Thread by @JamieWoodhouse

~4.3 million 🇬🇧 people may now have #COVID19

~6.3% or 1 in 16. ~13% in London

Assume: 4,326 dead (hosp +20%); IFR 1%; Adj. days to death ~18

So ~432k infected up to 18 days ago
~5.25 d to double so ~3.4 doublings since

So infections to date: ~4.7m, ~8% recovered so ~4.3m now

https://twitter.com/DHSCgovuk/status/1246060747727294466

Whole model is v.sensitive to inputs, so low confidence + wide ranges apply.

More deaths --> more infections
Higher IFR% --> fewer infs (don't need as many for same deaths)
Longer days to death --> more infs (more time to double)
Higher doubling rate --> more infs

This is a simple, top-down model.

For a pro version that also uses a "death, days to death, transmission speed" approach, read this from Imperial.

Already low given new deaths + non-hospital additions.

imperial.ac.uk/news/196556/co…

h/t @StefanFSchubert

It's heartening to finally see some press references to % infected, not just the "confirmed cases" iceberg tip.

unherd.com/2020/04/how-li… @TomChivers

news.sky.com/story/coronavi…

telegraph.co.uk/news/2020/03/2…

metro.co.uk/2020/03/30/cor…

msn.com/en-gb/news/new…

Way too late...

Numbers would imply total UK death toll of ~47k...

But only if IFR remains 1% + we see zero new infections.

Sadly, neither of the above are sound assumptions.

If our ICU capacity is overrun, the IFR% will spike hard.

New infections depends on hard #Lockdown + test/trace.

A wave is crashing over our ICUs.

We can't reduce the size of that wave because it's caused by infections from 2-5 weeks ago.

We can improve our readiness by scaling ICUs + protecting NHS staff.

We can stop the wave getting bigger + longer by hard #Lockdown + test/trace.

It's tempting to think... "There's 4326 deaths and #Covid19 kills about 1 in 100, so there must be ~432k infections".

The problem is you're nearly right. But those ~432k infections were 2-3 weeks ago. That's how long it takes to kill.

They've been doubling every ~5 days since.

This isn't the best way of estimating % infected!

We're waiting for people to die, assuming fatality rate to judge how many infections drove those deaths, then using doubling rates to judge infections today.

A better way? Random pop. sample testing!

https://twitter.com/JamieWoodhouse/status/1243875344916111360?s=20

Why is estimating # infected more important than just tested "cases"?

1⃣ Infected people are doing the infecting. The "cases" are in hospital/dead/immune

2⃣ If you know # infected and # deaths you can assess how deadly this thing really is (IFR%, not just "case" fatality)

3⃣ Telling public % infected helps us comply with hard #Lockdown.

BJ's speech should have started:

"There are infected people in every train, in every shop, in every park, in every tube... you might even be one of them... please take what I am about to say very seriously..."

This poll shows a dangerous level of complacency across the UK. I suspect this is worse in the real world.

Most people think less than a million are infected! That's what happens when govt + press only talk about the "confirmed cases" iceberg tip.

https://twitter.com/JamieWoodhouse/status/1245291470107672576?s=20

4⃣ "Total infected" tells us how many people are going to be coming through our ICU capacity in the next few weeks and whether that will be overrun.

If it is, #Covid19 will kill much more than 1%. When you run out of ICU beds / ventilators, many more die as in Italy/Spain.

If we'd started projecting total infected from day one (ideally via. regular pop. sample tests), we might have acted sooner.

We might also have persuaded people to comply better with our weak #Lockdown.

Instead... "a national scandal" @richardhorton1
thelancet.com/journals/lance…

A powerful visualisation of deadly UK #Covid19 complacency:

"Right things... at the right time..."?!?

H/t @Imperial_JIDEA

(I'll stop my amateur efforts as your weekly reports start to come out - critically important)

Amateur epidemiologists like me are annoying, but as long as we have pro epidemiologists saying things like:

"We have no way of knowing" or

"2-5x cases"... or

Refusing to even try to estimate this most critical number...

It feels sensible to have a top down sanity check.

I'm hoping our government knows all this in secret and is responding appropriately.

I don't know why they're not talking openly about it - as we need some calm fear to help us keep hard #Lockdown.

If, as early on, they're still underestimating this - we're in real trouble.

Thanks to those rare data journalists grappling with this most critical estimate - how many are really infected.

@TomChivers @EdConwaySky @alexwickham @Ashley_J_Kirk @PaulNuki @AlbertoNardelli @nicholascecil @jburnmurdoch

Wish you were asking questions at the daily pressers!

+ Thanks to the professionals who are trying to work "true infected" out instead of dodging.

Govt (+ public) need to listen.

@Imperial_JIDEA @imperialcollege @edge_health_ @georgebatchelor @ChristianMoroy @neil_ferguson

edgehealth.co.uk/post/covid23ma…

https://twitter.com/Imperial_JIDEA/status/1244607618989031424?s=20

When will mainstream journos start asking how many are really infected?

@maitlis @Peston @AndrewMarr9 @bbclaurak @adamboultonSKY @BethRigby @PaulBrandITV @Emmabarnett @mrjamesob @piersmorgan @pippacrerar @guardianheather @GaryGibbonBlog @carolecadwalla @Otto_English @IanDunt

When will mainstream journos start asking this most important #Covid19 question?

@rowenamason @JamesHockaday_ @alexwickham @NickCohen4 @jennirsl @tnewtondunn @lewis_goodall @faisalislam @AdamBienkov

+ Final thanks as ever to @cheianov who helped improve the maths.

Particularly modelling an adjusted "infected to death" duration given exponential skew over time.

On average, deaths to date are caused more by recent deaths than the 23.5 days from infection to death implies.