Dr. Michael Mina:

One very important point is rarely brought up. This gets to the question of why speed is so crucial. For example, PCR tests are used three days before a flight. When the virus grows in somebody's body everyone will be undetectable for the first couple of days after exposure. Once they become detectable on either test, usually the PCR is first, the virus will grow from barely anything to 1 million or 1 billion copies within a day. It grows very fast and that is the nature of exponential growth which is the only way viruses grow inside the body.

I bring that up because if somebody uses a PCR test two or three days before a flight and they get a negative result, by the time they walk onto that flight they could literally be at their peak of infection. They could actually have 1 billion or 1 trillion viral particles in their nose, completely undetected, because they got a test before they were detectable. The point is that the virus grows extremely quickly in the body. This is why we get super-spreaders. Someone at the peak of infectivity could actually be showing their negative PCR test from a swab taken two or three days earlier and walk onto a plane or into an event as a super spreader. An antigen test right there would detect those people without any problem. Those are the most important people to detect and have them rapidly removed from circulation. That is essential.

We then need to ask what is happening with vaccines. We saw what happened with the New York Yankees players and staff recently, and now we see the CDC back-pedalling. For about a year, I have tried to warn the CDC about this. If we start to relax our safeguards because the population is getting vaccinated, PCR testing is so sensitive that if somebody gets exposed, we are very likely to start picking up really low fragments of RNA.

When the viral load is that low, especially in a vaccinated person, it is often the case that the person will not be transmitting. Some people who have been vaccinated can transmit, and they will be protected on both tests. We do not want to quarantine people who are no longer transmissible; that is a net negative for public health, and PCR would cause that.

Antigen tests, because of their speed, which I explained, are safer and better when our goal is simply to limit transmission, especially in an acute event such as a flight, and also because they are much more accessible. They can be used on a more ongoing basis, if needed, over a longer period. It is much easier to do that than it is with PCR. Unfortunately, PCR, out the gate with this pandemic, was deemed to be the gold standard. As a public health test, however, it has few, if any, of the qualities we want in such a test. Rapid antigen testing, on the other hand, features speed, frequency of testing and many other qualities we would seek in a test that has a purpose of limiting transmission.