Top news, reports and insights for today:
- New cases seem to be peaking. Is it the epidemic or the testing bandwidth?
If you read my blog regularly, you know this is a big concern of mine. On the positive front, California, has just cleared out a big chunk of their “pending” tests. They are now reporting only 13,000 tests waiting in the cue (10% of all tests). But only 6 states are even reporting the numbers of “pending” tests. On the fantastic COVID Tracking Project website, the other 45 states show only “N/A”. And even in states that report (like New Hampshire), only the public health labs are releasing the data. The figure below comes from an ARS Technica story by Timothy Lee, which is based on C.T.P. data. The figure shows that testing in the U.S. grew exponentially along with the epidemic during the middle of March. It also shows that growth plateaued about March 26 and has failed to rise further despite very rapid growth of new cases in 21 states (see yesterday’s daily briefing). Multiple states are running low on staff and supplies needed to process the tests.
Why this is important? People are looking at new case curves and claiming that the epidemic is peaking. I am far from convinced. What the graph shows is that what has peaked is bandwidth for processing test results. At a time when we need more and better testing, we appear to have maxed out on that capacity. I don’t think we can know now whether we are near peak in new cases or not. I strongly urge states and major testing labs to #ReportPendingTests.
- False-negatives may be higher than we thought
My old colleague at Yale, Harlan Krumholz, M.D. had an important piece in the New York Times this week about the problem of false-negative tests. It’s a new disease and no test is perfect, but at this stage of the epidemic, false-negative results are especially problematic. A false-negative means a person who really has COVID-19 is tested and the result is negative. This relates to what epidemiologists refer to as the sensitivity of the test (what fraction of the true cases will be caught by the test). We would like that to be above 90%. The current tests have reported sensitivity that is quite high based on very carefully controlled testing in a lab setting. But in the real world, lots of things happen to weaken the performance of a test. Evidence suggests we may be looking at sensitivity in the real world closer to 70%. That raises the eye-brows of every epidemiologist – it’s a pretty bad score for such an important test.
Why this matters? A real-world sensitivity of 70% means that as many as 30% of tests may be false-negatives. Because most tests are negative (currently 81% of all tests done according to CTP), this matters a lot. But also, people who get tested and have a negative result are reassured they aren’t infected; they go back to their normal routine, and continue to spread the disease to others. Having a negative test leads them to be less careful. More study of how well the different tests are actually working is important right now.
The bottom line: if you have symptoms, and you are given a negative test result, please continue to assume you are a presumptive case and take the necessary precautions. - Evidence is hardening that those with no symptoms can spread the disease
Initially, the WHO said it was likely “rare” that those who had no symptoms (were asymptomatic) could transmit the infection to others. That was based on some very early work in China – seems like a decade ago. This was, at the time, a reasonable idea because most respiratory viruses (including other strains of coronavirus) don’t generally involve asymptomatic transmission. Now we have growing evidence that this is not true. If you follow the data on how this virus has spread, it gets harder to believe it could spread this fast and far in any other way. On Monday, the director of the CDC Dr. Robert Redfield told NPR that the number of asymptomatic infections may be as high as 25%. Some unknown fraction of those may be infecting others without knowing they have it.
It’s important to realize that asymptomatic is different than presymptomatic. People who test positive but don’t yet have symptoms but will later are presymptomatic. Based on studies of cruise ship passengers, 47% of those infected had no symptoms at the time of their test. Eventually, most of that group will feel sick, and hopefully change their behavior as a result. The asymptomatic group are people who are infected and can spread the virus, but have no fever, no cough, no shortness of breath, no other symptoms of COVID-19, and are therefore a lot less likely to limit their contact with others, wear a mask or call their doctor.
The bottom line: As epidemiologists, we struggle to tell the truth about this infection and inspire people to prepare and respond. If sometimes we overshoot the mark with dire-sounding predictions, it’s because we believe that is where the data is leading us, and because if we are doing our jobs right, people will later claim that we over-reacted. I can live with that. But of all the things that scare me about this pandemic, asymptomatic transmission is top on that list. If true, the control of the epidemic will be more difficult and the death toll will lean toward the higher end of the range of our models.
Thomas A. Glass, Ph.D. 