CovidGram: Testing and aerosol transmission – August 14, 2020

I. Testing

 What are the barriers preventing better testing in the U.S.?

  • How can the U.S. get faster accurate testing results?
  • How widespread is contact tracing?
  • Is the reason for paper tests’ low accuracy of about 50% sensitivity the quantitative threshold of virus units it can detect, or is there a more qualitative factor?
  • What kind of testing have model countries like South Korea, New Zealand, and Germany used?

Summary. Adequate testing and contact tracing—the key to other countries’ successes—continues to elude the U.S.

While test accuracy would seem to be the most important consideration, many manufacturers focused more on test speed since delayed results can make the test useless for pandemic control. Complexity of the testing process and equipment requirements are both key factors in determining how fast results become available. Some test makers focused on designing tests that allow at-home collection of samples; others designed colorimetric tests that require very little equipment or expertise in interpretation of results.

Molecular test – detects the presence of SARS-CoV-2 virus in nasal swabs or saliva by identifying RNA—nucleic acid sequences of various genes—specific to the virus.

Polymerase Chain Reaction (PCR) Test. ~95% accurate. Results in 24 hours. Some of the newest PCR tests claim to produce results in less than one hour, but confirmation of that fast turnaround is not found in the research literature. Used by CVS, Minute Clinic, Indiana State Health Dept, Marion County Public Health Dept

  • Nucleic Acid Amplification (NAA) Test. 70-80% accurate. Results in 15-20 minutes. Used by Walgreens, Passport Health Indianapolis
  • Loop-mediated Isothermal Amplification (LAMP) Test. 70-95% accurate. Results in 30 minutes.
  • Antigen Test. ~50-75% accurate. Results in 15 minutes.

Serologic test – detects antibodies against SARS-CoV-2 virus in a person’s bloodstream, which indicates previous Covid-19 infection. Not used to detect current infection.

Test reliability. Currently, performance data on tests is not well documented. But even in the case of the Abbott rapid NAA test used by Walgreens, the peer-reviewed evaluation of its performance represents just one data point, one set of findings by one team of U.S. researchers. Its finding that the Abbott NAA test gives false negatives at a 30 percent rate is therefore subject to some degree of uncertainty.

That level of false negatives means the results cannot be used to rule out the possibility of infection, only to confirm infection in the case of a positive test result. In other words, the test is not suitable to sort patients into different categories of isolation or quarantine.


  • The best way to get a highly accurate Covid test in Indiana at present is to take the PCR test offered by the county or state health departments or CVS.
  • PCR tests available in Indiana currently take at least 2 days for results. Recently available tests in certain other states can provide similar high accuracy, about 95%, with rapid 1-2 day turnaround, but they appear not to be available yet in Indiana.
  • The same-day results available from Walgreens testing have 70-80% accuracy. Recently, more experts are arguing that low accuracy tests can effectively control the pandemic when used frequently, every one or two days.


New England Journal of Medicine, August 6, 2020, False negatives in Covid-19 testing


The Atlantic, May 6, 2020, What’s South Korea’s Covid secret?


The Atlantic, August 12, 2020, How to conquer Covid-19 through strategic testing and the will of the U.S. government


University of Texas, Melamed Laboratory blog, August 11, 2020


Journal of Clinical Microbiology, May 29, 2020, Performance of Abbott ID NOW Covid-19 rapid NAA test


STAT, August 6, 2020, Faster, simpler Covid test


WIRED, August 7, 2020, Bill Gates on Covid: Most tests are garbage


JAMA Network, July 31, 2020, Screening strategies to permit safe reopening of college campuses.


Reuters, July 26, 2020, U.S. COVID testing falling short.


II. How the “most medically advanced” country became the world leader in Covid victims

Summary. A lengthy, probing article two weeks ago in The Atlantic points to our country’s neglected and uncoordinated public health system as a key cause of the Covid crisis, and in particular, of the ongoing failure in testing and tracing.

Once a source of national pride, our public health institutions produced nearly miraculous improvements last century in sanitation, nutrition, maternal and infant mortality, in addition to preparedness for epidemics. However, this success allowed the health care industry to shift away from a prevention-oriented mission to focus almost exclusively on the more glamorous goals of treatments and cures.

