Contact tracing must include asymptomatic period of index case

Evidence from researchers in Taiwan suggesting that Covid-19 patients were most infectious before symptom onset and within 5 days of onset makes contact tracing yet another challenge for those seeking to contain the pandemic.

“Our analysis of close contacts to confirmed Covid-19 cases revealed a relatively short infectious period of Covid-19 and a higher transmission risk around the time of symptom onset of the index case, followed by a lower transmission risk at the later stage of disease,” Hao-Yuan Cheng, MD, MSc, from the Epidemic Intelligence Center, Taiwan Centers for Disease Control, Taipei, and colleagues reported in JAMA Internal Medicine. “The observed decreasing transmission risk over time for Covid-19 was in striking contrast to the transmission pattern of severe acute respiratory syndrome (SARS), in which the transmission risk remained low until after day 5 of symptom onset in the index cases.”

Cheng and colleagues found that SARS-CoV-2 viral shedding in upper respiratory specimens was highest at the time of symptom onset before gradually decreasing to a low level after 10 days, and this was the case whether the patients with asymptomatic, minimally symptomatic, or symptomatic. And, they noted, the secondary clinical attack rate — the rate at which the disease infected a secondary host — was consistent with this viral shedding time frame.

“These findings underscore the pressing public health need for accurate and comprehensive contact tracing and testing,” Robert Steinbrook, MD, Editor-at-Large for JAMA Internal Medicine, wrote in a commentary accompanying the study. “Testing only those people who are symptomatic will miss many infections and render contact tracing less effective. The finding that asymptomatic people and those with minimal or fewer symptoms early in infection are those most likely to transmit Covid-19 strongly argues for maintaining social distancing and having people wear face masks to reduce the potential for transmission. Solely isolating patients symptomatic with Covid-19 will fail to control transmission during the infected but asymptomatic stage.”

Cheng and colleagues undertook their study, which was conducted from Jan. 15 to March 18, 2020, to help determine the transmission dynamics of Covid-19 during various exposure windows pre- and post-symptom onset.

They looked at 100 confirmed cases during this time period, as well as their close contacts (n=2,761), who were followed until 14 days after their last exposure to the index case. They noted that, of the 100 patients, there were 10 clusters of patients and 9 asymptomatic patients. The patients’ median age was 44, and more than half were men.

Of the close contacts, 5.5% were household contacts, 2.8% were non-household, and 25.3% were healthcare contacts. They found 22-paired index-secondary cases.

Other findings:

  • The overall secondary clinical attack rate was 0.7% (95% CI, 0.4%-1.0%).
  • The attack rate was higher among the 1,818 contacts with exposure to the index cases within 5 days of symptom onset (1.0% [95% CI, 0.6%-1.6%]) compared with those who were exposed later (0 cases from 852 contacts; 95% CI, 0%-0.4%).
  • 299 contacts with exclusive pre-symptomatic exposures showed an attack rate, 0.7% (95% CI, 0.2%-2.4%).
  • Those whose exposure was household and non-household family contacts had higher attack rates (4.6% [95% CI, 2.3%-9.3%) and 5.3% [95% CI, 2.1%-12.8%], respectively) compared to health care or other settings.
  • The age groups with the highest attack rates were age 40 to 59 years (1.1% [95% CI, 0.6%-2.1%]) and 60 years and older (0.9% [95% CI, 0.3%-2.6%]).

“To summarize the evidence, the decreasing risk for secondary infection over time in our study, the observed short serial interval, and the trend of decreasing viral shedding and viability after symptom onset strongly suggested high transmissibility of the disease near or even before the day of symptom onset,” Cheng and colleagues wrote. “Because the onset of overt clinical symptoms, such as fever, dyspnea, and signs of pneumonia, usually occurred 5 to 7days after initial symptom onset, the infection might well have been transmitted at or before the time of detection.”

Steinbrook noted that the study shows what Taiwan did correctly in its response to the pandemic, particularly its rapid and proactive approach.

“It is impressive, even astounding, that Taiwan not only conducted robust contact tracing and testing on the first 100 confirmed cases, but also quickly and comprehensively reported the results, thus meaningfully advancing knowledge of the transmission dynamics of the virus,” Steinbrook wrote. “Unfortunately, widespread testing was not available in the U.S. in February 2020, hampering the ability to identify people who were Covid-19 positive.”

Before the U.S. “reopens,” Steinbrook noted that sufficient testing and contract tracing must be available, as more outbreaks “will inevitably occur as social restrictions are removed [so they] can be successfully contained.”

He also noted that social distancing and wearing face masks should be continued until there is a treatment for Covid-19 or a vaccine.

The CDC advises that contact tracing should be part of the U.S. response to the Covid-19 pandemic. When contact tracing is undertaken and people who have been exposed to an infected Covid-19 patient are identified and notified, guidance suggests they should then be placed in quarantine. As previously reported by BreakingMED, contact tracing in the U.S will take “an army of disease detectives,” according to Crystal Watson, DrPH, of Johns Hopkins Bloomberg School of Public Health. It will also come at a cost of around $12 billion to expand the contact tracing workforce.

  1. A recent prospective case series from Taiwan found that patients were most infectious before and within five days of symptom onset.

  2. The study authors found that there was a decreasing transmission risk over time for Covid-19, which was in striking contrast to the transmission pattern of severe acute respiratory syndrome (SARS), in which the transmission risk remained low until after day 5 of symptom onset in the index cases.

Candace Hoffmann, Managing Editor, BreakingMED™

The study was funding by the Taiwan Ministry of Science and Technology.

Cheng disclosed no relevant relationships.

Steinbrook reported no relevant relationships.

Cat ID: 125

Topic ID: 79,125,254,930,287,500,932,730,933,125,190,520,926,192,927,151,928,925,934