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Adam Kucharski Profile picture
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22 Sep, 4 tweets, 1 min read
A question I often see: if COVID transmission continues, when will it reach a stable ‘endemic’ state? One way to look at it: the dynamics of endemic infections are typically driven by emergence of new susceptibility. A few thoughts… 1/
Many endemic infections continue to circulate because new susceptibility is gradually accumulated as unexposed children are born: 2/

For other pathogens (like seasonal coronaviruses) new susceptibility can also come from waning of existing immunity, or antigenic evolution of the virus - which has the effect of making previously immune people susceptible to infection again: 3/

Currently much COVID transmission is driven by people who haven’t had it before. If transmission continues, will eventually reach state where outbreaks are driven by new susceptibility, i.e. people who’ve previously had some immunity, or weren’t around when pandemic began. 4/4

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More from @AdamJKucharski

Adam Kucharski Profile picture
19 Sep
A Sunday thread on models of population interactions, in GIF form….

1. Random mixing en.wikipedia.org/wiki/Law_of_ma…
2. Strong network ties en.wikipedia.org/wiki/Interpers…
3. Non-assortative mixing en.wikipedia.org/wiki/Assortati…
Read 7 tweets
Adam Kucharski Profile picture
28 Aug
The terms ‘safe’ or 'unsafe' appear often in discussions of COVID threat, but they're vague, subjective words. What level of risk vs disruption is acceptable in long run (whether COVID or another disease)? 1/
As an example, I spent a chunk of my childhood paralysed and unable to walk thanks to a post-infection condition (GBS). Lingering effects for years. Not nice at all. But what should we as a society sacrifice to prevent a given level of risk? 2/
Vaccination is massively reducing COVID risk (at least in the countries that bought up most of the doses). So at what point are specific disruptive measures (whether at local or cross-border scale) no longer justified? 3/
Read 5 tweets
Adam Kucharski Profile picture
19 Aug
In discussions of future COVID dynamics and potential for ‘herd immunity’, need to remember there are four main routes to immunity that can reduce Delta transmission - even though coverage often focuses on just one or two of them. A quick thread... 1/
Because Delta transmissibility is so high, and current vaccine less effective in reducing Delta transmission, vaccination alone unlikely to get R below 1. But that doesn’t mean immunity can’t drive R below 1 in future... 2/

There are four main routes to immunity to Delta:
A. Cross-protective immunity from vaccine designed against earlier variant
B. Cross-protective immunity from infection with earlier variant
C. Immunity from infection with Delta
D. Immunity from Delta-specific vaccine
3/
Read 7 tweets
Adam Kucharski Profile picture
29 Jul
If - and it’s still a big if - exposure notifications from the NHS covid app/T&T were a major factor in driving the recent UK case decline, it’s worth considering what might happen next... 1/
It’s plausible that a rapidly growing epidemic + rapid testing availability + exposure notifications (both formal via T&T/app and informal among friends) has led to large numbers of people who would have been involved in transmission instead quarantining. 2/
If exposure risk is concentrated in time (e.g. because of Euro matches), quarantine timing would also have been concentrated in the period immediately after. Which means we can make a testable hypothesis about what might happen next… 3/
Read 7 tweets
Adam Kucharski Profile picture
15 Jul
Still see 70% quoted as level of vaccination required for 'herd immunity'. Important to note it's now likely to be much higher. The standard (albeit rough) calculation for herd immunity threshold is (1/E) x (1-1/R) where E is vaccine effectiveness in reducing transmission... 1/
In scenario where R is 6 (plausible for Delta in susceptible populations without any restrictions), and vaccination reduces infection/infectiousness such that onwards transmission reduced by 85%, above calc suggests would need to vaccinate (1-1/6)/0.85 = 98% of population. 2/
If transmission reduction is less than this (which is likely the case for some vaccines against Delta), or R higher, then herd immunity wouldn't be achievable through current vaccines alone. This leads to three possibilities... 3/
Read 6 tweets
Adam Kucharski Profile picture
12 Jul
One argument put forward for July 19th UK reopening is to bring infections forward to reduce winter wave. To be honest, I’ve always found idea that we could tailor a pandemic to get 'better' sized future waves a bit absurd - whether in spring 2020 or now. A few thoughts... 1/
For me, main issue now is medium term disruption vs medium term epidemic size. Many people now seem OK with R>1 in countries with relatively high vaccination % (at least implicitly, given they aren’t advocating for the strong measures required to guarantee R<1). 2/
Given R>1, much of Europe faces large epidemics likely to end with accumulation of immunity in next few months - much of it from infections. Reopening would accelerate this, but won't be difference between epidemic & no epidemic (unlike, say, reintroducing measures to get R<1) 3/
Read 10 tweets

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