The Department of Health today published headline results from its COVID-19 Infection Survey (CIS).

The findings set out in this report relate to modelled positivity estimates for Northern Ireland for the week up to the 31 December 2021. 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

Temporary publication of early results from the COVID-19 Infection Survey

During this period of high infections, we have decided to publish our headline results two days early. This means that the latest estimates on infection levels are available at their earliest opportunity and further breakdowns for the same period and a longer time series will be published on Fridays. These early estimates are provisional and are subject to change as we receive more data, but they have undergone sufficient quality assurance to ensure that they are based upon an acceptable number of test results received up to the end of the reference week. This week, we are publishing estimates for the week ending 31 December, based on a reference date of 28 December to account for anticipated additional test results for the last few days.

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 (25 December - 31 December), it is estimated that 72,900 people in Northern Ireland had COVID-19 (95% credible interval: 56,800 to 90,100). 
• This equates to 3.97% of the population (95% credible interval: 3.10% to 4.91%) or around 1 in 25 people (95% credible interval 1 in 30 to 1 in 20).
• Modelling suggests the trend in the percentage of people testing positive continued to increase in the week ending 31 December in Northern Ireland.
• COVID-19 infections compatible with the Omicron variant continued to increase rapidly across England, Wales, Northern Ireland and Scotland in the week ending 31 December 2021 and Omicron is now the most common variant across all UK countries (see below for more information on the variant analysis undertaken).

Variant analysis

The World Health Organization (WHO) have defined names for Variants of Concern. These are variants that the UK government has under surveillance. You can find out more in the SARS-CoV-2 variants of concern and variants under investigation in England briefing document (PDF, 2.51MB).

UK Variants of Concern:

• Alpha: B.1.1.7
• Beta: B.1.351
• Gamma: P.1
• Delta: B.1.617.2 and its genetic descendants
• Omicron: B.1.1.529 (which includes sublineages BA.1, BA.2 and BA.3)

The Omicron variant (B.1.1.529) of COVID-19 has changes in one of the three genes that coronavirus swab tests detect, known as the S-gene. This means in cases compatible with the Omicron variant, the S-gene is no longer detected by the current test. When there is a high viral load (for example, when a person is most infectious) absence of the S-gene in combination with the presence of the other two genes (ORF1ab and N-genes) is a reliable indicator of the Omicron variant (B.1.1.529). However, as the viral load decreases (for example, if someone is near the end of their recovery from the infection), the absence of the S-gene is a less reliable indicator of the Omicron variant.

More information on how variants from positive tests on the survey are measured can be found in the ONS Understanding COVID-19 Variants blog and in the methodology article.