UKHSA publishes update on
Omicron risk assessment, S gene target failure and local case
numbers
The assessment suggests that Omicron is displaying a significant
growth advantage over Delta, meaning that it is likely to
outcompete Delta in the UK and become the dominant variant.
This assessment is based
on analysis of UK data showing increased household
transmission risk, increased secondary attack rates (such as the
chance of each case infecting another individual) and increased
growth rates compared to Delta.
If the growth rate and doubling time continue at the rate we have
seen in the last 2 weeks, we expect to see at least 50% of
coronavirus (COVID-19) cases to be caused by Omicron variant in
the next 2 to 4 weeks.
The risk assessment also suggests that Omicron displays a
reduction in protection offered by having had a previous
infection or vaccination. Whilst there are insufficient data to
quantify either vaccine effectiveness or risk of reinfection in
the UK exactly, the observed growth, case distribution and early
analyses in both South Africa and the UK are consistent with some
loss of immune protection against infection. New studies are
being undertaken to assess this further.
There is insufficient data to make
any assessment of protection against severe disease, or
to assess the severity of illness caused by Omicron. Further
studies are underway in the UK and abroad.
In addition, UKHSA has published data
which shows the detection of cases exhibiting S-gene target
failure (SGTF) in
recent weeks across the country. Approximately half
of PCR tests in the UK are
able to detect SGTF.
SGTF is a useful
indicator of the presence of Omicron, because as a rule Delta
cases have the S-gene and Omicron cases do not. However, it is
not confirmatory as there are a number of other reasons that a
sample might exhibit SGTF. For example, there are still
a small number of cases of other variants, such as Alpha, in the
UK which would also result in S-gene dropout or there is a lower
amount of virus present in the sample where S-gene dropout cannot
be confirmed.
Positive tests with sufficient virus detected from people
arriving in the UK are sent for confirmation through Whole Genome
Sequencing, regardless of the presence or absence
of SGTF.
As part of UKHSA’s routine genomic
surveillance, approximately 15 to 20% of all
positive PCR tests are also sent
for sequencing.
Until the week beginning 23 November 2021, the weekly count of
cases with SGTF was routinely less than
150, making up less than 0.1% of all cases. Analyses of
sequenced SGTF samples has indicated
that until mid-November, more than 99% of these were Delta cases.
In the most recent week of data (specimen dates from 30 November
2021 reported as of 6 December), the number of cases
with SGTF has increased to 705.
The majority of these cases are located in London and the South
East.
Trends in SGTF over and time are
however affected by the coverage of laboratories contributing to
this surveillance data.
UKHSA Chief
Medical Advisor, Dr Susan Hopkins said:
It is increasingly evident that Omicron is highly infectious and
there is emerging laboratory and early clinical evidence to
suggest that both vaccine-acquired and naturally acquired
immunity against infection is reduced for this variant. It is
therefore absolutely critical that we all do everything that we
can to help break the chains of transmission and slow the spread
of this new variant.
Vaccination is critical to help us bolster our defences against
becoming severely ill from this new variant – please get your
first, second, third or booster jab without delay. Please also
make sure to follow all Government guidance to reduce the spread
of infection.
It remains vital that anyone with COVID-19 symptoms isolates and
gets a PCR test immediately.
