Grand Rounds

What is the Risk of SARS-CoV-2 Transmission Among Close Contacts?

BACKGROUND AND PURPOSE:

Ng et al. (The Lancet Infectious Diseases, 2020) determined asymptomatic attack rates and SARS-CoV-2 transmission risk factors among community close contacts of patients with COVID-19

Retrospective cohort study conducted in Singapore (January 23 and April 3, 2020)
- Singapore’s COVID-19 contact tracing program identified close contacts of COVID-19 cases
- These close contacts were then monitored during a 14-day quarantine at home
- Contacts who reported symptoms were admitted to the hospital for testing
Participants
- All close contacts of confirmed COVID-19 cases

Study Design

Definitions
- Household contacts: Individuals who shared a residence with the index COVID-19 case
- Non-household contacts: People who had contact for at least 30 min within 2 m of the index case
Contact tracing, quarantine and testing system
- All patients with COVID-19 in Singapore received inpatient treatment
- All close contacts were quarantined for 14 days with 3 times per day symptom monitoring via telephone
- Symptomatic contacts underwent PCR testing
Data analysis
- Secondary clinical attack rates were derived from the prevalence of PCR-confirmed SARS-CoV-2 among close contacts
- Consenting contacts underwent serology testing and detailed exposure risk assessment
- Bayesian modelling was used to estimate the prevalence of missed diagnoses and asymptomatic positive cases
- Univariable and multivariable logistic regression models were used to determine transmission risk factors

7,770 close contacts (linked to 1,114 confirmed cases) | 7,518 with complete data
- Household contacts: 1,863 individuals
- Work contacts: 2,319 individuals
- Social contacts: 3,588 individuals
Symptom-based PCR testing among close contacts identified 188 COVID-19 cases
Secondary clinical attack rate
- Household contacts: 5.9% (95% CI, 4.9 to 7.1)
- Work contacts: 1.3% (95% CI, 0.9 to 1.9)
- Social contacts: 1.3% (95% CI, 1.0 to 1.7)
Bayesian analysis of serology and symptom data performed on a subset of patients was used to estimate missed diagnoses by using symptom-based testing vs universal testing
- Estimated missed diagnoses: 62%
- Estimated asymptomatic cases: 36%

Factors associated with transmission

Household contacts
- Sharing a bedroom: multivariable odds ratio (OR) 5.38 (95% CI, 1.82 to 15.84; p=0.0023)
- Being spoken to for ≥30 min: OR 7.86 (95% CI, 3.86 to 16.02; p<0.0001)
Non-household contacts
- Exposure to >1 case: OR 3.92 (95% CI, 2.07 to 7.40; p<0.0001)
- Being spoken to for ≥30 min: OR 2.67 (95% CI, 1.21 to 5.88; p=0.015)
- Sharing a vehicle: OR 3.07 (95% CI, 1.55 to 6.08; p=0.0013)
The following was not independently associated with increased transmission risk
- Indirect contact | Meal sharing | Lavatory co-usage

Among household contacts, sharing a bedroom was associated with >5x increased risk and speaking with the primary case for at least 30 minutes was associated with almost 8x increased risk
Among non-household contacts, exposure to more than 1 primary case was associated with almost 4x risk and speaking with the primary case for at least 30 minutes was associated with 2.7x increased risk | Sharing a vehicle was associated with 3x increased risk
The authors suggest that based on the above data

…physical distancing and minimising direct verbal interactions would help reduce community transmission

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