Study population and setting
As part of an ongoing study of COVID-19 transmission within households in Tennessee and Wisconsin (United States), 185 household contacts of 105 index COVID-19 cases were recruited between April and September 2020 to complete daily symptom checks (self-reported) for 14 days and self-collect respiratory samples (nasal only or nasal/saliva) for SARS-CoV-2 RT-PCR testing. The investigators estimated the probability that household contacts would remain asymptomatic and test negative for SARS-CoV-2 on days 7 and 10 after last known contact with the index case, which was assumed to be the illness onset date of the index case.
Summary of Main Findings
A majority (59%) of household contacts tested positive for SARS-CoV-2 over the 14-day follow-up period, with a median time to first COVID-19 positive sample of 5 days (interquartile range: 3-7 days) following symptom onset for the index case in the household. A majority (76% and 86%) of these household contacts had detectable viremia within 7 and 10 days, respectively, of the index case’s symptom onset. Given a household contact was asymptomatic and RT-PCR-negative at day 7, the probability that the contact would remain SARS-CoV-2 negative through day 14 was 81% (95% CI: 67-90%). If the contact was asymptomatic and RT-PCR-negative on day 10, the probability of remaining asymptomatic and RT-PCR-negative on day 14 increased to 93% (95% CI: 78-98%). In a sensitivity analysis, where 45 household contacts were excluded due to multiple index cases within the household or possible tertiary COVID-19 transmission within the household, the probability of a household contact remaining asymptomatic and RT-PCR-negative through day 14 remained similarly high (95%, 95% CI: 81-99%), assuming the participant was asymptomatic and RT-PCR-negative on day 10.
This study conducted daily testing of household contacts of SARS-CoV-2 index cases for 14 days following last known contact with the index case. Multiple household transmission events were assessed and numerous sensitivity analyses conducted.
While household contacts are assumed to have repeated substantial exposure to index cases (particularly during the index patient’s pre-symptomatic period of COVID-19 infection, when transmission probabilities are highest), the inattention to clinical markers (i.e., viral load of index case at baseline) and other behavioral factors impacting COVID-19 transmission (e.g., number and duration of contacts with the index case during the period of infectiousness, adoption of mitigation measures within the household like mask use) makes it difficult to discern and compare COVID-19 acquisition risk among household contacts. Notably, because information on household contacts’ last known exposure to index case patients was unavailable, investigators used the date of index cases’ symptom onset as a proxy; this could have biased timing of specimen swabbing and subsequent calculations if contacts had subsequent exposures to the index case. Lastly, specimen self-collection for RT-PCR may have introduced measurement error into SARS-CoV-2 testing algorithms, resulting in systematic underreporting of COVID-19 test positivity by RT-PCR.
This study assessed the probability of SARS-CoV-2 infection among uninfected household contacts of index case-patients daily over a 14-day period, offering new insights into the risk of COVID-19 infection following a shortened (7 and 10-day) quarantine period.
This review was posted on: 22 January 2021