Person-to-person transmission of SARS-CoV-2 occurred between two people with prolonged, unprotected exposure while Patient 1 was symptomatic. Despite active symptom monitoring and testing of symptomatic and some asymptomatic contacts, no further transmission was detected.

Patient 1—a woman in her 60s—returned from China in mid-January, 2020. One week later, she was hospitalised with pneumonia and tested positive for SARS-CoV-2. Her husband (Patient 2) did not travel but had frequent close contact with his wife. He was admitted 8 days later and tested positive for SARS-CoV-2. Overall, 372 contacts of both cases were identified; 347 underwent active symptom monitoring, including 152 community contacts and 195 health-care personnel. Of monitored contacts, 43 became persons under investigation, in addition to Patient 2. These 43 persons under investigation and all 32 asymptomatic health-care personnel tested negative for SARS-CoV-2.

Patients with confirmed COVID-19 were defined as those with a positive SARS-CoV-2 test. Contacts were people with exposure to a patient with COVID-19 on or after the patient's symptom onset date. Contacts underwent active symptom monitoring for 14 days following their last exposure. Contacts who developed fever, cough, or shortness of breath became persons under investigation and were tested for SARS-CoV-2. A convenience sample of 32 asymptomatic health-care personnel contacts were also tested.

Coronavirus disease 2019 (COVID-19) is a disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), first detected in China in December, 2019. In January, 2020, state, local, and federal public health agencies investigated the first case of COVID-19 in Illinois, USA.

This Article describes the first person-to-person transmission of COVID-19 in the USA, including the clinical and laboratory features of both patients and the assessment and monitoring of several hundred individuals with potential exposure to SARS-CoV-2.

On Jan 23, 2020, Illinois, USA, reported the state's first laboratory-confirmed case (index case) of COVID-19 in a traveller who returned from Wuhan in mid-January, 2020. Subsequently, the first evidence of secondary transmission in the USA was reported on Jan 30, when the husband of the index patient, who had not travelled outside the USA, tested positive for SARS-CoV-2. Public health authorities did an intensive epidemiological investigation of the two confirmed cases.

In January, 2020, a novel virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was identified as the causative agent for a cluster of pneumonia cases initially detected in Wuhan City, Hubei province, China.SARS-CoV-2, which causes the disease now named coronavirus disease 2019 (COVID-19), had spread throughout China and to 26 additional countries as of Feb 18, 2020.Phylogenetic data implicate a zoonotic origin,and the rapid spread suggests ongoing person-to-person transmission. Several studies offer additional insight into person-to-person transmission.However, substantial knowledge gaps remain regarding the transmissibility between humans, including the level of exposure to a confirmed case at which transmission is more likely to occur.

Reconciling early-outbreak estimates of the basic reproductive number and its uncertainty: framework and applications to the novel coronavirus (2019-nCoV) outbreak.

WHO Statement on the meeting of the International Health Regulations (2005) Emergency Committee regarding the outbreak of novel coronavirus (2019-nCoV).

A familial cluster of pneumonia associated with the 2019 novel coronavirus indicating person-to-person transmission: a study of a family cluster.

Specimens were collected per CDC guidance.All specimens were refrigerated at 2–8°C before shipping on icepacks to CDC. CDC did real-time RT-PCR (rtPCR) to detect three separate genetic markers of SARS-CoV-2, as previously described.The cycle threshold value ranges for the three markers were interpreted as a semi-quantitative measure of the RNA concentration in the specimen.

CDC Interim guidelines for collecting, handling, and testing clinical specimens from persons under investigation (PUIs) for 2019 novel coronavirus (2019-nCoV).

For PUIs, specimens were collected and sent to CDC for testing. Specimens included upper (nasopharyngeal and oropharyngeal swabs) and lower respiratory specimens (sputum) if spontaneously produced. For patients with COVID-19, nasopharyngeal, oropharyngeal, serum, sputum, urine, and stool specimens were collected and sent to CDC for testing at initial presentation, and then every 2–3 days. Additionally, a convenience sample of 32 asymptomatic health-care personnel contacts had one-time nasopharyngeal and oropharyngeal specimens obtained at least 7 days from their highest-risk exposure. All health-care personnel contacts were offered testing, but laboratory capacity and availability of health-care personnel to undergo testing were limited in the setting of this urgent investigation. Before Patient 2 reported symptoms to public health investigators, nasopharyngeal and oropharyngeal swabs were also collected from Patient 2 owing to his high-risk exposures to Patient 1.

