Jeff Craven and Katie Lennon
June 08, 2020
An official with the World Health Organization (WHO) has stated that it appears to be "rare" that an asymptomatic individual can pass SARS-CoV-2 to someone else.
"From the data we have, it still seems to be rare that an asymptomatic person actually transmits onward to a secondary individual," Maria Van Kerkhove, PhD, WHO's COVID-19 technical lead and an infectious disease epidemiologist, said June 8 at a news briefing from the agency's Geneva headquarters.
This announcement came on the heels of the publication of an analysis in the Annals of Internal Medicine, which suggested that as many as 40-45% of COVID-19 cases may be asymptomatic. In this paper, the authors, Daniel P. Oran, AM, and Eric J. Topol, MD, of the Scripps Research Translational Institute in La Jolla, Calif stated: "The likelihood that approximately 40%-45% of those infected with SARS-CoV-2 will remain asymptomatic suggests that the virus might have greater potential than previously estimated to spread silently and deeply through human populations."
Dr. Van Kerkhove also made comments suggesting otherwise on Twitter, citing a new summary by WHO: "@WHO recently published a summary of transmission of #COVID19, incl. symptomatic, pre-symptomatic and asymptomatic transmission."
She also tweeted the following lines from the WHO summary: "Comprehensive studies on transmission from asymptomatic individuals are difficult to conduct, but the available evidence from contact tracing reported by Member States suggests that asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms."
In an additional post, Dr. Van Kerkhove added: "In these data, it is important to breakdown truly asymptomatic vs pre-symptomatic vs mildly symptomatic ... also to note that the [percentage] reported or estimated to be 'asymptomatic' is not the same as the [percentage] that are asymptomatic that actually transmit."
In the paper published in the Annals of Internal Medicine, Mr. Oran and Dr. Topol analyzed data of asymptomatic individuals from 16 cohorts between April 19 and May 26, 2020 — a wide-ranging group consisting of residents of cities, health care workers, individuals in homeless shelters, obstetric patients, residents of a nursing home, crew members of aircraft carriers, passengers on cruise ships, and inmates in correctional facilities. Each cohort had varying rates of asymptomatic or presymptomatic cases.
When residents of Iceland were tested, 43 of 100 individuals who tested positive for SARS-CoV-2 did not show symptoms. In Vo', Italy, 30 of 73 people (41.1%) with positive SARS-CoV-2 test results did not have symptoms in a first round of testing, and 13 of 29 (44.8%) had no symptoms in a second round of testing. Over half of residents of San Francisco's Mission District who received testing (39 of 74; 52.7%) did not have symptoms, while slightly less than half of Indiana residents tested showed no symptoms (35 of 78; 44.8%).
A majority of 41 individuals (65.9%) who were mostly health care workers at Rutgers University reported no symptoms of COVID-19 at the time of testing. Data from homeless shelters in Boston (129 of 147; 87.7%) and Los Angeles (27 of 43; 62.7%) also showed a high rate of individuals without symptoms. Among 33 obstetric patients in New York City who tested positive for SARS-CoV-2, 29 women (87.9%) were asymptomatic during a median 2-day length of stay. In a Washington state nursing facility, 12 of 23 individuals (52.1%) were positive for SARS-CoV-2 without showing symptoms in a first round of testing, with another 15 of 24 residents (62.5%) not showing symptoms in a second round of testing. Of these residents, 24 individuals (88.9%) later went on to show symptoms of COVID-19.
Most of the 783 Greek citizens who tested positive for SARS-CoV-2 after being evacuated from Spain, Turkey, and the United Kingdom showed no symptoms of COVID-19 (35 of 40; 87.5%). A group of 565 Japanese citizens evacuated from Wuhan, China, had a lower number of cases without initial symptoms – 13 people were positive for SARS-CoV-2, and 4 of 13 (30.8%) had no symptoms.
In closed cohorts, there appeared to also be a high rate of COVID-19 cases without initial symptoms. Of 3,277 inmates from correctional facilities in Arkansas, North Carolina, Ohio, and Virginia, 3,146 individuals (96%) had no symptoms at the time of testing. There was also a large percentage of passengers and crew of the Diamond Princess cruise ship (331 of 712; 46.5%) and an Argentine cruise ship (104 of 128; 81.3%) who were positive for SARS-CoV-2 without symptoms. On the aircraft carrier U.S.S. Theodore Roosevelt, 60% of 856 individuals, while on the French aircraft carrier Charles de Gaulle, nearly 50% of individuals were asymptomatic.
It is difficult to tell the difference between people who are presymptomatic and will later go on to develop symptoms of COVID-19 and those who will remain asymptomatic. "The simple solution to this conundrum is longitudinal testing — that is, repeated observations of the individual over time," but only 5 of 16 cohorts studied had longitudinal data on individuals, Mr. Oran and Dr. Topol said.
Seth Trueger, MD, an emergency physician and assistant professor of emergency medicine at Northwestern University, Chicago, who was not involved in the study, said it was important to see this information all in one place, even if the data isn't new.
"I think we've certainly kind of seen from the beginning there's some level of asymptomatic and presymptomatic spread," Dr. Trueger said. "In health care, we've been lucky to get those lessons early on and start to think of things like universal masking in hospitals, and unfortunate things like limiting visitors."
A more nuanced understanding of how SARS-CoV-2 spreads has been difficult to capture, in part because of operating under a shortened time frame and handicapped testing capacity, he noted. "[Even] in the best of possible circumstances, trying to figure out epidemiology in people who don't have symptoms is really tough," Dr. Truegar said.
"Even the best studies are still relatively decent samples, and not totally representative," he added.
Another limitation to capturing accurate data is method of testing. Real-time reverse transcriptase polymerase chain reaction using nasopharyngeal swabs can detect RNA fragments from SARS-CoV-2, which could potentially affect the results. "It's really hard to know what is actually infected virus versus just fragments of RNA that make the test positive," Dr. Trueger said.
If the rate of asymptomatic cases is higher than previously thought, it's a "double-edged sword," he noted. It may mean the infection fatality rate is lower than predicted, but "even at high levels of what we think community levels might be, we're far from herd immunity."
The study authors and Dr. Trueger reported no relevant conflicts of interest.
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