Criticism of prevalence data "absolute bunkum"

Saturday 15 May 2021

More information has emerged about the way that covid prevalence rates are calculated in Guernsey.

Following the introduction of category two regions on Friday, confusion reigned on social media about prevalence rates and a perceived discrepancy between Public Health Guernsey and Public Health England data.

Prevalence rates are used to inform the Civil Contingencies Authority on the risk posed to Guernsey’s population if travellers were to arrive from that region. Category two regions are defined as having less than 30 covid cases per 100,000 across 14 days. 

“It’s a very standard methodology,” said Dr Nicola Brink. “We take the number of cases in a region, divide it by its population and then multiply it by 100,000.”

Pictured: Dr Brink revealed a number of slides while explaining how Public Health calculates regional prevalence rates.

The constant comparing of Guernsey to other similar jurisdictions was panned by Dr Brink, who said it was unhelpful.

“I’m not a big fan of comparing jurisdictions – I think that everyone has to design a programme that is right for them,” she said.

“It doesn’t really mater what methodology you use, the important thing is to define what risk your population is willing to accept, if you’re using a prevalence rate to define border restrictions.”

This followed strong words from the CCA Chairman Peter Ferbrache for people spreading "misinformation" and rumours about data being misinterpreted by Public Health. 

“We are responding to the claims that we have made up data or we are fudging it; it’s absolute bunkum, absolute nonsense,” he said.

Pictured: Despite her reservations, Dr Brink did offer up some comparisons to our neighbours.

Dr Brink also sought to quash concerns about Public Health England publishing prevalence rates that don’t align with our own.

“Public Health England provide a seven-day rate, which allows them to compare regions within the UK and monitor trends there,” she said.

Additionally, a further seven days beyond the 14 day prevalence window are required before a region is re-categorised. This stops regions bouncing in and out of categories.