COVID-19 Science Update released: June 12, 2020 Edition 21

COVID-19 Science Update

The COVID-19 Science Update summarizes new and emerging scientific data for public health professionals to meet the challenges of this fast-moving pandemic. Weekly, staff from the CDC COVID-19 Response and the CDC Library systematically review literature in the WHO COVID-19 databaseexternal icon, and select publications and preprints for public health priority topics in the CDC Science Agenda for COVID-19 and CDC COVID-19 Response Health Equity Strategy.


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Asymptomatic Transmission

The prevalence of asymptomatic COVID-19 cases may vary by population. Examining duration of viral persistence, amount of virus and transmissibility of virus in asymptomatic individuals is relevant for infection control. Below we summarize three studies that discuss SARS-CoV-2 prevalence and clinical characteristics in asymptomatic and symptomatic persons.


A. Comparison of clinical characteristics of patients with asymptomatic vs symptomatic coronavirus disease 2019 in Wuhan, Chinaexternal icon. Yang et al. JAMA Network Open(May 27, 2020; Correction external iconon June 26, 2020).

Key findings:

  • Among 78 early patients in Wuhan with similar exposure histories and treatment course, 33 (42%) were asymptomatic.
  • Asymptomatic patients shed the virus for a shorter time than symptomatic patients.
  • Asymptomatic patients were more likely to be younger, have faster lung recovery per CT scans, and demonstrate less fluctuation of SARS-CoV-2 testing results.

Methods: 78 patients with RT-PCR-confirmed COVID-19 with exposure to the Hunan seafood market or close contact with a patient hospitalized for COVID-19. NP swabs for RT-PCR were collected every 24 to 48 hours. Limitations: Single hospital; clinical differences between asymptomatic and symptomatic patients may have been subject to provider bias; “fluctuation” not well defined but may refer to negative tests followed by positive tests..


Note: Adapted from Yang et al. Clinical features and prognosis of asymptomatic and symptomatic patients with COVID-19.

a Liver injury defined as serum alanine aminotransferase/aspartate aminotransferase levels more than 1.5 U/L; b Measured via nasopharynx swab; c Defined as when lung lesions began to improve by chest computed tomography; d Time from admission to the occurrence of 2 consecutive negative results for SARS-CoV-2 from nasopharyngeal swab. Licensed under CC-BY.


B. The natural history and transmission potential of asymptomatic SARS-CoV-2 infection.external icon Chau et al. Clinical Infectious Diseases (June 4, 2020).

Key findings:

  • Among a convenience sample of patients admitted to a single hospital, 43% were asymptomatic.
  • Asymptomatic patients had overall lower viral burdens (i.e., higher cycle threshold [Ct] values) that became undetectable faster than among symptomatic patients (Figure 1).
  • Asymptomatic patients transmitted infection to contacts (Figure 2).

Methods: Prospective study of 30 Vietnamese patients with RT-PCR-confirmed SARS-CoV-2 infection (13 asymptomatic and 17 symptomatic), from whom NP swabs were collected daily from study enrollment to discharge. Transmissibility in asymptomatic patients was assessed through contact tracing. Limitations: Small sample size, did not culture virus from RT-PCR positive results to establish infectiousness.

Figure 1:

Note: Adapted from Chau et al. Ct values over time for asymptomatic (green) and symptomatic (red) patients. Asymptomatic patients had overall lower viral burdens (higher Ct values) and cleared virus faster than symptomatic patients. Available via Oxford University Press Public Health Emergency Collection through PubMed Central.

Figure 2:

Note: Adapted from Chau et al. Epidemiologic links between patients. Asymptomatic patients are in blue and symptomatic patients are in red. Patients positioned on the circle attended an event; arrows indicate patients who tested positive for SARS-CoV-2 after contact with a patient who attended the event and whether they were later classified as symptomatic or asymptomatic. Available via Oxford University Press Public Health Emergency Collection through PubMed Central.


C. Pre-procedural surveillance testing for SARS-CoV-2 in an asymptomatic population shows low rates of positivitypdf iconexternal icon. Mays et al. Journal of Clinical Microbiology. (June 3, 2020).

Key findings:

  • <1% of asymptomatic individuals in the Seattle region seeking medical care without known exposure to COVID-19 tested positive for SARS-CoV-2 RNA.
    • This rate was lower than the rate observed among symptomatic hospitalized patients (10%) and asymptomatic patients with known exposure (8%) in the same region.

Methods: In the University of Washington hospital system, 2,056 asymptomatic individuals without known exposure had RT-PCR testing before undergoing a surgical or aerosol-generating procedure. Limitations: No follow-up of RT-PCR positive asymptomatic individuals to verify whether they developed symptoms later.

