I imagine some rather awkward discussions are taking place in Downing Street right now…

@RishiSunak @UKHSA @SteveBarclay @SRileyIDD Image
Once again, newspapers in the UK are filled with stories about a new Covid variant that’s suddenly causing cases to surge at a very rapid rate in India:

This is XBB.1.16, also known as ‘Arcturus’… Image
📍Covid cases are increasing at a rapid rate in India, with cases doubling every 5-6 days.

📍Wastewater surveillance in Bengaluru (a Southern India capital city) indicates that Covid levels in that area have just soared to the highest level since the start of pandemic. ImageImage
It’s still early days for the XBB.1.16 variant in India so the country-wide case numbers cannot be compared to the peaks of other big waves - it is the rapid exponential growth which is the big concern here…
Many people don’t truly understand the implications of exponential growth.

The problem is that, if something is doubling at a constant rate, the first few doublings don’t make a lot of difference…

But then… whoosh! 

1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048… Image
Doctors in India have warned:

“The XBB 1.16 variant is the primary cause of the rise in cases. This variant is showing immune escape & is able to evade the immune system. It is infecting those who had the Covid infection during previous waves.”

msn.com/en-in/health/m…
XBB.1.16 (‘Arcturus’) is very similar to the XBB.1.5 variant (the ‘Kraken’, previously the most transmissible COVID variant).

But some additional mutations have made it even more infectious…
A study from Japan has just been released showing that XBB.1.16 (‘Arcturus’) has the potential to spread ~30% more efficiently than its parental variant XBB.1 and ~20% more than XBB.1.5 (the ‘Kraken’).

It is thought to be the most infectious variant yet. Image
But XBB.1.16 (‘Arcturus’) is not just in India… it’s already been detected in many other countries around the world.

And it’s been in the U.K. for at least a month.

(Note: the last 2 weeks’ worth of data are incomplete and will be adjusted upwards as more data comes through). Image
In the US, they’re already starting to see the beginnings of exponential growth for XBB.1.16, with CDC Nowcast estimates for this variant more than doubling in the last week, from 3.4% to 7.2%…
It would be great to have visibility of what’s happening here in the UK…

However unfortunately, the arrival of Arcturus coincided almost perfectly with the axing of the ONS Infection Survey 🤦🏻‍♀️

This survey was the gold standard, the envy of the world.

independent.co.uk/news/health/co… Image
The beauty of ONS Covid infection Survey is that it was based on a random sample of households to provide a nationally representative survey.

This means it provided an unbiased estimate which was independent of how many people were testing themselves.

ons.gov.uk/peoplepopulati…
But sadly the survey was stopped just as Arcturus reached our shores… and just as Covid cases rose to the highest point so far this year.

The final ONS Infection survey indicated that 1 in 40 people were infected. That’s higher than in Jan 2021 when we went into lockdown! Image
Since then, with the new financial year upon us and Covid budgets slashed, we have entered a brave (or stupid) new world…

…the world of the data vacuum.

There is now NO reliable source of data for tracking Covid prevalence in the UK.
“Hang on, can’t we just look at Covid hospitalisation figures?” you may ask…

Sadly not. At the same time as stopping the ONS Infection Survey, big changes to Covid testing guidance in hospitals was also announced…

So Covid hospitalisation data is no longer reliable either👇🏻
“Surely we must still be doing genomic sequencing in the UK?”

Well yes, but since testing ended in hospitals, sample sizes have plummeted.

Last week, four XBB.1.16 samples were sequenced from a total of just 666.

A few months ago, we were typically doing 5,000 samples a week. Image
We do still have the Zoe Study which is modelled based on self-reported Covid test results from their user base.

But with self-testing dwindling to a trickle and no ONS Survey to optimise their algorithm against, I have doubts about how reliable this will be going forward…
Just a few weeks ago,
@mvankerkhove from the WHO gave an update on XBB.1.16 and stressed how important is is for countries to *strengthen* their surveillance systems of covid-19.

So why have we done the complete opposite?

We are flying completely blind now.
Meanwhile, India has been able to use their Covid surveillance as an early warning system to identify this incoming new wave and take action.

They’ve put hospitals on red alert & warned them to start preparing for potential surges in hospitalisations.

bbc.co.uk/news/world-asi…
The Welsh government have issued a written statement expressing their deep disappointment and concern about the UK government deciding to discontinue the ONS Infection Survey, given the importance of this surveillance study 👇🏻
…but at least Wales had the foresight to get their Covid wastewater surveillance up and running so at least they have something to fall back on. 💩 Image
Meanwhile, our Covid wastewater surveillance ended over a year ago.

So we are left wallowing in a complete data vacuum.
Come on @UKHSA, this must be a tad embarrassing now…

I’m sure many stakeholders are knocking at your door to enquire what the XBB.1.16 situation is like in the UK right now…

How do you tell them that you have absolutely no surveillance now?

When are you going to fix this?!
…and for anyone who is fed up of hearing about Covid and thinks all this is nothing to worry about, this thread is just for you 👇🏻
EDIT: I will keep adding to this thread as new information about the XBB.1.16 (‘Arcturus’) variant arises…
Some people have been downplaying the risks posed by XBB1.16 by saying that it may be no more severe than previous omicron variants.

We have limited data on its severity but we *do* have evidence that it’s ~20% more transmissible than the previous most transmissible variant.
A more transmissible variant (with the same severity) is still a very big issue.

Here’s why:

A 50% more transmissible variant will cause significantly more deaths than a 50% more severe strain because it will scale on an exponential growth curve instead of a linear curve. Image
Meanwhile, in Delhi (India), the exponential growth curve has been busy doing its thing:

📍Almost half of households in Delhi have one or more family member unwell.

