Proximity, verbal interaction risk factors for SARS-CoV-2 transmission

Proximity and verbal interaction are risk factors for SARS-CoV-2 transmission in household and non-household contacts of patients with COVID-19, according to a retrospective study from Singapore.

“[T]he household attack rate and individual-level transmission risk factors of SARS-CoV-2 suggest that physical distancing and minimizing direct verbal interactions would help reduce community transmission,” noted the researchers.

Using the contact tracing database of the Singapore Ministry of Health, the researchers looked at data of patients with PCR-confirmed COVID-19 (n=1,114) and all their close contacts (n=7,770; median age 33 years, 52.2 percent female) between January 23 and April 3, 2020. Close contacts comprised household contacts (ie, individuals sharing a residence with the index COVID-19 case; n=1,863) and non-household contacts (social or work contacts with ≥30 minutes of contact within 2 m of the index COVID-19 case).

All patients with COVID-19 received inpatient treatment with contact limited to healthcare staff. Close contacts underwent 14-day quarantine with symptoms monitored three times/day via telephone. Those who developed symptoms underwent in-hospital PCR SARS-CoV-2 testing; in this study, 188 tested positive for SARS-CoV-2. Rate of secondary clinical attack was higher in household compared with work or social contacts (5.9, 1.3, and 1.3 percent, respectively).

A total of 1,150 close contacts (median age 35 years, 54.2 percent female) who completed quarantine without testing positive for SARS-CoV-2 underwent serology testing and completed an exposure risk questionnaire. Of these, 3.8 percent (n=44) were SARS-CoV-2 seropositive, of whom 65.9 percent were asymptomatic.

From this cohort, secondary infection rates were 11, four, and five per 100 household, social, and work contacts, respectively. Bayesian analysis showed an estimated 62 percent of SARS-CoV-2–positive close contacts were missed by symptom-based PCR testing, and that 36 percent of household contacts with SARS-CoV-2 were asymptomatic. [Lancet Infect Dis 2020;doi:10.1016/S1473-3099(20)30833-1]

“[S]ymptom-based testing approach did not identify more than half of contacts with SARS-CoV-2 infection and … more than a third of infections were asymptomatic,” said the researchers. This highlights the importance of testing all household contacts, including asymptomatic ones, they said.

Independent risk factors for transmission

Speaking to an index case was an independent factor for SARS-CoV-2 transmission among both household (odds ratio [OR], 7.86, 95 percent confidence interval [CI], 3.86–16.02 and OR, 3.91, 95 percent CI, 2.09–7.34 for ≥30 and <30 minutes speaking times, respectively; p<0.0001 for both) and non-household contacts (OR, 2.67, 95 percent CI, 1.21–5.88; p=0.015 and OR, 2.50, 95 percent CI, 1.15–5.44; p=0.021, respectively).

SARS-CoV-2 transmission was more likely among household contacts who shared a bedroom with an index patient (OR, 5.38, 95 percent CI, 1.82–15.84; p=0.0023). Among non-household contacts, exposure to ≥1 case (OR, 3.92, 95 percent CI, 2.07–7.40; p<0.0001) and sharing a vehicle with an index case (OR, 3.07, 95 percent CI, 1.55–6.08; p=0.0013) were tied to SARS-CoV-2 transmission.

Indirect contact, sharing of meals, and using the same lavatories did not affect SARS-CoV-2 transmission in household and non-household contacts.

Results inform strategies

According to the researchers, the higher seroprevalence and secondary attack rates in household vs non-household contacts, potentially due to more prolonged contact in the former than latter, suggests that “priority for quarantine measures … be given to household contacts.”

With proximity and verbal interaction being independent risk factors for SARS-CoV-2 transmission, wearing of masks and physical distancing may help reduce community transmission, they added, though they acknowledged the difficulty in enacting this strategy in undiagnosed household contacts.

“[R]outine testing of close contacts regardless of symptoms will reduce missed diagnoses,” they said. A positive test in a household contact would allow for relocation of the contact or physical distancing.

They also noted the importance of early diagnosis of asymptomatic infection. “Detection of asymptomatic contacts with active SARS-CoV-2 infection is crucial both to determine contact-related secondary transmission events early and to avoid release of potentially infectious individuals from quarantine,” they said. Initiation of routine PCR at the start and end of quarantine, regardless of symptoms, may help address this issue.

The researchers also stressed the importance of seroprevalence studies to assess “the extent of infection in the community.” Identification of risk factors driving SARS-CoV-2 transmission is also crucial to develop public health strategies, due to the long-term infeasibility of community strategies such as lockdowns which have negative socioeconomic effects, they said.