If a contact developed fever, cough, or shortness of breath during active symptom monitoring, they were classified as a person under investigation (PUI; a standard case designation used by CDC during an outbreak)and were isolated and tested for SARS-CoV-2.

All health-care personnel and community contacts assessed to have had low-risk, medium-risk, medium-high-risk, or high-risk exposures were enrolled in active symptom monitoring, which continued for 14 days after last exposure to a patient with COVID-19. Active symptom monitoring was done using Research Electronic Data Capture software (Vanderbilt University, Nashville, TN). Contacts received automated, twice-daily emails inquiring about symptoms, including cough and shortness of breath, and a request for a self-measured temperature. If symptoms or fever (temperature of >38°C) were reported, or if contacts did not respond or declined email monitoring, public health officials telephoned contacts daily. For hospital-based health-care personnel not excluded from work, pre-shift symptom assessment for fever, cough, or shortness of breath was implemented by hospital occupational health services. To identify any contacts (including those that could not be reached for active symptom monitoring) seeking care for fever, cough, or shortness of breath at an emergency department, the Illinois Department of Public Health used locally available, near real-time surveillance data received from regional acute care hospitals, which included symptom and diagnoses data and personally identifiable information for matching.

Health-care personnel and community members with potential exposure to SARS-CoV-2 were interviewed using standardised contact questionnaires to assess exposure and whether the individual had true contact with a patient with COVID-19. Exposure risk was classified according to frameworks designed by members of the Illinois COVID-19 Investigation Team in consultation with CDC subject-matter experts ( table 1 ). These frameworks were based on published guidance for Middle East respiratory syndrome coronavirus and designed and implemented before interim risk assessment guidance for COVID-19 released by CDC.

CDC Interim US guidance for risk assessment and public health management of persons with potential 2019 novel coronavirus (2019-nCoV) exposure in travel-associated or community settings.

CDC Interim U.S. guidance for risk assessment and public health management of healthcare personnel with potential exposure in a healthcare setting to patients with 2019 novel coronavirus (2019-nCoV).

Patients with COVID-19 were interviewed using a standardised questionnaire to identify symptom history, locations visited while symptomatic, and individuals with whom they had contact while symptomatic. The Illinois COVID-19 Investigation Team, comprised of local and state public health staff and the CDC field team, worked with locations visited (eg, workplaces, retail establishments, or health-care facilities) by patients with COVID-19 to identify additional individuals who might have had exposures to SARS-CoV-2. To identify possible exposures in health-care personnel, patient logs and staffing records were obtained and reviewed for all health-care settings visited by patients with COVID-19. Security footage was reviewed to identify additional health-care personnel and patients who had contact with patients with COVID-19 during transport through the admitting hospital. Health-care personnel were defined as all people working in health-care settings who had the potential for exposure to infectious materials,including members of the Illinois COVID-19 Investigation Team. All other contacts were classified as community members, including patients in the same indoor environment in a health-care setting (eg, a hospital waiting room).

Patients with COVID-19 were defined as individuals with laboratory-confirmed SARS-CoV-2 infection. Contacts were defined as people who reported or were identified to have potential exposure to a case on or after the day of symptom onset of the case ( table 1 ). The earliest reported day with new symptoms was used as date of symptom onset. The date of symptom onset for the index case is considered day 0 for the purposes of this investigation, and all subsequent dates will be described by day of investigation (DOI), starting with DOI 0. In this Article, the numbers of contacts exposed to either case on or after the day of their first positive laboratory result are also presented.

Recommended PPE includes respiratory protection (ie, respirator), goggles or faceshield that covers the front and sides of face, gloves, and a gown.

† Recommended PPE includes respiratory protection (ie, respirator), goggles or faceshield that covers the front and sides of face, gloves, and a gown.

Recommended PPE includes respiratory protection (ie, respirator), goggles or faceshield that covers the front and sides of face, gloves, and a gown.

† Recommended PPE includes respiratory protection (ie, respirator), goggles or faceshield that covers the front and sides of face, gloves, and a gown.

Recommended PPE includes respiratory protection (ie, respirator), goggles or faceshield that covers the front and sides of face, gloves, and a gown.

