daily case numbers, or simply represent a blip.  Bases on current trends, it feels somewhat unlikely the government will release all measures on Monday.”



Latest PHE technical report on variants of concern:

PHE preprint on household transmission and the B.1.617.2 delta variant:



All our previous output on this subject can be seen at this weblink:



Declared interests

None received.

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