A: Testing individuals for COVID-19, while a necessary component of our public health strategy, doesn’t 100% prevent those who are infected with SARS-CoV-2 from transmitting the infection to others BEFORE they test positive.
The type of testing strategy used by the White House was to screen individuals for COVID-19 using a rapid antigen-based test (i.e., a test which detects proteins from the virus) which can deliver results in ~15-30 minutes. These tests, while highly sensitive in individuals with symptoms, were importantly not necessarily designed or approved for testing *asymptomatic* individuals for COVID-19. As a result, while you can be pretty confident that someone who tests positive with an antigen-based test truly has COVID-19 (and should not be admitted to a White House event, for example), a negative test result on this type of test, particularly among those who do not have symptoms, is not a guarantee that an individual is not currently infected and unable to transmit to others.
Indeed, as we recently highlighted, a key issue for both antigen-based tests as well as PCR-based tests (i.e., those that look for genetic material from the virus), is that a sufficient level of virus replication has to take place between the time when someone gets infected and someone is tested, for the virus to be detected. Testing too early (i.e., before symptom onset for the antigen-based test or before day 4-5 after exposure for the PCR-based test) increases the chances that you will get a false negative result. There is also the possibility that you will get a false negative result due to an error in how the sample is collected or in the test process itself, which is true at any point in the course of illness.
While this draw-back of rapid antigen-based tests is not inherently bad, it requires that these types of tests be used IN CONJUNCTION with and not INSTEAD OF other measures that serve to prevent spread of COVID-19 from those who are infected to others BEFORE they develop symptoms or test positive. In other words, those who test negative with a rapid antigen-based test, should still operate under the assumption that they might still be infected and engage in other practices that prevent transmission to others such as social distancing, mask use, increased ventilation, restriction on the number of people gathering and the duration of time people gather. In other words, we still have to #StaySMART. Unfortunately, these precautions were not implemented during recent events at the White House, which enabled transmission of SARS-CoV-2 from people who were infected but tested negative that otherwise might have been prevented.
Bottom line: Like Nerdy Girl in Spirit, Dr. Megan Ranney stated in the NBC News article linked below, “Just like a pregnancy test cannot take the place of birth control, Covid-19 tests should not be seen as substitutes for robust strategies to reduce community transmission.” And we would add, just like a pregnant woman can pee on a stick and get a negative test several days in a row…before getting that plus sign, individuals with COVID-19 can test negative several days in a row…before testing positive too. The difference being that pregnant women can’t transmit pregnancy to others in the meantime 🙂
Overall, when and if rapid antigen-based tests become more widely available for use, these tests should be considered an additional tool for stopping transmission of COVID-19, not the *only* tool in our toolbox.
For more information on the utility, purpose and drawbacks of different types of tests see NBC News story by Dr. Megan Ranney here, NPR story here and/or Times article here.
