Unsupported: The study’s results have also been used to claim that the virus is not contagious or that asymptomatic carriers do not spread COVID-19. Given that all 455 contacts were already protected to some degree from respiratory droplets, the main mode of COVID-19 transmission, the results cannot be used to make conclusions about asymptomatic transmission in an unprotected community.
FULL CLAIM: Asymptomatic carriers cannot spread COVID-19; COVID-19 is not contagious; masks are not needed to reduce COVID-19 transmission from asymptomatic carriers
A meme being spread by Facebook posts in late May 2020 states that a study which “exposed” an asymptomatic COVID-19 carrier to 455 people found that none of the 455 people were infected. Some of the earliest instances of the meme were shared in Facebook pages dedicated to conspiracy theories and vaccine opposition. Based on a study published in the journal Respiratory Medicine on 13 May 2020, its results have been interpreted in several ways according to Facebook posts, with some claiming that these results show the virus is not contagious, or that asymptomatic people cannot spread the virus, or that there is no need to wear a face mask or covering, or that COVID-19 test results produce many false positives. Posts containing the link to the PubMed citation of the study showed more than 46,000 interactions on Facebook, while the meme has received more than 15,000 interactions to date.
We examined what the study actually did and whether its results support these claims. The study can be read in full on PubMed. (Note that PubMed is a repository or library of scientific literature, not the publisher or funder of studies listed in its database, despite what some Facebook posts have claimed.)
The study was conducted at a hospital in Guangdong, China, and its stated aim was “to analyze the infectivity of asymptomatic carriers.” To that end, the researchers examined 455 people who had contact with one asymptomatic carrier, a 22-year-old woman with a medical history of congenital heart disease, after she was admitted to hospital due to worsening shortness of breath on January 13, 2020. She was hospitalized until February 11.
Before admission, she had tested positive for SARS-CoV-2 using the RT-PCR nucleic acid test, and was therefore placed in a quarantine ward. According to the study, while in isolation, she did not exhibit any symptoms associated with viral infection, such as fever and muscle aches. Doctors also did not find abnormalities in her chest CT scan. It should also be noted that she received antiviral and interferon therapy during her stay in the hospital.
Given her positive COVID-19 test and lack of clinical symptoms, she was diagnosed as an asymptomatic carrier. Her diagnosis led all individuals who had been exposed to her during her hospital stay—comprising 224 hospital staff members, 196 family members, and 35 patients—to be routinely screened.
The study stated that “All patients … wore masks except for eating or drinking and were admitted to [the] infectious department for medical isolation.” Patients’ beds were also spaced apart by 1.2 meters. About 47% of the patients were hospitalized for cardiovascular or digestive diseases. Some patients developed respiratory symptoms such as fever and cough, although all of them tested negative for COVID-19 by RT-PCR.
In the case of the 196 family members, the study stated that “The situation on wearing masks was the same as that in patients.” In other words, all 196 were also wearing masks except while eating or drinking. One of them developed fever, cough, and a sore throat, but tested negative on four successive COVID-19 tests, and was later diagnosed with acute tonsillitis.
As for the 224 hospital workers, about 45% were nurses, 26% were doctors, and the remaining members of the group were security guards, cleaners, or transportation personnel. The study stated that “all personnel working in [the emergency department] must take medical protection—namely, using [an] N95 mask, putting on [an] isolation gown and wearing goggle[s].” All 224 were tested for COVID-19 on two occasions; all results were negative.
In the end, the study concluded that “Infectivity of some asymptomatic SARS-CoV-2 carriers might be weak,” but also that “Effective prevention and control measures are helpful to prevent COVID-19 spread of asymptomatic carriers.”
Thus, taking the study as a whole, it is clear that the results do not support the various claims made in many Facebook posts citing the study. In fact, the results contradict these claims, as they suggest that widespread use of face masks could reduce COVID-19 transmission among contacts.
The claim that the results show that masks are unnecessary demonstrates a failure to grasp the significance of these observations and/or cherry-picking of information to suit one’s preferences. As the study’s authors acknowledged, widespread use of face masks may have played a role in curbing the chain of transmission. In addition, given that all contacts traced in the study wore some form of protection, mainly face masks, its results cannot be extrapolated to an unprotected community—as many of the Facebook posts strongly imply—or used to claim that asymptomatic transmission does not occur.
Numerous studies have already shown that asymptomatic and presymptomatic (delayed appearance of symptoms post-infection) transmission is a cause for concern regarding COVID-19 transmission in a community[3,4,5]. In the case of the Diamond Princess cruise ship, about 18% of infected people showed no symptoms.
Mathematical modeling based on epidemiological data in China estimated that about 86% of undocumented infections were responsible for around 80% of documented infections, although one caveat is that undocumented infections were not limited to asymptomatic infections, but also included mild infections. Undocumented infections, explained the authors, “go unrecognized owing to mild, limited, or lack of symptoms and thus, depending on their contagiousness and numbers, can expose a far greater portion of the population to the virus than would otherwise occur.”
Claims that the nucleic acid tests are inaccurate and lead to mostly false positives are unsupported. Health Feedback covered this claim in a previous review, showing that such tests are highly specific for SARS-CoV-2 and able to distinguish between the novel coronavirus and other viruses. In fact, the nucleic acid test that some of these Facebook posts call into question is based on the same principle as the test used in the Respiratory Medicine study. It is not clear why these posters consider the study’s results and conclusion valid, but claim the COVID-19 nucleic acid tests used in screening the community are not reliable. These claims are inconsistent with each other.
In summary, the study’s results suggest that masks may be useful in reducing COVID-19 transmission, contrary to claims in many Facebook posts. However, it is important to keep in mind that the study does come with several limitations. For example, the viral load in the asymptomatic carrier was not measured in the study, so it is possible that the carrier had a low viral load that would have made transmission less likely to begin with. More significantly, the study only monitored one asymptomatic carrier, which may not be generalizable to other asymptomatic people. Larger studies of asymptomatic carriers are needed to obtain more conclusive results on the risk of transmission.
- 1 – Gao et al. (2020) A study on infectivity of asymptomatic SARS-CoV-2 carriers. Respiratory Medicine.
- 2 – Prather et al. (2020) Reducing transmission of SARS-CoV-2. Science.
- 3 – Bai et al. (2020) Presumed Asymptomatic Carrier Transmission of COVID-19. JAMA.
- 4 – Arons et al. (2020) Presymptomatic SARS-CoV-2 Infections and Transmission in a Skilled Nursing Facility. New England Journal of Medicine.
- 5 – Mizumoto et al. (2020) Estimating the asymptomatic proportion of coronavirus disease 2019 (COVID-19) cases on board the Diamond Princess cruise ship, Yokohama, Japan, 2020. Eurosurveillance.
- 6 – Li et al. (2020) Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV-2). Science.