In response to calls for Covid-19 testing of the entire U.S. population, several large universities, and even some employers, have announced plans for extensive Covid-19 testing of their employees (and students) to support a safe return to work and school. These efforts are based on expert recommendations, some of which call for repeated testing of the entire population (using the PCR test) every 14 days, or even more often. Such initiatives could require millions of tests in the U.S. each and every day.
With each announcement, pressure grows on other organizations — and public health departments — to follow suit, potentially triggering a testing arms race.
While mass Covid-19 testing might seem intuitive, its benefits are unlikely to meet the high expectations for it.
Proponents of universal Covid-19 testing, meaning testing the entire population, have highlighted its value for contact tracing — a widely touted approach for controlling the spread of an infection by identifying those who had been in contact with an infected individual and quarantining them. Contact tracing has been effective in containing other outbreaks, such as Ebola, and countries such as South Korea and Germany squelched initial Covid-19 outbreaks using aggressive testing and contact tracing.
Frequent testing of the entire population would help identify so-called hidden hidden carriers — individuals infected with SARS-CoV-2, the virus that causes Covid-19, but who have no symptoms of it. They seem to play an important role in the spread of Covid-19. Identifying these silent spreaders could help public health workers be more effective at contract tracing by identifying others who have been exposed and may require quarantine.
But this argument isn’t as strong as it might seem. Asymptomatic spread is contact tracing’s Achilles’ heel. Even if testing the entire population was able to identify silent carriers, there would be a delay in putting this information to use because testing wouldn’t occur continuously and there would be a lag in test results. This is particularly true since SARS-CoV-2 readily spreads during a short interval of a few days, typically in the early stages of infection. Even with testing every two weeks and a 24-hour lag in results, universal testing would catch less than half of asymptomatic carriers during their most infectious period.
According to a model developed by Imperial College London and the World Health Organization, based on these optimistic assumptions the impact of universal Covid-19 testing would reduce the number of Covid-19 cases by less than 10%. More conservative (and realistic) parameters would suggest a smaller impact.
There are also the harms of Covid-19 testing to consider.
Perhaps the biggest challenge is false negatives — tests signaling no infection in those who are indeed infected. Covid-19 testing is typically performed on samples from behind the nose or in the back of the mouth, and will be positive only if the sample happens to contain the virus. In some people, the virus may be present in high quantities only deep in the lungs. False negatives occur surprisingly often — perhaps as often as one-third of the time — which could lead to a false sense of security among those with such results.
Testing those without symptoms can also lead to false alarms. PCR testing for the virus, which is the best way to identify an active infection, can detect the presence of SARS-CoV-2 — or remnants of it — for weeks, even when the infection is unlikely to be transmitted to others. Testing the entire population would undoubtedly identify a large number of such individuals, unnecessarily sidelining them from work and society.
Other important harms of widespread testing include the real possibility that testing locations might serve as sites for congregation and thus promote the spread of SARS-CoV-2; the need for testers to don scarce personal protective equipment; and the fact that testing is resource intensive, time-consuming, and uncomfortable.
Importantly, not testing the entire population would not represent a nihilistic acknowledgment that Covid-19 cannot be controlled. To the contrary, reducing the focus on testing could enable greater attention on higher-yield efforts. Emerging evidence suggests that consistent use of face masks might substantially reduce infection rates — perhaps by more than 90% (though these findings still require peer review) — based on data from countries like Japan that experienced sustained low infection rates despite limited tracing but regular mask use. In contrast, other nations like South Korea and Germany that suppressed initial outbreaks with aggressive testing and tracing have experienced resurgences after reopening their economies.
In addition to using face masks, meticulous hand-washing, regularly disinfecting surfaces, and continued social distancing could have substantial benefits, particularly for high-risk populations.
I believe that the most important role of testing will be for monitoring Covid-19 within communities to guide the tightening and loosening of social distancing restrictions. During influenza season, the Centers for Disease Control and Prevention regularly conducts such sentinel surveillance to alert health care providers when cases are on the rise. It may even prove there are better ways to conduct sentinel surveillance than testing, such as disease weather maps and wastewater sampling to identify leading indicators of outbreaks.
Though well-intended, when it comes to Covid-19 testing for the entire population, the juice may not be worth the squeeze. We can’t test our way out of this pandemic. Instead, we would get better bangs for our bucks by coupling smart, parsimonious testing with simple, high-value interventions such as face coverings and good hand hygiene.
Michael Hochman is a primary care physician, associate professor of clinical medicine at the Keck School of Medicine of USC, and director of the Gehr Family Center for Health Systems Science and Innovation at Keck.