You've no doubt seen talk about "reopening" the country. In Georgia, it's even happening. Is this a good idea? We say that we need more testing in order to know. We definitely need more testing, but even that has limitations in preventing the spread of #COVID19. Why?

A thread.

A super important thing to know up front is that when we talk about the “COVID test” we are really talking about a set of different tests that have varying ability to detect a current or past infection. I wrote a thread about that here: https://twitter.com/jenheemstra/status/1244043096301731845

While some tests are more sensitive than others, none of them are perfect. We don’t know how long after becoming infected before a person can spread the virus to others, but we do know that none of the “nasal swab” tests can detect infection immediately.

You can think of this similarly to pregnancy tests. When someone becomes pregnant, their body ramps up production of a key hormone called hCG, and that is what a pregnancy test is detecting. However, it takes some time for the level to get to the point that it can be detected.

This is why even the most sensitive of tests can’t tell someone if they’re pregnant until several days after they become pregnant. Same goes for infections. When someone is first infected, it starts with a small number of virus molecules, and then those replicate over time.

Different nasal swab tests will be able to detect and diagnose an infection at varying times post-infection, but all of them take some time. And, just like how you’re either pregnant or you’re not, you’re either infected or you’re not, independent of what the test says.

So, where does this leave us? Are the tests useless? Definitely not! It’s just critical that we understand their limitations. If you get a positive test result, there’s a good chance you have the virus and should get treated and isolate yourself.

But, if you get a negative result (and especially if you have symptoms) this does not mean you can safely go about normal life without still having some risk of spreading the virus. You might just be too early in the infection to test positive.

Everything I’ve talked about above is for the “nasal swab” test that is intended to detect a current infection. There is also a blood test that should be able to tell people whether they have been infected previously and have now recovered.

The blood test is important since we don’t currently know how many people have been infected and just didn’t have symptoms. Right now, the data suggest that people who have had COVID-19 cannot get it again, so the blood test might tell how many people are immune.

As we talk about reopening, it’s important to realize that “getting back to normal” should probably mean “starting up with a new normal.” If we care about the most vulnerable people in our population, we need to realize the risks that no test can fully eliminate.