1/ Stunning insights from the NHS symptom-reporting app. It shows a dramatic decrease in live symptomatic cases, starting on 1st April, just 8 days after the UK lockdown began. 
2/ The % decrease is vastly greater than the slight decrease in hospital admissions over the same period.
Why? Here's my proposed explanation: (THREAD)
Why? Here's my proposed explanation: (THREAD)
3/ There are two key facets to this. First, a delay effect. Hospital admission comes maybe 1-2 weeks after symptoms appear. Thus the admissions curve probably peaks a week later than the symptomatic curve, just from this effect.
4/ More importantly, there is a demographic effect. Prior to lockdown COVID-19 was rampant among the working-age, socially active, young end of the population distribution who are resilient to the disease except in rare cases.
5/ Older people during that period were cushioned from the main thrust of the disease growth by caution, fewer close social interactions, and even retirement.
The lockdown dramatically changed this balance between young and old.
The lockdown dramatically changed this balance between young and old.
6/ Just 8 days after the lockdown began, live symptomatic cases (shown by the app, first tweet in this thread) peaked at about 2.1 million before dropping steeply to 360,000 by 23rd April. What had changed? Rapid transmission between the socially active young had almost ended.
7/ However, hospital admissions, dominated throughout by older people (who have weaker immune response) fell much more slowly. Why? Because the disease is now concentrated in that part of the population - both more susceptible to infection and more likely to become severely ill.
8/ What inferences can be drawn?
(A) The average reproduction ratio R taken over the whole population distribution, under the current UK lockdown, looks to be much lower than has been estimated from hospital admissions. That gives our decision-makers some powerful new options.
(A) The average reproduction ratio R taken over the whole population distribution, under the current UK lockdown, looks to be much lower than has been estimated from hospital admissions. That gives our decision-makers some powerful new options.
9/
(B) During lockdown the disease has likely remained embedded in the older portion of the population, which is now a potential infection source for the wider population, where the disease has been rapidly clearing. Going forward, demographically targeted measures will be key.
(B) During lockdown the disease has likely remained embedded in the older portion of the population, which is now a potential infection source for the wider population, where the disease has been rapidly clearing. Going forward, demographically targeted measures will be key.
10/
(C) With this insight we can see that a strong lockdown, combined with strong test-and-trace measures and quarantining of international arrivals, has the capability to not only control the epidemic, but eliminate the disease entirely, as has been done in New Zealand.
(C) With this insight we can see that a strong lockdown, combined with strong test-and-trace measures and quarantining of international arrivals, has the capability to not only control the epidemic, but eliminate the disease entirely, as has been done in New Zealand.
11/ This is by far the best and fastest way to restore our economy to health, as well as saving thousands more lives over the next few months.
12/ Correction: The app is not from the NHS, it has been produced by ZOE in collaboration with academic institutions across the UK.
covid.joinzoe.com
covid.joinzoe.com