† Recommended PPE includes respiratory protection (ie, respirator), goggles or faceshield that covers the front and sides of face, gloves, and a gown.

Recommended PPE includes respiratory protection (ie, respirator), goggles or faceshield that covers the front and sides of face, gloves, and a gown.

† Recommended PPE includes respiratory protection (ie, respirator), goggles or faceshield that covers the front and sides of face, gloves, and a gown.

Implemented after identification of the second case of laboratory-confirmed COVID-19 in Illinois on Jan 30, 2020.

* Implemented after identification of the second case of laboratory-confirmed COVID-19 in Illinois on Jan 30, 2020.

CDC Interim US guidance for risk assessment and public health management of persons with potential 2019 novel coronavirus (2019-nCoV) exposure in travel-associated or community settings.

Implemented after identification of the second case of laboratory-confirmed COVID-19 in Illinois on Jan 30, 2020.

* Implemented after identification of the second case of laboratory-confirmed COVID-19 in Illinois on Jan 30, 2020.

CDC Interim U.S. guidance for risk assessment and public health management of healthcare personnel with potential exposure in a healthcare setting to patients with 2019 novel coronavirus (2019-nCoV).

Recommended PPE includes respiratory protection (ie, respirator), goggles or faceshield that covers the front and sides of face, gloves, and a gown.

† Recommended PPE includes respiratory protection (ie, respirator), goggles or faceshield that covers the front and sides of face, gloves, and a gown.

Implemented after identification of the second case of laboratory-confirmed COVID-19 in Illinois on Jan 30, 2020.

* Implemented after identification of the second case of laboratory-confirmed COVID-19 in Illinois on Jan 30, 2020.

Person-to-person transmission of SARS-CoV-2 occurred between two people with prolonged, unprotected exposure. No further transmission was detected, despite monitoring contacts for symptoms and testing all who developed fever, cough, or shortness of breath and testing a convenience sample of asymptomatic health-care professional contacts. Further detailed reports of contact investigations associated with cases of SARS-CoV-2 could improve understanding of the transmissibility of this novel virus.

We detail prolonged, unprotected contact between a travel-related index case who was symptomatic and her husband, who subsequently acquired infection. This represents the first known person-to-person transmission of SARS-CoV-2 in the USA. We also detail a thorough contact investigation related to these cases. We identified, risk-stratified, and actively monitored almost 350 contacts of both cases. 43 contacts developed symptoms of fever, cough, or shortness of breath in the 14 days following their last exposure to either case and were tested for SARS-CoV-2, and 32 asymptomatic health-care professional contacts who had exposures across a range of risk levels were also tested for SARS-CoV-2. All 75 tested negative.

We searched PubMed for articles published between database inception and Feb 18, 2020, describing transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) using the search terms “severe acute respiratory syndrome coronavirus 2”, “SARS-CoV-2”, “novel coronavirus”, “2019-nCoV”, or “COVID-19”; and “transmission”, “person-to-person”, or “human-to-human”. We found 34 articles, of which 13 were primary reports of person-to-person transmission. None provided full details of the contact investigation and none were from North America.

The Illinois Department of Public Health, Chicago Department of Public Health, Cook County Department of Public Health, and DuPage County Health Department consulted with the US Centers for Disease Control and Prevention (CDC) for technical assistance and invited a CDC field team to assist with onsite investigations after laboratory confirmation of the first case of COVID-19.

Results

Patient 1 is a female in her 60s who travelled to Wuhan on Dec 25, 2019, and returned to Illinois on Jan 13, 2020, and who was not symptomatic while travelling. In Wuhan, she visited a hospitalised relative regularly and visited other family members who had undiagnosed respiratory illnesses, one of whom was later hospitalised with viral pneumonia. No contacts had laboratory-confirmed COVID-19, but it is unknown whether any were tested for SARS-CoV-2.

Figure Symptoms and results of rtPCR testing for SARS-CoV-2 by day of investigation Show full caption Gradient shading indicates unclear period of symptom onset from patient report. inc=inconclusive result. rtPCR=real-time RT-PCR. SARS-CoV-2=severe acute respiratory syndrome coronavirus 2. *Patient 1 and Patient 2 in home isolation. On DOI 6, she sought care at an outpatient clinic for fever, fatigue, and cough and was hospitalised that day for pneumonia. She was reported to public health authorities as a PUI on DOI 7. Retrospectively, she reported that her symptoms, which also included nausea, abdominal discomfort, and dizziness, started as early as 6 days before admission ( figure ).