📍 Test positivity rates in Delhi have shot up to over 32%.

ndtv.com/delhi-news/del… Image
Here in the UK, we’re now starting to see early signs of exponential growth of the XBB.1.16 (‘Arcturus’) variant from the small number of Covid tests that are still being sequenced.

Beyond this, we are pretty much flying blind, in a complete data vacuum... Image
With these early warning signs of exponential growth, it would be great to have some proper Covid surveillance back in place right about now…

How about it, @UKHSA ?

If you don’t want to take my word for it, please listen to the advice of the World Health Organisation.

👇🏻

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More from @_CatintheHat

Jan 4
COVID & CHILD DEVELOPMENT

New US study showing impact on child brain development when their mothers are infected with COVID during pregnancy.

Results show:
🔎 Structural differences in regions of the brain
🔎Lower cognition & social-emotional scores

sciencedirect.com/science/articl… x.com/harryspoelstra…Image
And here’s another study, this time from Brazil (published Jun 2025), which also shows the impact of maternal COVID infection on child development.

At 24 months:
🔎 36% of infants exhibited cognitive delays
🔎 64% communication delays
🔎 57% motor delays

nature.com/articles/s4139…Image
And another study, just published this week (Jan 2026) in ‘Obstetrics & Gynecology’, came to similar conclusions…

“Maternal SARS-CoV-2 infection in pregnancy was associated with increased risk of adverse neurodevelopmental diagnoses by age 3 years”.

journals.lww.com/greenjournal/f… x.com/jama_current/s…Image
Read 4 tweets
Dec 17, 2025
“When it comes to flu, the focus is often on droplet transmission, but there’s also evidence of aerosol transmission. That means that ventilation & air filtration are HUGELY important.

“Are the Govt looking to improve that to help deal with all the respiratory infections?”

/1
It’s absolutely brilliant to hear Baroness Bennett raising this crucial question in the House of Lords this week.

Thank you, @natalieben 🙏🏻

You can read a transcript of the full question and response received below ⬇️



/2 hansard.parliament.uk/Lords/2025-12-…Image
Given the shocking state of ventilation in UK hospitals ⬇️…

3/
Read 4 tweets
Dec 11, 2025
This feels like an important breakthrough moment…

On the BBC News this evening, Medical Editor @BBCFergusWalsh clearly stated:

“As for facemasks, simple surgical masks are *not* good at stopping viruses. You really need a properly fitted tight respirator mask for that”…

/1
…which begs the question, why does the NHS infection control guidance STILL only recommend surgical masks for treating patients with airborne viruses like flu & Covid… and not proper FFP3 masks?

Even Baroness Hallett was rather perplexed by this during the Covid Inquiry.

/2
The IPC experts (Dr Warne & Dr Shin) who provided independent specialist advice to the Covid Inquiry both stated that IPC guidelines should be updated to recommend routine use of FFP3 masks when caring for patients with ANY respiratory virus.

So why has this not been done?

/3
Read 7 tweets
Dec 10, 2025
CEO, NHS Providers, @danielelkeles:

“It’s a v nasty variant of flu that we have this year.”

“We need to get back into the habit that, if you’re coughing & sneezing […] then you must wear a mask when you’re in public spaces.”

I’m so pleased to hear Daniel promoting masks BUT…
…I have 3 questions for @danielelkeles:

1️⃣ He only mentions that people who are sick “must wear a mask in public spaces”…

…but why did he not also suggest that people who are *not* sick should wear a mask to prevent themselves getting sick in the first place?!

Like this ⬇️ Image
2️⃣ Why didn’t @danielelkeles mention FFP masks?

Surgical masks DON’T protect against airborne transmission of flu.

“Live viruses could be detected in the air behind ALL surgical masks tested. By contrast, properly fitted respirators could provide at least a 100-fold reduction.” Image
Read 5 tweets
Dec 8, 2025
UK ‘SUPERFLU’ OUTBREAK

Schools in England & Wales have been forced to close amid escalating flu outbreaks with hundreds of pupils off sick at a time.

Simon Kidwell, headteacher of Hartford Manor Primary told BBC Breakfast: “The winter bugs spread really easily in schools”…

/1
Headteacher Simon Kidwell is not wrong.

Children are crammed into poorly ventilated classrooms, shoulder-to-shoulder with up to 29 other kids, for around 6 hours a day, 5 days a week.

It’s the perfect environment for airborne diseases to spread…

/2 Image
We had a big wave of Covid in Sept/Oct, followed in very quick succession by the current wave of flu and RSV.

It’s been one perpetual cycle of illness hammering schools.

I often hear people say: “oh, but it’s always been like this”.

But it *hasn’t* always been like this.

/3 Image
Read 18 tweets
Dec 4, 2025
NHS England: “It will not be possible to halt the spread of a new pandemic virus, and it would be a waste of public health resources and capacity to attempt to do so.”

This has got to be one of the most 🤯 things I’ve ever seen written down in an official document. Image
And this is an important point from @mdc_martinus ⬇️

The NHS constitution states that they’re meant to PREVENT health problems…

…so how can their pandemic response strategy say it’d be a WASTE of public health resources to attempt to halt the spread of a new pandemic virus?!
@mdc_martinus Here is the full letter from CATA (an alliance of medical organisations, royal colleges & trade unions) raising the alarm about the NHS pandemic response strategy to the Chair of the Covid Inquiry.

For more details, please read the excellent thread from @cv_cev linked below ⬇️ Image
Read 5 tweets

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