U.S. Department of Health & Human Services

11/17/2020 | Press release | Distributed by Public on 11/17/2020 18:45

COVID-19 Science Update released: November 17, 2020


Transmission of SARS-CoV-2 on mink farms between humans and mink and back to humans.external icon Munnink et al. Science (November 10, 2020).

Key findings:

  • Of 97 mink farm employees tested, 43/88 (49%) were positive by RT-PCR, 38/75 (51%) had detectable antibodies, and 66/97 (67%) had evidence of SARS-CoV-2 infection by RT-PCR or serology.
  • The mink and human strains clustered closely together (Figure).
    • There was greater phylogenetic relatedness between farm-related human and mink cases than between farm-related human and community cases within the same postal code.
  • Potential mink-to-human transmitted SARS-CoV-2 infections did not have differences in signs, duration, or severity compared with human-to-human transmitted infections.

Methods: Outbreak investigations were performed when respiratory signs and increased mortality for minks were reported to the Dutch Agriculture authorities. Investigations at 16 mink farms sought to identify the source of the outbreak, trace contacts and track movement of animals, vehicles, and people. SARS-CoV-2 diagnostic testing for mink and human samples was performed using RT-PCR and by antibody testing. Viral genomes were sequenced for PCR-positive samples with Ct values ≤32. Limitations: The investigation could not exclude the role that exposures from untested animals or humans might have played in transmission; temporary farm workers not tested.

Implications: SARS-CoV-2 outbreaks in mink farms are well-documented and resulted in recently-abandoned plans to cull Denmark's 17 million minksexternal icon . This paper provides phylogenetic data suggestive of mink-to-human SARS-CoV-2 transmission; however, directionality of transmission is challenging to definitively demonstrate using phylogenetics. Concerns about potential mutations in SARS-CoV-2 from animal-to-human transmission that may lead to more rapid spread or jeopardize vaccine efficacyexternal icon may be premature.


Note: Adapted from Munnink et al. Phylogenetic tree of mink and human sequences found in a single farm (NB1) and the closest matching human sequences from the national SARS-CoV-2 database (NA 134 2020, and NA 376 2020). Numbers in black represent the bootstrapping values using all Dutch SARS-CoV-2 sequences. Permission request in process.

Repeated cross-sectional sero-monitoring of SARS-CoV-2 in New York Cityexternal icon . Stadlbauer et al. Nature (November 3, 2020).

Key findings:

  • A rapid rise in SARS-CoV-2 seroprevalence (6.2%) was detected in the urgent care (UC) group (Figure, A) compared with the routine care (RC) group, coinciding with the surge of new confirmed SARS-CoV-2 infections and related hospitalizations in New York City.
    • The uptick in seropositivity lagged approximately 1-2 weeks behind the increase in new cases (Figure A, C).
  • There was a longer delay between the increase in confirmed cases and the increase in seroprevalence in the RC group (Figure B, C).
  • Seroprevalence in the UC group declined to levels seen in the RC group coinciding with a drop in incidence of SARS-CoV-2 cases between May and July 2020 (Figures A, C).

Methods: Weekly cross-sectional analysis of anti-SARS-CoV-2 spike antibodies on 10,691 plasma samples randomly collected from patients at Mount Sinai Hospital in New York City from February 9 to July 5, 2020. Seroprevalence was compared between an UC group (meant to capture seropositive individuals) and a RC group (meant to capture visits unrelated to COVID-19 and resemble the general population). Limitations: Non-random, non-representative sampling.

Implications: This will be the first of a series of repeated cross-sectional seroprevalence studies to understand the dynamics of SARS-CoV-2 transmission, seroconversion, and the stability of antibody responses over the course of a COVID-19 outbreak.


Note: Adapted from Stadlbauer et al. Serum antibody prevalence in the UC group (A) and in the RC group (B), and number of confirmed cases/day and number of deaths per day, February-July 2020 (C). Permission request in process.

Outcomes of contact tracing in San Francisco, California-test and trace during shelter-in-placeexternal icon . Sachdev et al. JAMA Internal Medicine (November 2, 2020).

Key findings:

  • COVID-19 contact tracing program reached more than 85.4% of cases (Figure) and 83.8% of contacts.
  • The secondary attack rate was higher among household compared with non-household contacts (111 of 983 [11.3%] and 9 of 231 [3.9%], respectively; p <0.001).
  • Median time from symptom onset for a case to notifying and testing the contact was 6 days.
    • The median time from symptom onset to receipt of test results was 5 days.
    • Case interviews occurred on average 1 day after the test result and the first contact was notified on average 1 day after that.

Methods: Case investigation and contact tracing for 1,394 COVID-19 cases identified in San Francisco, California between April 13 and June 8, 2020. The study period included dates after which universal testing for COVID-19 contacts, regardless of symptoms, was recommended. The proportion of people who were interviewed and for whom close contacts were identified, with at least 1 contact being notified, was calculated. Median number of days (with IQR) taken to process each step was reported. Limitations: Underreporting of close contacts.

Implications: In order to maximize the impact of contact tracing on mitigating the spread of COVID-19, metrics such as these are needed, and efforts must be made to address testing delays and to improve contact identification.


Note: Adapted from Sachdev et al. Percentages for people with COVID-19 at selected stages of contact tracing implementation. Reproduced with permission from JAMA Internal Medicine. Sachdev et al., Outcomes of contact tracing in San Francisco, California-test and trace during shelter-in-place. DOI:10.1001/jamainternmed.2020.5670. Copyright© 2020 American Medical Association. All rights reserved.

Proportion of asymptomatic infection among COVID-19 positive persons and their transmission potential: A systematic review and meta-analysisexternal icon . Yanes-Lane et al. PLOS One (November 7, 2020).

Key findings:

  • 24 studies report that the proportion of asymptomatic individuals among individuals testing positive for SARS-CoV-2 ranged from:
    • 20%-75% for the general population.
    • 2%-50% for contacts of known cases.
    • 45%-100% of obstetric patients presenting to hospitals.
    • 42%-66.7% of nursing home residents; 0%-50% of nursing home staff.
    • 50%-87.8% in congregate settings such as temporary homeless shelter.
    • 2% of retail workers (single study).
  • 8% (18/96) of contacts of asymptomatic index patients were SARS-CoV-2 positive in pooled analysis of 5 transmission studies.

Methods: Review of 28 moderate or high-quality studies systematically testing for COVID-19 through June 22, 2020, assessing the proportion of infections occurring in asymptomatic individuals and transmission. Limitations: Included mainly small studies of people with COVID-19 (n <100), limiting estimate precision; contact tracing in studies was limited which may bias the results; study heterogeneity limited ability to analyze by age group or sex.

Implications: The proportion of asymptomatic infections appears high in many groups. As asymptomatic individuals may have considerable transmission potential, symptom-based testing is insufficient to eliminate transmission. High quality studies in representative general population samples are needed to better understand the role of asymptomatic individuals in the transmission of SARS-CoV-2.