Before hospitalisation, she had frequent, close contact with her husband on DOI 0–6 when she had an active cough. Her husband had not travelled to Wuhan. She and her husband live together, eat together, share a bed, and have frequent face-to-face interactions. Facemasks or other personal protective equipment (PPE) were not used at the home. Her husband was classified as having high-risk exposures and began active symptom monitoring on DOI 7 with specimen collection on DOI 11, before his report of any new symptoms.

Patient 2 has chronic obstructive pulmonary disease, with a chronic, productive cough and baseline dyspnoea; therefore, the timing of symptom onset related to COVID-19 was difficult to determine ( figure ). When first interviewed as a contact on DOI 7, he reported no fever or change in chronic respiratory symptoms. Later, he reported increased dyspnoea and sputum production starting on DOI 11, which was also the first day of specimen collection as a contact in Patient 1's investigation. Upon further interview of Patient 2's contacts, it was suggested that some non-specific symptoms might have started as early as DOI 3, with fatigue and worsening cough. On DOI 14, he reported new haemoptysis and worsening dyspnoea through active monitoring. He was promptly admitted to the hospital and placed in an airborne infection isolation room (AIIR). Nasopharyngeal and oropharyngeal specimens from DOI 11 tested positive for SARS-CoV-2 on DOI 15.

On hospital admission, vital signs, and physical examination for Patient 1 were within normal limits. Her chest radiograph demonstrated no abnormalities, but a CT scan of her chest revealed bilateral multifocal infiltrates and mediastinal and hilar lymphadenopathy. On admission, Patient 2 had mild tachypnoea and coarse breath sounds with mild wheezes bilaterally, although whether these signs represented a change from his baseline status is unclear. Patient 2's chest radiograph showed emphysematous changes and right lower lobe infiltrates consistent with pneumonia. For both patients, testing for other viral and bacterial respiratory infections was negative. Both experienced mild leukopenia (Patient 1 white blood count nadir 3·0 × 103 cells per μL, Patient 2 nadir 3·4 × 103 cells per μL), lymphopenia (Patient 1 absolute lymphocyte count nadir 0·7 × 103 cells per μL, Patient 2 nadir 0·8 × 103 cells per μL), and mild elevations in aspartate aminotransferase and alanine aminotransferase (Patient 1 peak 46 units per L and 66 units per L, Patient 2 peak 47 units per L and 75 units per L). No other remarkable laboratory results were noted.

Both patients recovered and were discharged to home isolation on DOI 23. Hospital admission was extended while arrangements were made for home isolation. Home isolation for both patients was lifted on DOI 33, following two sets of negative respiratory specimens collected 24 h apart.

16 CDC

Infection control basics. Patient 1 wore a facemask in the emergency department waiting room and was placed on droplet precautions in the emergency department and for the first 10 h after admission. She was subsequently transferred to an AIIR, where health-care personnel entering the patient's room were required to adhere to Standard, Contact, and Airborne Precautions, including hand hygiene, gloves, gown, respirator, and eye protection.Health-care personnel were enrolled in active monitoring, and potential breaches were recorded and investigated to determine risk classification. Patient 2 was immediately evaluated and admitted to an AIIR and placed on Transmission-Based Precautions as described for Patient 1.

For Patient 1, initial nasopharyngeal, oropharyngeal, and sputum specimens collected on DOI 7 were positive, whereas serum and urine were negative. Her initial sputum rtPCR cycle threshold values ranged between 24–25, indicating high viral burden before isolation. Sputum specimens remained positive longer than all other specimens for both cases ( figure appendix pp 3–4 ). Stool specimens collected for Patient 1 also remained positive longer than nasopharyngeal and oropharyngeal specimens; however, Patient 2 had no positive stool specimens. Neither Patient 1 or 2 had serum or urine specimens that tested positive for SARS-CoV-2.

