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Posts Tagged ‘antigen testing

more on rapid antigen testing, and the vaccine race

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So to continue with this issue of rapid at-home testing, there are/were many tests of a more simple and potentially cheaper type being manufactured, but they were all diagnostic tests (i.e tests that require expert interpretation as part of a diagnosis), and even if they’d been scaled up fairly rapidly they wouldn’t meet the kind of demand Dr Mina was envisaging. That’s to say, not doubling the tests available but multiplying those tests by a hundred or more, for nationwide availability in the US. 

I want to get clear here, for myself, about the difference between an antigen test and a PCR test. An antigen test detects viral proteins. The paper strip test Dr Mina refers to contains antibodies that will bind to the antigens, or proteins, if those antigens are present in sufficient numbers. The presence of those antigens, or viral proteins, indicates that the virus is active – it is producing the antigens via the ribosomes of host cells. The PCR test detects viral RNA, whether or not the RNA is active. And so the antigen test reveals infectivity. The PCR test more often than not finds inactive viral fragments, since this RNA remains in the cell for some time after the period of infectivity, the upswing, which is relatively short. 

Dr Mina has this to say about the sensitivity of the two test types. The PCR test will pick up virus from a few days to six weeks (or more) after infection, but the subject may be infective, that is, able to spread the virus, for the first two weeks (or less) after acquiring it. So its sensitivity to detecting an infective subject is not so great as its sensitivity to the virus itself (living and reproducing, or dead, or disabled). An antigen will be testing negative, both in the very early phase of infection, when the virus isn’t yet producing enough viral protein to show up on the test, and in the long phase when the virus, or parts of it, are still present but no longer replicating and infecting. So it is actually more sensitive to infectivity, which is exactly what’s required. And this essentially has to do with the frequency with which the antigen test can be used, because the PCR test has this lag time built into it. 

It’s hard to believe that it’s this simple and straightforward, and that supposedly smart regulators aren’t jumping on this and getting these tests out there. Could I be missing something? I note that Dr Mina uses transmissible rather than infective, by the way.

So why aren’t such tests available? In the USA, it’s because it sounds a lot like a diagnostic, which requires approval or licensing from an organisation called CLIA – but that’s for them to work out. As to the situation here in Australia, which hasn’t had to deal with anything like the mess they’ve made for themselves in the USA, such a testing system would still help to detect spreaders, providing there was blanket use, and this would mean fewer lock-downs and less economic impact. As would be the case globally. An ABC article from late October features an interview with Prof. Deborah Williamson, director of clinical microbiology at Melbourne’s Doherty Institute, who recognises the value of rapid antigen testing, but feels that we need ‘to better understand their effectiveness as a screening tool in different epidemiological contexts’. This is understandably cautious, but then there isn’t the urgency in Australia that there so obviously is in the USA. But the USA has another major problem, which is almost incomprehensible considering the disaster that has unfolded there – and that is lack of compliance. Even if rapid antigen testing – cheap and in such supply that it could be utilised on a daily basis by the whole population – even if this was made available, there’s surely a major question as to whether most people would use the test any time they looked a bit peely-wally [under the weather], let alone when they were completely asymptomatic. So you could say that Americans are paying the price for their ‘rights without responsibility’ ideology – not shared by all Americans of course, but apparently shared by too many for them to escape from this, or any other pandemic, lightly. 

Anyway, if we imagine a world, or a country, of largely compliant, responsible individuals, and widely available, cheap or free antigen testing, there would be no need for the quite onerous contact tracing mechanisms that we now have – signing in by phone or by hand at restaurants, pubs and the like – because those testing positive at home wouldn’t be attending those places until they tested negative again. Businesses could run, schools, airlines, etc. Economies could function almost as normal. 

Of course now we have the vaccine, or almost. So far though it’s the Pfizer/BioNTech two-shot vaccine, which needs to be kept at way below zero (celsius) temperature, so, difficult to scale up and make available to those without proper facilities. No sign of that one coming to Australia for a while. I read an article yesterday, ‘The Amazing Vaccine Race’, in Cosmos mag. It outlines some of the contenders – the companies and the vaccine types. It points out that some companies are trying to play the long game, to try not for the first vaccine, or one of the first, but the best. The problem though, says, Nicolai Petrovsky, whose company Vaxine is based here in Adelaide, is that ‘the first runners end up getting all the resources’. And it may take quite a while to work out the best, and if the early runners turn out to be good enough, we may never find out which would’ve been the best. Vaxine is currently trialling a covid19 vaccine which combines the virus’s spike protein with an adjuvant (a treatment which enhances the immune response of the vaccine) based on a plant polysaccharide. And there are some 160 other contenders, according to the article. One in Sydney is combining the spike protein with bacillus Calmette-Guerin (BCG) which has been shown to reduce mortality from a range of viral respiratory infections. And there are others, just sticking with Australia, some with a degree of complexity that defeats me, for now. However, there are scant resources for local production here.

Although phase 3 trials of the current front-runners tested for safety among many thousands, it’s unlikely that scaling up to the millions will be without casualties, however minimal. And there’s the question of long-term immunity, which can’t really be tested for in this rushed situation. So it will be very interesting to see which of the current contenders wins out in the ultra-long run, or if something we’ve barely heard of yet finally proves the best option. 

References

Rapid Coronavirus Testing – At HOME (COVID-19 Antigen Tests) with Dr. Michael Mina (video)

https://www.abc.net.au/news/health/2020-10-24/rapid-antigen-tests-for-coronavirus-screening/12808176

Dyani Lewis, ‘The Amazing Vaccine Race’, in Cosmos: the science of everything, issue 88, September-December 2020.

Written by stewart henderson

December 9, 2020 at 5:44 pm