How UK variant could spread

The UK variant of Covid-19 found in Auckland is much more infectious. In assessing the risk, it’s all about the reproduction number. Keith Lynch, Kate Newton and Aaron Wood report.

The UK Covid-19 variant has been found in the community, prompting a snap lockdown in Auckland. The variant behaves differently and is much more infectious. It makes the risk of an outbreak much greater. How, exactly?

Let’s start with the R number

You’ve probably heard of the Reproduction or R number. This is the average number of people to whom one infected person will pass the virus.There are actually two R numbers.

Think of R0 as the starting point of a pandemic. It is, on average, the number of people to whom an infected person will pass the virus, assuming there is nothing to mitigate the spread.

So, for example, no-one has developed immunity, no-one is wearing a mask, or practising social distancing.

Re is the transmission rate once a virus is embedded in a community and when public health measures, such as mandatory social distancing and vaccines, are put in place.

The Re, therefore, will be lower than the R0. The Re is typically reported as the R number. So, for example when you read that the UK’s R number is between 0.7 and 0.9, that’s the Re number.

Why the UK variant is so worrying

The UK or B.1.1.7 strain, which was discovered in late 2020, is thought to be 50-70 per cent more infectious than the original strain of Covid-19.

The genetic make-up of the UK variant makes it much better at latching on to human cells and therefore at making more people sick and infectious. In short, it drives the R number up.

A higher R number means more infections and subsequently more seriously ill people.

How the new variant spreads

The fact the variant is 50-70 per cent more infectious obviously sounds bad, but what does that actually mean? To answer that question, we’ve run four different scenarios:

Scenario 1: There is one case of the original Covid-19 variant in the community. There are no public health measures in place, no-one has immunity, and noone has been vaccinated. In this example, the R is 2.5.

Scenario 2: There is one case of the UK variant in the community. There are no public health measures in place, no-one has immunity, and no-one has been vaccinated. In this example, the R is 3.75, or 50 per cent more infectious than the original variant.

Scenario 3: There is one case of the original Covid-19 variant in the community. There are public health measures in place, similar to level 2 restrictions in New Zealand. In this example, the R is 1.2.

Scenario 4: There is a single case of the UK variant in the community. There are public health measures in place, similar to level 2 restrictions here. In this example, the R is 1.8, again a 50 per cent increase.

Before we go on with these scenarios, a few things.

We didn’t just pluck these numbers out of the air. We picked those four R numbers in consultation with University of Canterbury mathematics professor Michael Plank.

Plank has been part of a group devising maths-based models to help the Government with its decision-making throughout the coronavirus pandemic.

The R numbers will obviously vary country to country, dependent on a broad range of factors.

These are hypothetical scenarios to illustrate the potential spread of the more virulent Covid-19 variant. We are not attempting to predict what will happen in real life.

The two graphs on these pages show the difference between a controlled and uncontrolled spread of the UK variant.

But remember, nothing is ever certain with Covid-19. Each outbreak is unique, Plank says.

An outbreak with an R number of 1.8 may well become established in the community, and that very quickly becomes an enormous number of cases (assuming there has not been further mitigation). But you can get lucky, and it could just fizzle out.

‘‘The higher the R number, the less likely it is to fizzle out, but it can still happen,’’ Plank says.