Table 2 Actively monitored contacts and PUIs owing to contact with a patient with COVID-19, Illinois, USA, 2020 Since first reported date of symptom onset On or after date of first positive specimen Total contacts Did not become a PUI Met PUI criteria * * US Centers for Disease Control and Prevention PUI criteria for contacts of a confirmed case: fever (subjective or objective) or signs or symptoms of lower respiratory illness (eg, cough or shortness of breath). PUIs positive for COVID-19 † † PUIs were tested for COVID-19 using real-time RT-PCR for severe acute respiratory syndrome coronavirus 2. Only results from PUIs tested for COVID-19 in this investigation are presented here. Total contacts Did not become a PUI Met PUI criteria * * US Centers for Disease Control and Prevention PUI criteria for contacts of a confirmed case: fever (subjective or objective) or signs or symptoms of lower respiratory illness (eg, cough or shortness of breath). PUIs positive for COVID-19 † † PUIs were tested for COVID-19 using real-time RT-PCR for severe acute respiratory syndrome coronavirus 2. Only results from PUIs tested for COVID-19 in this investigation are presented here. Community contacts High risk 1 0 1 1/1 1 0 1 1/1 Medium high 7 5 2 0/2 1 1 0 ·· Medium 28 24 4 0/4 0 0 0 ·· Low 116 111 5 0/5 65 61 4 0/4 Total 152 140 12 1/12 67 62 5 1/5 Health-care personnel contacts High risk 32 28 4 0/4 22 20 2 0/2 Medium high 39 30 9 0/9 29 24 5 0/5 Medium 12 6 6 0/6 9 5 4 0/4 Low 112 99 13 0/13 95 84 11 0/11 Total 195 163 32 0/32 155 133 22 0/22 Total contacts 347 303 44 1 ‡ ‡ The index patient, Patient 1, is excluded from this total 222 195 27 1 ‡ ‡ The index patient, Patient 1, is excluded from this total Data are n or n/N. PUI=person under investigation. COVID-19=coronavirus disease 2019. 372 contacts of either Patient 1 or Patient 2 were identified. Public health investigators were able to assess exposure risk and actively monitor symptoms for 347 (93%) of the 372 contacts, including 222 (94%) of 236 contacts with exposure on or after the date of first positive specimen collection. There were 25 people that had insufficient contact information to complete active monitoring. None of these individuals were found to have emergency department visits with fever, cough, or shortness of breath using near real-time surveillance data received from regional acute care hospitals for 14 days after their last exposure. Data presented are for those actively monitored. Of these 347 contacts, 195 (56%) were health-care personnel and 152 (44%) were community members. Although the majority of monitored contacts (228 [66%] of 347) had low-risk exposures, 119 (34%) had exposures of medium risk or greater ( table 2 ).

Although Patient 1 and 2 live together and were hospitalised in the same facility, and therefore shared several common contacts (65 shared community contacts from emergency department or outpatient waiting rooms and 28 health-care personnel who interacted with both patients), they also had many unique contacts. Patient 1 had 92 unique health-care personnel contacts and 16 unique community contacts, including one household contact (Patient 2). Patient 2 had 75 unique health-care personnel contacts and 71 unique community contacts, including 51 from outpatient waiting rooms.

The majority of contacts (303 [87%] of 347 total monitored contacts and 195 [88%] of 222 monitored contacts on or after the date of first positive specimen collection) did not develop symptoms consistent with PUI criteria. Additionally, surveillance data from Illinois acute care hospitals indicated that no asymptomatic monitored contacts or other contacts who could not be reached for active symptom monitoring presented to an emergency department with fever, cough, or shortness of breath during DOI 6–30.

During active symptom monitoring, 44 (13%) of 347 total contacts became PUIs, including 27 (12%) of 222 monitored contacts who had exposures on or after the date of first positive specimen collection.

As a household contact, Patient 2 was the only community member who had a high-risk exposure. He became a PUI and subsequently the only other patient with COVID-19 in this investigation. Of the remaining 43 PUIs, all tested negative for SARS-CoV-2 while symptomatic; 32 of these PUIs were health-care personnel and 11 were community contacts. Although 18 (41%) of 44 PUIs had low-risk exposures, 26 (59%) had exposures of medium risk or greater.

32 health-care personnel contacts who were not PUIs had one-time nasopharyngeal and oropharyngeal specimens collected 7–14 days after their highest-risk exposure. All of these exposures occurred on or after the date of first positive specimen collection of a patient with COVID-19. 21 (66%) of these asymptomatic health-care personnel had exposures of medium risk or greater. All were negative for SARS-CoV-2 at the time of testing.