Assessing Household Coronavirus Exposure and Contact Tracing

May 12, 2020

Contact tracing allows epidemiological response by not only notifying people of exposure so they can quarantine, but also understanding potential exposure dynamics that allowed for disease transmission.

More and more, we’ve been hearing about the critical role of contact tracing in helping to break the chain of SARS-CoV-2 infections. In fact, this capability is one of many requirements for truly and safely opening back up businesses.

Contact tracing allows epidemiological response by not only notifying people of exposure so they can quarantine, but also understanding potential exposure dynamics that allowed for disease transmission. For understanding interactions, the role of environmental contamination, and how fomites function in this pandemic, this piece of epidemiology is vital.

A recent study was published in JAMA Internal Medicine regarding a contact tracing assessment in Taiwan, which highlighted not only exposure periods in regards to index case illness and symptoms, but also how household contacts tend to be higher risk.

From January 15 to March 18, 2020, investigators reviewed 100 confirmed SARS-CoV-2 cases.

All 2761 close contacts were quarantined at home for 14 days after their last known exposure to the index case. The period of investigation began at the date of symptom onset and could be extended to 4 days prior to symptom onset when epidemiologically indicated.

Ultimately, they defined close contact as anyone who had more than 15 minutes of face-to-face contact with the confirmed case while not wearing a mask. In a healthcare environment, close contact was defined as contact with an index case involving less than 6 feet without personal protective equipment that was appropriate for that time.

While researchers only had contacts report relevant symptoms such as fever, cough, or other respiratory symptoms, they did test anyone that reported those relevant symptoms.

Unfortunately, as we have learned more, that limited list of triggering symptoms makes it potentially limited.

From those 100 patients, they reviewed 2761 close contacts (5.5% were households, 2.8% were non-household family contacts, and 25.3% were health care contacts). Ultimately, 22 secondary cases were identified and interestingly, none of the secondary cases were related to the 9 asymptomatic index cases.

The median incubation period was estimated to be 4.1 days and of those 22 secondary cases, 4 were asymptomatic. The attack rate for secondary cases was 0.7%.

“All of the 22 secondary cases had their first exposure before the sixth day of the index case’s symptom onset," investigators wrote. "By comparison, only 68% of noncase contacts had their first exposure before day 6. The secondary clinical attack rate was higher among those whose initial exposure to the index case was within 5 days of symptom onset than those who were exposed after day 6 (zero transmission of 852 contacts [95% CI, 0%-0.4%]).”

When reviewing the varying levels of contacts, the research team found that the attack rate for household contacts was 4.6% and 5.3% in nonhousehold family contacts. The attack rate for healthcare exposure was 0.9%. Ultimately, they also found that there was a higher transmission risk around the time of symptom onset from the index case and that there was a lower risk at the tail-end of disease.

This is consistent with studies showing that more viral shedding in the upper respiratory system tends to happen early in the disease progression and wanes over time.

The authors highlight 2 takeaways—first, that the observed short duration of infectiousness with lower risk of transmission 1 week after symptom onset has important implications for redirecting the efforts to control COVID-19.

"Given the nonspecific and mostly mild symptoms of COVID-19 at presentation, patients are often identified and hospitalized at a later stage of disease when the transmissibility of infection has started to decrease," they wrote.

Second, their study observed a low transmission rate, which is good news.

Given the highest risk for transmission to be in the initial days of symptoms, it is critical to rapidly isolate symptomatic patients and work to reduce transmission through isolation and infection control measures.