Initially, the thought was that only about 2% of COVID cases had co-infection with other viral illnesses, so the guidelines were to do a PCR respiratory virus panel that tests for a bunch of common viruses. If something came up positive, you then weren't allowed to test for COVID. This never made any sense to me. During the winter, it's not uncommon to see preschoolers test positive for 3 viruses. These likely represent multiple infections starting at different times, and explains waves of illness but constant runny noses in this age group. Probably 3 or 4 times a year, I'll see 4 concurrent positives. One of my colleagues recently had a patient with 5, but that's almost as rare as the pan-positive urine drug screen (marijuana, cocaine, meth, opiates, benzos, barbiturates and (highest degree of difficulty these days) PCP).
Since early in the outbreak, with more sampling, the current thinking is that there is up to a 20% co-infection rate in all comers. One study in critically ill patients in China at multiple different hospitals showed rates between 20 and 80%. There does seem to be a trend toward higher co-infection rates in sicker people. Not sure if co-infection worsens severity of illness (though this seems quite plausible) or severe illness increases susceptibility to other infections.
We are are starting to realize that the tests are about 75% sensitive. No bueno. It's not clear if this is a problem with the PCR test itself, getting an adequate sample to test (which should have been controlled for in the attached article "singapore testing") or due to some characteristic of the virus itself. Take a few minutes to look at figure 1 at the top of page 4 - lots of data crammed in, but very informative. Pay close attention to the red and green dots. Positive, then negative, then positive again. Patients sick enough to be in the ICU with negative tests on random days. It's definitely frustrating, and means that a negative test still requires the same quarantine or a second test if still symptomatic. The other concerning thing is the number of days that virus continues to be shed after folks are asymptomatic.