Implications for 3 studies (Yang et al., Chau et al. and Mays et al.): Prevalence of SARS-CoV-2 in asymptomatic persons may depend on the level of community spread and is likely to vary by region. Asymptomatic persons can transmit to others although virus in the respiratory tract of asymptomatic persons appears to be present in lower concentrations and for a shorter period than in symptomatic persons, suggesting a shorter period of infectiousness. Infection control practices, including self-isolation and contact tracing, remain necessary for asymptomatic infected persons.

See Oran et al.external icon for a summary of findings from studies on asymptomatic SARS-CoV-2 infection.



Characteristics of U.S. nursing homes with COVID ‐19 casesexternal icon. Abrams et al. Journal of the American Geriatrics Society (June 2, 2020).

Key findings:

  • In an analysis of 9,395 nursing homes, 31.4% had > 1 documented COVID-19 case.
    • Among nursing homes with at least one COVID-19 case, average number of cases per facility was 19.8 (Figure 1).
  • Larger facility size, urban location, greater percentage of African-American residents, non-chain status, and state were associated with having COVID-19 cases.
  • Five-star rating, prior infection control violation, Medicaid dependency and ownership (i.e., for-profit, nonprofit, government), were not associated with having COVID-19 cases.
  • Outbreak size was associated with facility size, for-profit status, and state.

Methods: A retrospective analysis of US nursing homes using publicly available data sources. Limitations: Lack of a centralized national reporting system for nursing homes, incomplete and variable reporting of COVID-19 and related characteristics.

Implications: Nursing homes with certain characteristics (e.g., larger size, urban location, higher proportion African-American residents) were at an increased likelihood of having COVID-19 cases and may merit heightened vigilance for infections and their prevention to protect the health of residents and staff.


Note: From Abrams et al. Frequency distribution of COVID-19 cases in 2,575 U.S. nursing homes with > 1 case reported through May 2020 located in 21 states. Available via Wiley Public Health Emergency Collection through PubMed Central.


Racial disparities in knowledge, attitudes and practices related to COVID-19 in the USA.external icon Alobuia et al. Journal of Public Health. (June 3, 2020).

Key findings:

  • Although African American and Hispanic persons were less knowledgeable about COVID-19 when compared with White persons, they were more likely to practice social distancing and other interventions that reduce the spread of infection (Figure).
  • Persons with higher levels of education or higher income were more knowledgeable about COVID-19 but were not more likely to engage in public health interventions to reduce transmission (Figure).

Methods: A multivariate regression analysis using phone survey results from 1,216 persons who participated in the March 2020 Kaiser Family Foundation ‘Coronavirus Poll’ to identify predictors of COVID-19-related knowledge, attitudes and practices. Limitations: Response bias, cross-sectional survey.

Implications: Racial and socioeconomic disparities exist in COVID-19-related knowledge and practice. Expanded messaging on the public health benefits of social distancing may be needed. Educational resources on COVID-19 should be tailored to minority racial and ethnic groups and those of lower socioeconomic status.


Note: Adapted from Alobuia et al. Adjusted odds ratio and 95% confidence intervals on COVID-19-related knowledge (blue) and practicing public health interventions (red) stratified by various demographic characteristics. Odds ratio calculations adjusted for race, sex, age, marital status, education level, income, insurance coverage and political views. Reproduced by permission of Oxford University Press on behalf of the Royal Colleges of Physicians of the United Kingdom. Please visit: icon.


African American children are at higher risk for COVID‐19 infectionexternal icon. Bandi et al. Pediatric Allergy and Immunology (May 29, 2020).

Key findings:

  • 5.3% (25/474) children tested positive for SARS-CoV-2 RNA.
    • 5 were hospitalized; 3 of whom were admitted to pediatric ICU (PICU).
      • 80% (4/5) of hospitalized children and all children admitted to PICU were African American (AA).
    • Age and race/ethnicity were associated with positive SARS-CoV-2 RNA results.
      • Children with positive results were older than those with negative results (9.3 vs. 4.9 years, respectively).
      • Percent testing positive was higher for AA (6.8% (14/250)) and Hispanic (6.6% (2/119)) compared with non-Hispanic white children (1.7% (2/119)).
    • Asthma was not associated with positive test result.

Methods: 474 children (<18 years old) in Chicago with fever, cough, dyspnea, myalgia, sore throat, or anosmia were tested by RT-PCR. Demographics, risk factors, and asthma status were recorded. Associations were evaluated by unadjusted and adjusted (age, race, gender) logistic regression and random step-wise matching. Limitations: Small sample size; asymptomatic children not tested.