As a result, funding for public health in this country is now a mere 2.5 percent of all health expenditures. This low priority funding status helps explain the remarkable lack of coordination and efficiency in ramping up testing and contact tracing that has plagued the U.S. public health response to Covid from the beginning.

Takeaway. An adequate public health safety net will be an essential tool for the current Covid fight and for health challenges of the future.


The Atlantic, August 4, 2020, How the pandemic defeated America


III. Aerosol (persistent airborne) transmission of virus

  • Can I catch Covid from walking 6 feet behind an infected person or standing downwind of infected people because the virus particles they are breathing out remain suspended in the air (in aerosol)?
  • What precautions can I take to avoid exposure to virus particles in aerosol?

Summary. Recent studies provided evidence that virus particles remain suspended in the air on microdroplets long after an infected person has shed them through normal breathing and conversation activities. However, future studies are still needed to demonstrate a statistically valid correlation between exhaled aerosols and infectivity.

Takeaway. Exhaled coronavirus remains suspended in the air for minutes, possibly even hours, so whether you are walking behind someone outside of your bubble or simply sitting on your porch downwind of someone within 20 feet, mask use is warranted.


New York Times, August 11, 2020, “A smoking gun”: Infectious coronavirus retrieved from hospital air


Medrxiv, August 4, 2020, Viable SARS-CoV-2 in hospital air


New York Times, July 30, 2020, Evidence for aerosol transmission aboard the Diamond Princess


Medrxiv, July 13, 2020, Mechanisms of aerosol transmission


Environment International, May 27, 2020, How can airborne transmission by minimized


IV. Contagion: What makes the coronavirus infectious?

 How much exposure to the virus causes infection?

  • What kinds of situations account for the most COVID-19 spread? For example, is there any evidence of contracting Covid from handling groceries?

Summary. All reports advise that the risk of virus transmission from handling groceries is very low compared to the risk of airborne transmission, but none has yet cited specific cases of transmission on food surfaces.

A JAMA study last week showed that out of three hundred Covid cases in a South Korean hospital, roughly half of whom were asymptomatic, the asymptomatic cases shed the virus in an amount similar to the symptomatic cases. Therefore, both asymptomatic and symptomatic cases should be isolated.

Takeaway. No virus transmission via groceries has yet been documented. Asymptomatic cases of Covid need to be isolated just as symptomatic cases are.

A bulletin issued by the Centers for Disease Control (CDC) advises that patients with mild to moderate Covid symptoms become non-infectious after 10 days and can be released from quarantine four days earlier than previously believed.


CDC, July 22, 2020, Mild to moderate Covid cases not infectious after 10 days.


JAMA Internal Medicine, August 6, 2020, Viral shedding among symptomatic and asymptomatic patients in South Korea.


V. Can T-cell immunity help prevent infection?

Summary. Aside from the highly competitive effort to develop a safe vaccine and effective antiviral drugs, considerable scientific effort is focused on understanding the specific immunology of COVID-19.  Recent studies are starting to show that the body’s T-cells may be providing some innate immunity due to their ability to “remember” specific viral agents and attack the invasion before it can take hold in the body.

  • Does a sub-infectious dose of coronavirus exposure—for example less than 100 particles in a near pass with an infected person who is simply talking and not sneezing—have the potential to induce a certain level of immunity via T-cell memory?


The Atlantic, August 5, 2020, Update on coronavirus immunology


Additional data

“…paper antigen tests aren’t SARS-CoV-2 tests at all, not like PCR tests are. They are rapid, cheap COVID-19 contagiousness tests. That shift in thinking, Mina argues, should undergird a shift in our national strategy.”

“While antigen tests need the equivalent of 100,000 viral strands per milliliter, a typical PCR test can detect a positive from as little as 1,000 strands per milliliter. [A milliliter amounts to about one level teaspoon.] There is only about a day at the beginning of an infection when the two tests would give different results—when there are more than 1,000 viral strands per milliliter of your saliva or snot but fewer than 100,000, according to Dan Larremore, a mathematician at the University of Colorado at Boulder. During that period—approximately day two or day three of an infection—antigen tests are truly inferior to PCR tests.”