Study: Unclear but present danger: An asymptomatic SARS-CoV-2 carrier

Superspreaders are critical infectious resources in multiple infectious diseases. They can be asymptomatic or present mild symptoms but can transmit pathogens to susceptible populations, leading to severe symptoms, and even death. Early identification of this population is extremely important to inhibit the spread of infectious diseases. Right now, the whole global world is suffering from a devastating infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In this article, a superspreader cluster event in coronavirus disease 2019 (COVID-19) was identified by tracking contacting histories of infected patients. This cluster was found to be originated from an asymptomatic SARS-CoV-2 carrier, which resulted in 13 secondary cases getting infected. All the secondary patients presented with non-typical symptoms of COVID-19, such as fever, dry cough, and myalgia, one of which died of respiratory failure at the end. From this cluster, we learn that people with older ages, low immunity, multiple underlying diseases, especially pulmonary diseases, can contribute to a poor prognosis. Thus, asymptomatic superspreaders of COVID-19 can be extremely dangerous and must be handled time-efficiently.

Coronavirus is an enveloped single-stranded and positive-sensed helical RNA virus belonging to the Orthocoronavirinae family. It can be widely distributed in humans, other mammals, birds, and bats, leading to acute and persistent infections. In December 2019, there were multiple cases of non-typical pneumonia found in Wuhan, China, many of them were found having an exposure history to Huanan Seafood Supermarket. The pathogen was finally determined to be a novel coronavirus through virus isolation and gene identification. On 2020, January 12th, the World Health Organization (WHO) denominated this virus to be 2019 Novel Coronavirus (2019-nCov). Until Jun 17th, 2020, more than 8,214,571 cases have been confirmed in the global world and 444,853 patients died of novel coronavirus disease (COVID-19) in 176 countries from Johns Hopkins Coronavirus Research Center, a real-time updated data bank. A total of 2,141,306 cases are confirmed and 117,033 cases are dead of COVID-19 in the United States. COVID-19 now has been a global pandemic, millions of people are struggling to survive. Innumerable people are facing up to unemployment, and the global economy is suffering a devastating setback.

In the early hypothesis related to the interactions between host and pathogens, scientists believe that infected people may have an equal chance to transmit pathogens to secondary cases. However, some scientists observe there is a 20/80 rule existing in infectious diseases, which is believed that 20% of infected cases contribute to 80% of infected cases. This 20/80 rule is applicable in many transmissions of infectious pathogens among different species. A basic reproductive number named R0 is a fundamental epidemiological parameter to characterize infectious disease outbreaks, it indicates an average number of secondary infections originated from an index individual during the whole period of infectious disease, suggesting the severity of infectious diseases. R₀ of COVID-19 is estimated to be 2 to 4 which is quite similar to influenza. However, a group of people named superspreaders can transmit pathogens disproportionately to more than an average number of secondary cases, and are likely to promote the speed and scale of outbreaks. Usually, superspreaders transmit at least 10 individuals, sometimes even up to 100 secondary cases.

Superspreaders are recorded in the histories of multiple infectious diseases. Two superspreaders are reported in Hong Kong epidemic in coronavirus induced severe acute respiratory syndrome (SARS) in 2003. The first index patient was estimated to infect at least 138 secondary cases since March 3rd, 2003 in the Prince of Wales Hospital. The number of people infected by the second index patient was not clear, however, it was estimated at least 300 people were infected with the contact of this patient. Three superspreaders also occurred in Middle East respiratory syndrome coronavirus (MERS-CoV) occurred in 2015. The first superspreader transmitted the virus to a total of 27 secondary cases in the same hospital, and the second and third superspreaders infected at least 73 and 24 tertiary cases, respectively. Ebola virus belongs to the filovirus family and leads to severe hemorrhagic fever in human beings with a mortality rate of 90%. It had a devastating outbreak in Western Africa in 2014–2015. This outbreak can be traced back to several superspreaders who contributed to the secondary cases. There are a few case reports about the superspreaders in COVID-19 as well. From recent news, an England businessman, who might be infected in Singapore when having a conference, headed to France, Switzerland, and England consequently, resulting in at least 11 secondary cases infected. Moreover, a superspreader in Korea infected at least 37 people during a church-related and daily-routine activity. Another Indian preacher, who just had a trip from COVID-19 epicenter Italy and Germany, went preaching in at least 10 villages in Punjab State without any self-isolation until this patient felt ill and died. Because of him, 19 people have been confirmed 2019-nCov positive, more than 200 people are waiting for the testing results, and at least 15,000 people have to be under a strict quarantine.

Although superspreaders are rare cases in plenty of infectious diseases, early identification of superspreaders is very significant in reducing infected cases and preventing outbreaks. Here, we report a superspreader cluster from Chongqing, an asymptomatic index patient transmitted to 13 persons unintendedly, and whether other individuals get infected are still unknown. This superspreader patient A was not found first. However, many secondary cases were discovered initially from the same family, all of them had a close contacting history with patient A who had a recent travel history from Wuhan and did not show any symptoms. Thus, this asymptomatic patient A was excavated and deemed to be a superspreader of COVID-19. From learning this superspreader event cluster, we highly recommend identifying superspreaders timely by tracking contacting histories from infected cases as early as possible to prevent consequent transmission and outbreaks.