Implications: AA and Hispanic children might be at higher risk for COVID-19 infection, and AA children may have a more severe infection course. Older children might also be at higher risk for COVID-19 infection. Studies are needed to corroborate these findings.

Modeling & Transmission


Evaluating the effectiveness of social distancing interventions to delay or flatten the epidemic curve of coronavirus disease. Matrajt et al. Emerging Infectious Diseases. (April 21, 2020).

Key findings:

  • In a modeling exercise, social distancing implemented early in the epidemic would delay the time to the peak number of cases while social distancing implemented later would decrease the number of cases (Figure 1).
  • A 25% reduction in social contacts among adults age < 60 years coupled with substantial social distancing in children and adults age ≥ 60 years could avert an estimated 90% of COVID-19 cases, hospitalizations and deaths (Figure 2).
  • Reducing social contacts among children as part of social distancing interventions could decrease overall number of cases, hospitalizations, and deaths by 60─70% (Figures 1 and 2).
  • Cases would rebound after the end of all interventions, a finding that emphasizes the need for a multipronged public health approach that includes testing, isolation of infected cases and contact tracing once social distancing restrictions are eased (Figure 1).

Methods: Modeled effectiveness of four 6-week social distancing interventions to reduce COVID-19 cases, hospitalizations, and deaths in Seattle, WA during the first 100 days of the epidemic:

  • 95% reduction in social contacts in adults age ≥ 60 years (first scenario)
  • First scenario and 85% reduction in social contacts in children (second scenario)
  • First scenario and 25%, 75% or 90% reduction in social contacts in adults <60 years (third scenario)
  • Second scenario and 25%, 75% or 90% reduction in social contacts in adults <60 years (fourth scenario)

Limitations: Did not consider transmission from asymptomatic individuals.

Implications: Social distancing alone is insufficient to reduce COVID-19-associated morbidity and mortality and should be coupled with increased testing, self-isolation, when appropriate, and contact tracing. The relative infectiousness of children compared to adults is important to understand and incorporate into models.

Figure 1

Number of projected COVID-19 cases if social distancing interventions started 50 (A) or 80 (B) days after the first case was reported.

Figure 2

Number of averted COVID-19 cases (A), hospitalizations (B) and deaths (C) due to social distancing interventions

Note: Adapted from Matrajt et al. Figure 1: Number of projected COVID-19 cases if social distancing interventions started 50 (A) or 80 (B) days after the first case was reported. Dotted lines indicated the start and end of 6-week social distancing interventions. As per scenarios mentioned in Methods, green indicates the first scenario; red indicates the second scenario, orange and purple indicate the third and fourth scenarios, respectively with 25% reduction in social contacts in adults <60 years. Figure 2: Number of averted COVID-19 cases (A), hospitalizations (B) and deaths (C) due to social distancing interventions. Same color schema as in Figure 1. Open access journal; all content freely available.

Clinical Treatment & Management


Pathological findings in the testes of COVID-19 patients: Clinical implicationsexternal icon. Yang et al. European Urology Focus (May 31, 2020).

Key findings:

  • In a series of 11 male autopsies, seminiferous tubular injury was mild (<10% seminiferous tubules affected), moderate (10%-50% tubules affected) or severe (>50% tubules affected) (Figure 1).
  • The ACE2 receptor (a target for SARS-CoV-2 virus) was expressed on cells in testicular tissues (Figure 2).
  • SARS-CoV-2 RNA was detected in 1 of 11 testicular samples; virus was not visualized by electron microscopy in any samples.

Methods: A postmortem analysis of testicular samples from 11 COVID-19 cases from Tongji Medical College in China. Samples were analyzed by light and electron microscopy, immunohistochemistry and RT-PCR for SARS-CoV-2 RNA. Limitations: Limited samples from one hospital.

Implications: There may be a pathological effect of SARS-CoV-2 infection on testicular tissue. Further study is needed to determine the mechanism of injury.

Figure 1

Extent of seminiferous tubular injury in postmortem testicular samples from COVID-19 cases

Note: Adapted from Yang et al. Extent of seminiferous tubular injury in postmortem testicular samples from COVID-19 cases. Licensed under CC-BY-NC-ND 4.0.

Figure 2

ACE2 cells expressed in Sertoli cells and strongly expressed in Leydig cells (long arrows) according to immunohistochemistry. Spermatogonia are negative (short arrows).

Note: From Yang et al. ACE2 cells expressed in Sertoli cells and strongly expressed in Leydig cells (long arrows) according to immunohistochemistry. Spermatogonia are negative for staining (short arrows). Licensed under CC-BY-NC-ND 4.0.


