The COVID-19 R number in Northern Ireland has drifted back up towards 1 since the partial lifting of the lockdown restrictions

The COVID-19 basic reproduction number, or R, has become the most crucial metric for policymakers as they start to tentatively reopen the economy following the lockdown measures that were implemented in March to halt the spread of the disease. Lockdown measures will only be relaxed when the R value is below one.

To attempt to calculate the R value for Northern Ireland, I used Imperial University’s EpiEstim app, which provides estimates for the reproduction number from daily case count data and serial interval estimates from analysis on a COVID-19 outbreak in Singapore. The tool estimated Northern Ireland’s R value as of the 28th of May to be 0.88, close to 0.9 which is the current official estimate.

The chart at the top of the post shows how Northern Ireland’s estimated R value has changed since the start of the outbreak. The tool uses a 7 day window to calculate R. Seven days after the lockdown restrictions were implemented in Northern Ireland R was around 2.6; it fell below 1 around the 24th of April.

The first phase of the lifting of lockdown restrictions commenced on the 19th of May. Since the start of the lifting of restrictions, the R value has increased from around 0.64 to 0.88; although the figure was as high as 0.99 earlier this week.

The figure for R is higher in Northern Ireland than it is in other parts of the UK and Ireland. The chart below shows data for the Republic of Ireland, England and Scotland (I couldn’t find easily accessible data for Wales). The R value dropped below 1 in all these regions around the middle of April, and is now sitting at 0.75 in England, 0.58 in Scotland and 0.67 in the Republic.

Whilst the estimation of R using this method is not an exact science, the figures here are in line with other estimates such as the regional R numbers published here.

Across Northern Ireland as a whole the R value is around 0.9, but there are significant regional variations even within Northern Ireland’s comparatively small area. The chart below shows estimates for R across each of Northern Ireland’s council areas since the 7th of May.

The count of new cases in the Derry & Strabane and Fermanagh & Omagh council areas has dwindled down to zero over the course of May. In Belfast, Lisburn & Castlereagh, and Ards & North Down the R value has stayed well below 1, even as restrictions have been relaxed.

In Newry, Mourne & Down and Antrim & Newtownabbey, the R value appears to be very close to 1. More concerning are the four council areas where the R value appears to be higher than 1.35, with the Causeway Coast & Glens area appearing to display a rapid increase in the R value since restrictions were partially lifted.

The R number appears to vary significantly within Northern Ireland, and it may be appropriate to consider lifting restrictions in some areas such as Derry, West Tyrone and Fermanagh before other areas such as the north coast. With the R value in Northern Ireland so close to 1, a blanket lifting of restrictions could cause the R value to increase significantly above 1, risking another wave of infections.

In England, the R value is higher now (0.75) than it was in Northern Ireland when restrictions began to be lifted. With the lifting of restrictions in England planned for Monday, an increase in R similar to what was seen in Northern Ireland could see R rising above 1 yet again.

The R value rising above 1 could force the government to re-impose restrictions on meeting people from outside your household and on non-essential retail. It is imperative that restrictions are eased in such a way that R stays comfortably below 1.

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