Eight new COVID-19 cases today. It’s no surprise when you look at some numbers

So, as I sit at home with a very, very slight headache (i.e. not at work when I would otherwise be so), the now familiar figure of Ashley Bloomfield reports eight new confirmed cases of COVID-19  including two in Waikato. A surprise, given that we had just twelve yesterday? No. A worry? Maybe, but no more so than it was yesterday.  Here are some numbers to make the point.

All the cases today are imports – recent arrivals from overseas. Let’s try to estimate the probability of any arrival from overseas carrying the virus. Western Europe has a population of roughly 500 million (depending on which countries you count), and, according to the latest WHO bulletin, have confirmed about 60 thousand or so cases of COVID-19, around half in Italy. That’s roughly speaking about 1 person in eight thousand confirmed with the virus. Excluding Italy it’s about 1 in  15,000. For the US, it’s 3500 cases in a population of about 330 million, or 1 person in about a hundred thousand. For Australia, it’s 375 in about 25 million, or about 1 in 70,000.

That’s the numbers of confirmed cases yesterday. But how many actually are carrying the virus? Well, it’s likely to be much higher. First, given that many people have very mild symptoms, there will be many cases ‘missed’ by the counting. How much by? It’s hard to say, but it could be substantial.  Secondly, the figures report what was happening a week or so ago. The numbers carrying it today, assuming the doubling of three-to-four days has continued unabated (though surely with the tightening restrictions in Europe and US there will be some significant decline?) will be maybe four times what they were last week. One could easily imagine then in Europe it’s close to 1 in a thousand people actually having the virus.

How many people have entered NZ from Western Europe in the last week? I don’t have figures, but Auckland Airport reports for a typical month around 400,000 arrivals, maybe 13,000 arrivals a day. Now traffic has declined in the last month, but hadn’t collapsed until Monday.  An Airbus A380 arriving from Dubai is typically loaded with passengers from Europe. Imagine counting off every 1000th arrival from Europe and sticking a COVID-19 tag on them. There are likely to be several a day, and these cases will flow on to reported cases in the next week or so. Expect some higher numbers.

Another way to look at it is in terms of exponential growth. We could expect imported numbers to double every three or four days, corresponding to that rate being seen across the world as a whole, until the border ‘closure’ hits the numbers (perhaps in a week or so). Thus the four cases over last weekend might, very roughly, ‘balloon’ to about 16 imported cases next weekend. That’s a vague estimate, and I could be very wrong. But I would expect numbers will grow quickly in the next week.

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