## COVID-19:Still Looking good, but stiiiiillll not over

Click for a larger image. Logarithmic graph showing positive caseshospital admissions and deaths since the start of the pandemic. The blue arrows show the dates of ‘opening’ events. See text for further details

Friends, so here we are, May 17th 2021, and I can finally resume the natural pastime of old men: sitting in cafes.

Obviously, matters pandemical are not ideal, but they are in my estimation looking good.

To explain my uncharacteristic positivity, allow me to remind you how this summer differs from last.

I know what happened last summer…

As we look at the figure above covering positive cases, hospital admissions and deaths across the pandemic, we see that the second wave began in July 2020 – when cases began to rise. This was before the end of the first wave as judged by the minimum in the rate of deaths.

And after cases started to rise hospital admissions and deaths both fell for a further two months!

So now we should focus our attention on cases for first signs of a problem.

However what matters is:

• not the absolute number of cases,
• not the population prevalence of cases
• not even the rate of change of cases

What matters is this:

• Is there the potential for the pandemic to expand into the general population and kill hundreds of thousands of people?

Last summer the answer was definitely ‘Yes’.

This summer the answer is probably ‘No’.

Why? Because 55% of the entire population, including practically all of the most vulnerable groups have received a first dose of the vaccine.

Together with the 10% (roughly) of the population who have had the disease, we are close to herd immunity.

Herd Immunity is not an on-off thing.

As the prevalence of immune people approaches a critical prevalence – probably around 66% for the coronavirus – the speed of viral transmission slows as the virus finds it increasingly difficult to move from an infected individual to a vulnerable individual.

Once the prevalence of immune people has passed the critical prevalence – chains of viral transmission decay with increasing rapidity as immunity prevalence approaches 100%. For any physicists reading – it’s a continuous phase transition.

Probably

Careful readers may have noticed that I slipped in an italicised  ‘probably’ a few paragraphs back. Things could still go wrong.

Most notably, one variant of the virus or another could acquire the ability to escape the immunising effects of the vaccine.

This is possible, but it is – as far as I can tell – not a very likely outcome.

What will happen next?

Click for a larger image. Logarithmic graph showing positive cases, since the start of 2021. The blue arrows show the dates of ‘opening’ events. See text for further details

The figure above shows that the ‘openings’ (shown as blue arrows) are having an effect, because after each ‘opening’ step, the rate at which cases are falling slows down, because viral transmission chains can spread further in the more liberal environments.

And since the start of May, the daily rate of positive cases has been rising slowly.

So after today, with one has to expect that the daily rate of positive cases will rise faster, and that numbers will probably not decline for several weeks or months.

Also, the potential for tourists and returning tourists to re-seed infections of different variants around the country is a concern.

And many people will be distressed by this rise in cases and the (probably) ineffective controls at the borders.

But, as I said above, in my opinion the key question is:

• Is there the potential for the pandemic to expand into the general population and kill hundreds of thousands of people?

And as far as I can tell, the answer is ‘No’. Probably.