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Altering the COVID-19 Case Definition

At first of the pandemic, little was recognized concerning the novel coronavirus we now know as SARS-CoV-2 – or as its generally known as COVID-19.

Two of the issues we didn’t know had been how lengthy an an infection lasts or whether or not you might be contaminated greater than as soon as. We now have been monitoring take a look at outcomes since June 2021 to achieve solutions to those questions and have printed evaluation within the weekly Influenza and COVD Surveillance studies.

Though reinfections had been initially very uncommon, we’ve seen the quantity rising slowly over the past two years, as immunity from prior an infection wanes and new variants emerge.  Throughout the Omicron variant wave, the quantity and proportion of individuals being reinfected with SARS-CoV-2 has elevated. Moreover, as so many individuals have now skilled an infection since 2020, there’s a bigger pool of individuals during which reinfection can happen.

To adapt to this altering image, UKHSA has modified the case definition for COVID-19.

How are episodes of COVID-19 outlined?

Till now, COVID-19 instances have been reported on the particular person stage: each constructive take a look at taken and reported by one particular person has been thought-about a part of a single case report, initiated by their first constructive take a look at.

Most infectious ailments sadly have the potential to contaminate individuals greater than as soon as. The frequency at which this occurs varies between ailments, as does the definition of an episode, however the precept stays the identical.

Primarily based on proof from the SIREN examine (on the SIREN portal, and in The Lancet), an episode definition has now been created.

Briefly, constructive assessments belonging to the identical particular person are grouped collectively and thought of one episode if they’re inside 90 days. Constructive assessments outcomes which can be 90 days aside (no matter unfavourable assessments in between) shall be thought-about as a separate episode of an infection, and subsequently the particular person is counted as a case greater than as soon as. These subsequent episodes are known as ‘attainable reinfections’.

For instance, if an individual assessments constructive at some point (day zero) and once more 100 days later (day 101), they might be deemed as having two case episodes of an infection. Nevertheless, in the event that they examined constructive on day zero, day 30, and day 101, all these assessments can be grouped right into a single case episode of an infection.

This episode definition relies solely on time and doesn’t contemplate unfavourable take a look at outcomes, nor whole-genome sequencing outcomes. Present proof and evaluations counsel that this has minimal affect on the case episode figures. As all the time, UKHSA and our companions will proceed to observe and replace our methodologies as applicable.

What this implies for the COVID-19 response

The case definition that has existed because the starting of the pandemic  (particular person stage) has been the surveillance definition on which the UK Coronavirus Dashboard evaluation, and studies have been based mostly. Nevertheless, contact tracing has undertaken a really protected follow of following all constructive instances, no matter whether or not they had been attainable reinfections or instances of extended an infection.

The change within the case definition to at least one based mostly on episodes signifies that case charges will change and higher mirror present charges of an infection locally. From 31 January 2022, the time of this variation, the entire enhance within the variety of instances in England is round 4.0%, representing 588,114 instances over your complete interval of the pandemic. Nevertheless, a lot of the added episodes have been because the emergence of Omicron within the UK; presently round 10% of recent episodes every day are reinfection episodes.

Because of the statistically small numbers concerned till now, it is extremely unlikely that had these instances been beforehand included within the surveillance database and studies, they might have had any substantial affect on situational assessments and nationwide or regional stage resolution making.

Moreover, when contemplating ranges of transmission, case charges are not often utilized in isolation and are supported by a variety of different related metrics – similar to positivity charges, ONS prevalence estimates and R consensus statements by area.

Adjustments have now been made to many metrics on the UK Coronavirus Dashboard, particulars of which might be discovered right here – Metrics replace.

How the change is being made

COVID-19 take a look at outcomes from laboratories throughout the nation, together with NHS and UKHSA labs, UK Authorities lighthouse laboratories, and personal testing suppliers, are reported to UKHSA’s Second-Era Surveillance System (SGSS). SGSS collects and organises constructive take a look at outcomes for all notifiable ailments from over 225 laboratories throughout the UK.

When a constructive take a look at result’s reported by a laboratory to SGSS, it goes via linkage processes to validate who the pattern was taken from utilizing the NHS affected person information after which goes into the deduplication logic to type a part of the case dataset.

Up till now, this deduplication logic signifies that if a person has not had a earlier constructive COVID-19 take a look at, they enter the case dataset (recognized in epidemiological lingo as a “linelist”). Nevertheless, if that particular person has already had a constructive Covid-19 take a look at outcome reported they are going to exist already within the linelist and so shall be deduplicated. This ensures that the person solely exists within the linelist as soon as. This course of occurs to all new constructive take a look at outcomes every single day.

Within the new course of, this deduplication logic considers the episode definition and assigns constructive assessments outcomes to episode information (reasonably than particular person information) as applicable.

To make this variation, the crew that run SGSS have to undo all of the deduplication that has occurred to constructive COVID-19 take a look at outcomes, after which redo it utilizing the brand new episode definition. This entails 19,833,233 constructive take a look at outcomes being deduplicated into 15,177,316 episodes (as of 31st January), all in at some point, so we’ve a seamless transition of knowledge going into surveillance, evaluation, and naturally the UK Coronavirus Dashboard.

Public Well being Companies in Wales, Northern Eire and Scotland are equally updating their case definitions to this episodes-based (90-days aside) definition.  They’ll make the modifications to the info they supply to the UK Coronavirus Dashboard sooner or later.

Wanting ahead, UKHSA will proceed to observe take a look at outcomes and reinfections to make sure that we work to our duty to defending each member of each group from the affect of infectious ailments.


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