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Dublin: 18 °C Thursday 6 August, 2020

Explainer: The R0 is now 'at or above 1', but what does that mean for Covid-19 in Ireland?

Acting chief medical officer Dr. Ronan Glynn said that the situation remains stable.

Prof Philip Nolan, Chair of NPHET Irish Epidemiological Modelling Advisory Group.
Prof Philip Nolan, Chair of NPHET Irish Epidemiological Modelling Advisory Group.

DESPITE ACTING CHIEF medical officer Dr. Ronan Glynn saying last night that Covid-19 is “stable” in this country, health officials are nonetheless voicing “a level of concern” about the current situation. 

The country has seen an increase in weekly virus cases over the past couple of weeks, with particular concern being raised about cases related to travel

At last night’s Covid-19 briefing, Professor Philip Nolan, chair of NPHET’s Irish Epidemiological Modelling Advisory Group, said that reproduction number (R0) has also gone up in recent weeks and is now “at or just above 1″.

“For several weeks now, we’ve been looking at declining disease and reduced transmission and R0s in the region of 0.5. Our best estimate is, as you would expect from the number of cases we’re seeing this week compared to last week, the reproduction number is at or just above 1,” he said. 

The R0 has become a highly publicised metric in gauging the prevalence of Covid-19 across populations. The number refers to the number of other people infected by one confirmed case.

If people with Covid-19 are infecting an average of one person the R0 is 1, if they are infecting two people the R0 is 2, and so on. 

In the early stages of the pandemic in this country, the R0 was somewhere between 2 and 4. Immediately after the first restrictions were introduced it then decreased to 1.5 and 3.

Last month, after a couple of months of severe restrictions and widespread changes in public behaviour, the R0 was estimated at somewhere between 0.4 and 0.5

If the R0 is kept below 1 then the virus will eventually die out in a population but, as Nolan has previously explained, until there is a vaccine for the virus there is always the potential for the virus to be reintroduced into a population. 

Speaking back in April about the R0, Nolan said: 

If the more relaxed measures also succeed in containing the disease, and R0 doesn’t go above 1.1, the reduction number just keeps above 1, and we get a small increase in cases, the reimposition of measures will suppress the disease again.If the reproduction number goes a little bit higher to 1.2, we’ve got a slightly larger but manageable peak in cases, and a reimposition of measures will again suppress the disease if the reproduction number goes up to 1.6, imagine what that means, that means means if one person gets infected, they infect one other person, but the second person infects two people.


The use of the R0 as a gauge has several drawbacks, however. One of them is that there are various ways to estimate it and various models used to do so. 

One of the models used by NPHET is the one developed by Imperial College London, which also takes account of the number of deaths within a population. A figure that has been in decline here in recent weeks. 

The estimation of the R0 is also dependent on the number of cases within a population and NPHET has noted that this can be difficult when case numbers are low. 

For reference, on the April day when Nolan made the above statement there were 629 new cases reported. Yesterday, 23 new cases were reported and it’s expected that the number will be somewhere above 90 for the week. 

When cases themselves are low, it is also clear that the effect of the R0 is less pronounced because there are fewer people with the virus. 

“Broadly speaking the last couple of weeks we’ve had very low levels of disease, very low case numbers confirmed per day,” Nolan said last night. 

We’d a particularly low case count in week 26, that’s two weeks ago, with only 61 cases in total being confirmed. And then last week we saw a higher number, 93, and at the end of this week we’ll probably see around that number again. So the last two weeks, we’ve seen just a little bit more little bit more incident cases than we saw three weeks ago.

“And if you’re seeing an increase in cases, then you are going to see an increase in reproduction number. We’ve said before that this estimate is really quite unreliable when you’ve got very low numbers, and therefore the estimate is varying quite widely,” he added. 

009 Dept of Health Acting chief medical officer Dr Ronan Glynn. Source:

When numbers are low, it can cause the R0 to spike, an fact we observed recently in Germany.  This occured when the number jumped to 2.88 before reducing again. 

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In that case it came on the back of 687 new cases being reported on one day, 549 of which were in one region and were connected to an outbreak in a meat-processing plant.

The Robert Koch Institute noted that because national infections were still low, that single outbreak had a big effect on the country’s R0.

“If we are able to keep the number of cases low the whole time, then we will be able to contain and end outbreaks locally, and that will remain the case,” the institute’s head Lothar Wieler said at the time.

A similar message is now being shared by Irish health officials, who are repeating that greater importance should be placed on managing local clusters. 

“I’d focus on the increase in cases and their clustering, rather than the reproduction number,” Nolan told RTÉ’s Morning Ireland today. 

And this is the point at which we need to re-sound the message about going about our business with great caution, so that there is no opportunity for the virus to spread, and to encourage people very strongly to remember that. We’re seeing very low numbers, very low levels of the virus and very low numbers of cases still, and it’s at this early point that we want to remind and encourage the public that we can resume some level of social activity and some level of economic activity. But with great caution.

Speaking at last evening’s briefing, Dr Glynn made a similar call.

“I think the situation overall remains stable,” he said.

“The best estimate of R would suggest that it’s still at or about 1 and we’ve seen that as we’ve eased down through the various measures that, because the vast majority of people are doing precisely what we’ve asked them to do, we’ve not seen advance in the figures.”

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Rónán Duffy

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