It is 9 months now — why have we no mass testing for Sars-Cov-2 yet?

On the surface this is such an obviously good idea. Nobel laureate Paul Romer has made some calculations already in March. The math is clear — even with imperfect tests, even with imported cases, with frequent mass testing we could have the pandemic under control in a month. The cost would be big — but actually much less than than the stimulus packages. So why this did not happen yet? Why it is not even a common knowledge that this should be the plan? My theory is that there were multiple filters each reducing confidence in it. First two, maybe the most impactful, are just strange perception accidents:

• At first look the costs to test everybody seems prohibitive. The so called ‘gold standard’ PCR tests, that everybody uses now, are at least about $100 a test. It is quite surprising that tests can be 20 or maybe 200 times cheaper (the antigen tests can cost $5 and similar tests for detecting malaria even 50 cents).
• The antigen tests are commonly confused with antibody tests. Not only by amateurs or journalists — I haves seen that mistake in official correspondence quoted by journalists. Antibody tests are not very good for stopping the virus spread because antibodies appear in the blood with a big delay and if we relied on antibody tests people could walk around for a week spreading the virus before we could detect it and tell him to isolate. This is reinforcing the first point — because when someone notices a ‘cheap rapid test’ with something ‘anti’ he automatically assumes that it is about detecting antibodies. I know — I did that.

Then we have the bureaucratic complication, that is both a real block and is also reinforcing the perception that the rapid tests are useless:

• The agencies approving tests for distribution require them to have accuracy close to the PCR tests. In particular the American FDA requires the antigen tests to have 80% sensitivity of PCR for point of care use and 90% of PCR sensitivity for home use. We now know that for catching the infectious people (who have high viral loads) the sensitivity of antigen tests is enough — but the agencies are big beaurocracies and setting standards is their job.

Then we have some real problems, with satisfying solutions:

• With mass testing there will be a huge problem of false positives. For example if you test a million people with current tests you would get about 20k false positive results. It might not be that unreasonable price for stopping the epidemy — but it might destroy compliance. Fortunately there are ways to reduce it — if we get two tests with uncorrelated false positives — then could expect only 400 of double false negatives — much more reasonable, and at these numbers you can add PCR testing to do a triple check. This is not yet scientifically evaluated — but we can expect that in tests looking for different parts of the virus the false positives should not be very correlated.
• Population wide testing will be difficult organizationally. Even if the tests themselves are cheap — then what about the highly qualified lab workers and nurses to take the samples and process them, working in a dangerous conditions? That would never be cheap. Fortunately the antigen tests can be so simple that they can be applied at home by the patients themselves.
• It might be difficult to produce the tests in needed quantities and at the required quality. This is a real problem, but the tests are not complicated machines — they are simple paper slips infused with the proper chemicals. Once the basics are worked out — like finding the right substances — it seems to be well suited for mass production.

And finally:

• If people test themselves — then how can we make sure they comply with the procedures and isolate themselves and report the results? Isolating is the more important part — but I am afraid that if people don’t report the results — then they might skip the costly isolation. Especially if they remain asymptomatic. People can invent all kind of theories to justify doing things for their benefit. How can we motivate people to isolate? I have no ready answer to that. Maybe we need a hybrid system where there will be some testing at schools, at work and other gatherings with proper reporting and procedures plus some at home testing where we more rely on people.

On a more meta level — how similar it is to the masks situation: an idea that for a lay person looks obviously good is attacked by experts. Here with that additional antigen - antibody confusion.

Additional info on these tests and advocacy: https://www.rapidtests.org/