Three weeks was never going to be enough

As a country we’ve been in lockdown for almost three weeks. The government said they’d review it after three weeks, which is on Easter Monday.

Why Three Weeks?

Why did the government pick three weeks as the review period?

Well, when you look at how people seem to progress through being infected the vast majority, well over 90%, are no longer infectious after that time. Pretty much everyone that was infected before the lockdown started will have either recovered or died after three weeks.

So three weeks in what you think you should be seeing, if the lockdown has worked, are the people that lived with those that were infected before the lockdown developing symptoms. They’ll also be passing it onto the people they come into contact with, but far fewer than before.

What you see isn’t what you get

However there’s a time lag in there, and we’ve only tested people admitted to hospital. So there are a load of people infected before lockdown that only get tested 7-10 days after they develop symptoms at the point they get admitted to hospital. Symptoms start about 5-7 days after they were infected. So overall the people being tested in hospital were infected 12-17 days earlier.

From the published stats, there seems to be up to a three to four day lag in processing tests, especially over a weekend. So today’s reported new cases were probably infected between 2-3 weeks earlier. Before the lockdown in most cases. That’s why it still looks like cases are going up. What we’re reporting today is the pre-lockdown infection rate.

This is apparent in the official estimate of the rate of infection. Before the lockdown the estimate was that every infected person passed it on to 3.3 other people. That took us from a 3-4 new cases per day in late February to over 2,000 per day on 26 March. Those also have the 2-3 week delay mentioned above.

The estimate on 31 March, less than a week after the lockdown started, was that it had slowed to between 0.6-0.9 infections. Certainly when you look at the daily increase in confirmed cases the rate of increase starts to slow in mid-March.

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