Effect of convalescent plasma therapy on time to clinical improvement in patients with severe and life-threatening COVID-19external icon. Li et al. JAMA (June 3, 2020; Correction external iconon August 4, 2020).

Key findings:

  • Overall, no difference was observed in clinical improvement with administration of convalescent plasma: treatment group 51.9% (27/52) vs control group 43.1% (22/51), hazard ratio (HR), 1.40 (95% CI, 0.79-2.49).
    • In patients with severe disease, convalescent plasma treatment was associated with improved outcome (HR 2.15, p=0.03) but not in patients with advanced life-threatening disease (HR 0.88, p=0.30).
  • Overall, convalescent plasma treatment was associated with higher rates of negative NP RT-PCR at 24, 48, and 72 hours.

Methods: Open-label, randomized trial of convalescent plasma versus standard treatment in 103 participants with severe or life-threatening laboratory-confirmed COVID-19 in 7 medical centers in Wuhan, China. The trial was terminated early after 103 of 200 planned enrollments due to the containment of COVID-19 in Wuhan. Limitations: Small sample that did not reach predetermined enrollment; open label; median time between symptom onset and randomization was 30 days; standard therapy in both groups was not uniform.

Implications: Convalescent plasma did not result in clinical improvement within 28 days of administration; higher rates of conversion to negative RT-PCR may indicate antiviral activity of convalescent plasma.


Rate of cumulative clinical improvement over time for all patients (A), those with severe disease (B) and those with life-threatening disease (C) for convalescent plasma and control groups.

Note: from Li et al. Rate of cumulative clinical improvement over time for all patients (A), those with severe disease (B) and those with life-threatening disease (C) for convalescent plasma and control groups. Reproduced with permission from JAMA. doi:10.1001/jama.2020.10044. Copyright©2020 American Medical Association. All rights reserved.

Laboratory Science


Swabs collected by patients or health care workers for SARS-CoV-2 testingexternal icon. Tu et al. NEJM (June 3, 2020).

Key findings:

  • Using healthcare worker (HCW)-collected NP samples (NPS) as the standard, the sensitivities of self-collected mid-turbinate samples (MTS), tongue samples (TS), and anterior nares or nasal samples (NS) were high and comparable (Figure 1).
  • RT-PCR Ct values for self-collected MTS (r = 0.86) were more highly correlated with Ct values for HCW-collected NPS than TS (r = 0.48) or NS (r = 0.78) (Figure 2).

Methods: A prospective study in Washington State of 530 participants designed to compare self-collected TS, NS and MTS to HCW-collected NPS for detection of SARS-CoV-2 RNA. Limitations: Small number of positive SARS-CoV-2 RNA results.

Implications: This study and a study presented in a previous May 22, 2020 Science Update by Wehrhahn et alexternal icon. indicate that self-collected samples can provide a reliable alternative to HCW-collected samples. Use of self-collected samples can lower HCW exposure to SARS-CoV-2 and reduce demand for PPE.

Figure 1

Note: Adapted from Frazee et alexternal icon Sample collection sites for use in SARS-CoV-2 RT-PCR. Nasopharyngeal (NPS) samples were collected by healthcare workers and tongue (TS), nasal (NS) and mid-turbinate (MTS) samples were self-collected. This article was published in Annals of Emergency Medicine, Vol 71, Frazee et al., Accuracy and Discomfort of Different Types of Intranasal Specimen Collection Methods for Molecular Influenza Testing in Emergency Department Patients, P509-517, Copyright American College of Emergency Physicians 2020. This article is currently available at the Elsevier COVID-19 resource center: icon.

Figure 2

Note: Adapted from Tu et al. Cycle threshold values for SARS-CoV-2 RT-PCR results from healthcare worker-collected nasopharyngeal swabs (y-axis) and self-collection of tongue, nasal and mid-turbinate swabs. From NEJM. 383:494-496. DOI: 10.1056/NEJMc2016321. Copyright ©2020 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.

In Brief

Lockdowns and School Closures

COVID-19 Data Quality

Other Topics

Disclaimer: The purpose of the CDC COVID-19 Science Update is to share public health articles with public health agencies and departments for informational and educational purposes. Materials listed in this Science Update are selected to provide awareness of relevant public health literature. A material’s inclusion and the material itself provided here in full or in part, does not necessarily represent the views of the U.S. Department of Health and Human Services or the CDC, nor does it necessarily imply endorsement of methods or findings. While much of the COVID-19 literature is open access or otherwise freely available, it is the responsibility of the third-party user to determine whether any intellectual property rights govern the use of materials in this Science Update prior to use or distribution. Findings are based on research available at the time of this publication and may be subject to change.


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Page last reviewed: December 17, 2020
Content source: Office of the Chief Science Officer - COVID-19