The R-Factor is the crucial figure keeping NSW in lockdown

There's one calculation health authorities around the world are using to determine the length of lockdowns, and as NSW Premier Gladys Berejiklian hinted today, the state is some way from being on the right side of it.

The R-Number or R-Factor ("r" for "replication"), predicts the potential for disease spread in the community – that is, how many people one person with a disease could be reasonably expected to infect.

Ms Berejiklian said today that the R-Factor for COVID-19 in NSW showed that each person with the virus was spreading it at the moment to about 1.3 people.

A quiet Bondi Beach in Sydney.

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Thus, today's 633 new cases could infect a further 822.9 people – 823 rounding up.

"We need that number to be below one," Ms Berejiklian said.

"We need to make sure that whoever has the virus doesn't spread it to anybody else."

How is the R-Factor calculated?

The first thing to know is that the R-Factor will differ between communities nationally and around the world.

The BBC reported earlier this year, as the UK was experiencing new cases in March, that people could expect to infect seven to nine other people – not unexpected given the UK's much denser population.

But that much higher figure preceded the first recorded case of the Delta variant in the UK by about a month.

The R-Number is a necessarily reactive figure, calculated through ongoing case numbers.

READ MORE: New Zealand COVID-19 infection came from NSW: Ardern

Contact tracing and hospitalisations provide the data for researchers to determine the spread of a disease.

As the New Scientist has reported, this retrospective data building means the data is imprecise – but it does indicate a worrying trend.

The R-Factor for measles in an unvaccinated population is estimated to be about 15 – another testament to the efficacy of vaccinations.

READ MORE: 'We haven't seen the worst of it': 633 new local cases in NSW

Ms Berejiklian was clear that bringing the R-Factor below 1.0 was key to reopening NSW.

"That's what will turn the corner," she said today.

"And until we hit that corner, we're going to see case numbers rise and the way we stop case numbers rising is stopping mobility."

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