Dr. Tony Holohan:

I will ask Professor Nolan and Dr. Glynn if they wish to come in. I previously tried to answer the question briefly in general terms. I apologise for using up so much of Deputy Lahart's time in answering the question, but it is important that we get the opportunity to articulate our position. We have given that advice on antigen tests. They have a particular sensitivity and specificity. An antigen test is not as good as a PCR test, which is regarded as the gold standard. However, it has the advantage of being rapid, easily accessible and so on. In the hands of healthcare professionals, their use is better than in the hands of the public. Certain tests have been evaluated for their use as self-care tests, so it is based on that.

In general terms, the advice we give is that antigen tests are useful in certain circumstances. They are useful, in particular, in the circumstance of high incidence. Where what we call the “pre-test probability” of the disease being present is high, meaning the chances of the person in the first instance having the disease, the results of the test can be trusted much more in those kinds of situations than in a situation where the disease incidence is low.

The behaviour that we have always been concerned about and that has given us reason to express concern - and we have some data to back this up - is the situation where somebody does the test, particularly a person who is symptomatic, gets a negative result, concludes that means they do not have Covid and then goes on about their business without restricting their movements and so on, thereby putting other people at risk and contributing to disease transmission. That is the use - members may be familiar with this categorisation - as a “green light”. In other words, somebody does the test to give themselves permission to do something as if the test result is negative. We have always advised against their use as green light tests, and I think most international authorities advise against this as well.

The real value of an antigen test is in a high-incidence situation when a person does the test, gets a positive result that they would not have known about otherwise and decides not to go out to the pub, to work, to a wedding or whatever it might be. That is the real value. We have evidence through the Amárach tracker, which is a survey we do every week and have been doing over the course of the pandemic. The committee might be aware that we do this . We ask different questions, depending on what is topical at a point in time. When we go back to about three or four months ago, we see that where a significant portion of people who had symptoms were using an antigen test and had a negative result, that was not being followed up in a number of people by a restriction of movements and a confirmation through PCR, which was our advice at the time.

The public health messaging was focused on trying to improve that performance, and we did see an improvement in that performance. However, the most recent set of data that we have from earlier this week - and I can share these figures with the committee - shows us that about 30% of the adult population report using antigen tests in the previous week. That was as high as 55% or so in the first week after Christmas. Of the 30% using the antigen tests, many of them are in the situation where they have symptoms, and then when they have a negative result they are reporting that they are not self-isolating. I am generalising because I do not have the figures in front of me right now, but this happens in a significant majority of circumstances. We know this situation was also happening in schools, where people using the antigen test when children were symptomatic and a negative result was used as a basis to send children back into school. That is exactly the concern.

It is not about the antigen test per se, but the way in which people interpret and use the result. When it is used in the right circumstances and the interpretation of the result and people's resulting behaviour is appropriate, then we do not have a concern. Our concern is that it must be used in the right circumstances, where disease incidence is high, and then the interpretation of both the positive and negative result needs to be right and inform people's behaviour. As I said, for a period time we can see that a substantial portion of the population with symptoms were using the test inappropriately and then using the negative result to not self-isolate. That is the reason we have expressed concern about their use.