Two new Omicron sub-variants have caused concern amid a recent increase in Covid cases across the country.
The Office of National Statistics (ONS) shared rising stats showing that UK infections have grown by 43 per cent weekly (that's 1.4 million people, up from 989,800 the week before), as new NHS data shows an increase in Covid patients being hospitalised in England.
It is thought these sub-variants are behind the substantial increase - so what are they and how are they different from previous Covid variants?
Here is a breakdown of everything you need to know.
What are the new Omicron sub-variants experts are concerned about?
In April, the World Health Organisation began to monitor the two strains named BA.4 and BA.5 that were discovered in South Africa around January and February this year.
Then the European Centre for Disease Prevention and Control labelled BA.4 and BA.5 "variants of concerns" in mid-May.
The sub-variants are being investigated to see if there are dangerous, since they are thought to be a product of the Omicron variant which was transmitted globally in late 2021.
How do BA.4 and BA.5 compare with other variants?
The difference between Omicron and the new sub-variants are that they have three mutations to the spike proteins that mean it has developed a way to attack lung cells - in a similar way to previous variants Alpha and Delta. Omicron was a milder form of Covid which didn't infect lung tissue.
Preliminary data from Professor Kei Sato of the University of Tokyo appears to back this theory up.
"Altogether, our investigations suggest that the risk of [these] Omicron variants, particularly BA.4 and BA.5, to global health is potentially greater than that of original BA.2," Professor Sato said.
What is the Covid case rate in the UK currently?
The infection numbers are at the levels they were in early May, though they are not near the highest rate of 4.9m recorded in late March when Omicron BA.2 peaked.
The ONS believes the recent rise is "likely caused by infections compatible with Omicron variants BA.4 and BA.5", which are now thought to be the most dominant variants in much of the UK.
Though experts are still learning more about the new sub-variants as they continue to monitor the case numbers.
Dr Meera Chand, UKHSA director of clinical and emerging infections, said: "The reclassification of these variants as variants of concern reflects emerging evidence on the growth of BA.4 and BA.5 internationally and in the UK.
"Whilst the impact of these variants is uncertain, the variant classification system aims to identify potential risk as early as possible.
"UKHSA is undertaking further detailed studies. Data and analysis will be released in due course through our regular surveillance reporting," she added.
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