SARS-CoV-2 Seroprevalence and Risk Factors Among Close Contacts

Virus cells. Coronavirus Covid-19 concept image.
Intensive health surveillance of close contracts of patients with COVID-19 provided a rare opportunity to determine asymptomatic attack rates and SARS-CoV-2 transmission risk factors.

Symptom-based polymerase chain reaction (PCR) testing strategy missed over half of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-positive close contacts, according to results published in The Lancet Infectious Diseases.

Between January 23 and April 3, 1114 cases of coronavirus disease 2019 (COVID-19) were PCR-confirmed. Investigators identified 7770 close contacts of these cases, of which 1863 were household contacts, 2319 from work contacts, 3588 from social contacts. Of these, 7582 contacts completed quarantine without a COVID diagnosis and 188 contacts tested positive for COVID-19.

Data determined that the secondary clinical attack rates were 5.9% (95% CI, 4.9-7.1) in household contacts, 1.3% (95% CI, 0.93-1.9) in work contacts, and 1.3% (95% CI, 1.0-1.7) in social contacts. Sex of the contact and symptom duration of the index case before hospital admission were not significantly associated with the secondary clinical attack rate. There was evidence of clustering of secondary infections in homes and workplaces (P <.0001 for both) but in social contacts (P =.40).

The overall secondary infection rate was estimated to be 11 (95% credible interval [CrI], 9-14) per 100 household contacts. Secondary infection rates were 4 (95% CrI, 3-5) per 100 social contacts, and 5 (95% CrI, 3-8) per 100 work contacts. Bayesian modelling also estimated that the symptom-based testing strategy missed 62% (95% CrI, 55-69) of infections. Over 36% (95% CrI, 27-45) of household contacts with SARS-CoV-2 were asymptomatic. There was a lower rate of missed infections among household contacts than social or work contacts.

Exposure risk factors associated with infection for household contacts included sharing a bedroom (odds ratio [OR], 5.38; 95% CI, 1.82-15.84; P =.0023) and being spoken to by a contact positive with COVID-19 for 30 minutes or longer (OR, 7.86; 95% CI, 3.86-16.02; P <.0001). For social contacts, risk factors were having contact with more than one person positive with COVID-19 (OR, 3.92; 95% CI, 2.07-7.40; P <.0001), being spoken to by the index case for 30 min or longer (OR, 2.67; 95% CI, 1.21-5.88; P =.015), and sharing a vehicle (OR, 3.07; 95% CI, 1.55-6.08; P =.0013).

Recall bias for symptoms during quarantine was a potential study limitation as was the lack of consent to serological testing from all contacts.

“Targeted community measures should include physical distancing and minimizing verbal interactions. Testing of all household contracts, including asymptomatic individuals, is warranted,” investigators concluded.


Ng OT, Marimuthu K, Koh V, et al. SARS-CoV-2 seroprevalence and transmission risk factors among high-risk close contacts: a retrospective cohort study. Lancet Infect Dis. Published online November 2, 2020. doi: 10.1016/S1473-3099(20)30833-1.