Asymptomatic COVID-19 Infection: A Review of Cases Entering China

Infectious diseases test our response mechanisms constantly. For many, the non-specific symptoms make early detection and isolation hard, resting solely on travel or recent exposure. When dealing with novel pathogens, it’s essentially building the bridge as you run across it, meaning that our guidance and protocols are developed from previous outbreaks of similar pathogens and continuously refined as more data is analyzed.

One of the more challenging pieces to coronavirus 2019 (COVID-19), and there have been many, is the frequency at which people are asymptomatic and pre-symptomatic. Unlike response to Ebola virus diseases back in 2014, in which our focus could be on travel and symptoms, responding to COVID-19 means accounting for those who are entirely asymptomatic.

Such infections are exceedingly difficult to manage from a public health perspective as they require people to act as if they might be contagious—stay home, wear a mask when you leave, clean your hands, avoid indoor spaces with those outside your household and with poor ventilation, clean/disinfect, etc. This can be difficult though, as so much of our habits regarding infectious diseases are in regards to known exposures and obvious symptoms, which trigger us to stay home and isolate. Moreover, understanding the true burden of asymptomatic infections is nearly impossible as it relies on testing of individuals who might not seek care or testing.

In an effort to understand the potential burden of asymptomatic SARS-Cov-2 infections in a traveling population, a group of researchers studied those entering China from April 16 to October 12, 2020. Such attention to travel has been ongoing since the beginning of the pandemic, with heavy emphasis on border closures and strict pre-travel testing. More recently, many countries are requiring testing upon arrive in additional to quarantine.

This retrospective cohort study utilized data from the Chinese Center for Disease Control and Prevention which involved RT-PCR tests performed at China’s border checkpoints in the aforementioned timeframe. Since April 1, 2020, China has utilized mandatory testing and post-travel quarantine for those negative on arrival for all traveling into China via air, sea, or land.

Asymptomatic individuals were defined as those who had a positive test result but did not develop symptoms during their 14 quarantine (as they were tested upon arrival and on day 13). The authors noted that, “of the 19 398 384 international travelers who entered China during the study period, 3103 had SARS-CoV-2 infection. Most were male (75.5%) and were aged 20 to 49 years (80.8%). Among all SARS-CoV-2–positive entrants, 1354 (43.6%) had symptoms at entry (symptomatic) and 137 (4.4%) developed symptoms (presymptomatic; median time to symptom onset, 1 day; interquartile range [IQR], 0-5 days; 95th percentile, 10 days) and were categorized as confirmed COVID-19 cases, whereas 1612 (51.9%) never developed symptoms through day 13 and were considered to have asymptomatic SARS-CoV-2 infection.”

Interestingly, when looking at all international entrants, the research team found that while the proportion of SARS-CoV-2-positive persons remained stable, those who were deemed presymptomatic or asymptomatic actually increased over time. The proportion of asymptomatic entrants grew from 27.8% from April 16-30 to 59.4% during September 28-October 12.

As the authors note, these findings are worrisome and might signal an increase in asymptomatic individuals, which stresses public health efforts and emphasizes the need for continued infection prevention efforts.