Here’s a good picture illustrating the difference between the PCR and Rapid Test. A PCR amplifies DNA and so if taken at the right time it will detect the virus before a rapid test will. But this happens when there isn’t much viral load and too little of the virus to be transmissible. Moreover, at these times, the virus is increasing rapidly so the rapid test will find the virus tomorrow. The PCR test will also pick up fragments after transmissiblity has passed which also isn’t very useful. A rapid test is very sensitive for doing what it is supposed to do, identifying periods of infectiousness.
Michael Mina has done a great job promoting rapid tests and I do think we are beginning to see some recognition of the difference between infected versus infectious and the importance of testing for the latter. What is frustrating is how long it has taken to get this point across. Paul Romer made all the key points in March! (Tyler and myself have also been pushing this view for a long time).
In particular, back in March, Paul showed that frequent was much more important than sensitive and he was calling for millions of tests a day. At the time, he was discounted for supposedly not focusing enough on false negatives, even though he showed that false negatives don’t matter very much for infection control. People also claimed that millions of tests a day was impossible (Reagents!, Swabs!, Bottlenecks!) and they weren’t impressed when Paul responded ‘throw some soft drink money at the problem and the market will solve it!’. Paul, however, has turned out be correct. We don’t have these tests yet but it is now clear that there is no technological or economic barrier to millions of tests a day.
Go yell at your member of Congress.