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Professor Philip Nolan has defended the National Public Health Emergency team from a claim that it wrongly estimated the R (reproductive) number of Covid-19 in mid-October.
The chairman of NPHET’s Epidemiological Modeling Advisory Group reacted today to a tweet from independent TD Michael McNamara, who was the chairman of the Dáil’s special committee on coronavirus.
McNamara said on Twitter: “On October 22, NPHET declared ‘the current estimate of R is 1.3-1.4’. That was clearly wrong.”
Professor Nolan replied: “The estimate was made on October 20 based on confirmed cases up to midnight on October 18. It might be helpful to establish the fundamental principles behind the different ways of estimating R.”
He explained: “R is an estimate of the rate of viral transmission: if a person has the virus, how many people on average will they infect.”
“We cannot know R for today or yesterday; if people are transmitting the virus today, it takes time for the resulting infected to become symptomatic or to be traced, tested and confirmed as cases. We can only reliably estimate R for today in 7-14 days. “
Professor Nolan gave an example of how 10 people infected with SARS-CoV-2 on day zero, then infecting 14 others, would give an R of 1.4.
The average time it takes one person to infect another is known as the serial interval, which is estimated at about 4.5 days.
“We don’t know about any of these infections until they become symptomatic or are found, tested and confirmed,” said Professor Nolan.
“This takes a variable amount of time, typically taking 5 to 7 days to develop symptoms and 7 to 12 days to become a confirmed case.
“So we cannot measure infections, we can only estimate infections from the pattern of cases over time.
“Cases do not arise at the same time as infections, there is a lot of variability. And some infections will not be detected at all.”
Interactive map: Covid-19 cases in your local area
He added: “The different ways of estimating R use statistics and models to infer the underlying patterns of infection from the known pattern of cases, but the most important limitation is that the estimate relates to infections (and social contacts) one to two weeks. before.
“There are ways to ‘forecast R,’ but they have significant limitations. So we don’t rely solely on R estimates to understand the likely trajectory of the epidemic, but rather use a variety of indicators and analyzes.”
Yesterday, Health Minister Stephen Donnelly greeted some “good news” about the 14-day Covid-19 incidence rate here.
“In other good news, Ireland’s 14-day incident rate is 186 per 100,000 today. It was over 200 on Thursday and over 300 not long ago,” he said on Twitter.
“This is the result of one nation working together to roll back this virus and protect each other. Let’s keep going!”
Meanwhile, seven more people have died from coronavirus in Northern Ireland, bringing the official death toll to 781.
And a village in Co Roscommon has completed a Covid-19 rapid testing community pilot program, with 150 residents participating over a six-week period.
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