Coronavirus (Covid-19) Infection Survey Northern Ireland – 12-18 December 2020

Date published: 24 December 2020

The Department of Health today published the next in the series of weekly results from its COVID-19 Infection Survey (CIS).

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The findings set out in this report relate to modelled positivity estimates for Northern Ireland for the week up to the 18 December 2020. The aims of the CIS are to estimate how many people have the infection and the number of new cases that occur over a given time as well as estimating how many people have developed antibodies to COVID-19.

The survey over time will help track the extent of infection and transmission of COVID-19 among people in the community population (those in private residences).

Key Findings:

Due to the relatively small number of tests and positive swab results within our sample, credible intervals are wide and therefore results should be interpreted with caution.

  • During the most recent week of the study (12 December – 18 December), it is estimated that 10,100 people in Northern Ireland had COVID-19 (95% credible interval: 6,100 to 14,900).  
  • This equates to 0.55% of the population (95% credible interval: 0.33% to 0.81%) or around 1 in 180 people (95% credible interval 1 in 300 to 1 in 125).
  • This is based on statistical modelling of the trend in rates of positive nose and throat swab results. Modelling suggests that there are early signs that the percentage of people testing positive in Northern Ireland has increased in the most recent week.
  • In the latest six-week period, there were 16,629 swab tests taken in total from 7,672 participants. Of these, 84 participants tested positive from 68 different households.
  • In the latest two-week period, of the 4,496 participants in the study, 26 tested positive from 22 households.

An analysis was produced by Sarah Walker at the University of Oxford to look at the prevalence of the new variant of the virus across the UK. Swabs are tested for 3 genes present in the coronavirus: N protein, S protein and ORF1ab. Each swab can have any one, any two or all three genes detected. Positives are those where one or more of these genes is detected in the swab other than tests that are only positive on the S-gene which is not considered a reliable indicator of the virus if found on its own.

The new variant of Sars-Cov-2 has genetic changes in the S gene. This means the S-gene is no longer detected in the current test, and cases that would have previously been positive on all three genes are now positive only on the ORF1ab and the N gene (not the S gene). There are also other reasons why a swab may be positive for only these two genes, including lower viral load in the sample, which is why we have always seen a small percentage of this type of positive result. Absence of the S-gene appears to have become a reliable indicator of the new variation from mid-November. Prior to that, the data should not be read as being an indicator of the variant. There has recently been an increase in the percentage of positive cases where only the ORF1ab and N genes were found and a decrease in the percentage of cases with all three genes. We can use this information to approximate the growth of the new variant.  

Over the last few weeks, decreases in Northern Ireland have been due to cases which have a pattern of gene positivity which is not compatible with the new variant of the virus. However, the early signs of an overall increase appear to reflect an increase in new-variant compatible positives in the most recent week. 

It should be noted that there is considerable uncertainty around these estimates due to the small numbers of new variant compatible positives detected in Northern Ireland.

Notes to editors: 

1. The Department of Health has been working along with the Public Health Agency, Northern Ireland Statistics and Research Agency and the Office for National Statistics (and its various survey partners) to extend the COVID-19 Infection Survey to Northern Ireland. Fieldwork in Northern Ireland began on 27 July 2020.

2. All results are provisional and subject to revision. Due to relatively small number of tests and positive swab tests within the sample, credible intervals are wide and therefore results should be interpreted with caution.

3. These statistics refer to infections reported in the community (i.e. private households). These figures exclude infections reported in hospitals, care homes and/or other institutional settings.

4. Estimates of the total national proportion of the population testing positive for COVID-19 are adjusted to be representative of the population of Northern Ireland that live in private residential households in terms of age, sex and region.

5. Weekly reports are to be published with findings from the COVID-19 Infection survey. It is anticipated that new and further analyses will be added to the weekly reports over time.

6. Further information about quality and methodology associated with the survey can be found on the ONS website.

7. This publication is available on the DoH website.

8. Additional information is available from:
Information Analysis Directorate
Department of Health
Annex 2, Castle Buildings
Belfast BT4 3SQ
Telephone: 028 9052 2340

9. For media enquiries please contact the DoH Press Office by email

10. Follow us on Twitter @healthdpt.

11. The Executive Information Service operates an out of hours service for media enquiries between 1800hrs and 0800hrs Monday to Friday and at weekends and public holidays. The duty press officer can be contacted on 028 9037 8110.

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