People with no symptoms responsible for nearly 45% coronavirus infections as they transmit it silently: Study
Asymptomatic persons — those who do not show any symptoms — seem to account for approximately 40% to 45% of Covid-19 infections, and they can transmit the virus to others for an extended period, perhaps longer than 14 days. This is according to a review of 16 cohorts of Covid-19 and asymptomatic infections globally.
The absence of Covid-19 symptoms in persons infected with SARS-CoV-2 (the virus that causes Covid-19) might not necessarily imply an absence of harm, says a research team from Scripps Research Translational Institute, US. "The likelihood that approximately 40% to 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," says the study published in Annals of Internal Medicine.
From April 19 through May 26, 2020, using the keywords Covid-19, SARS-CoV-2, symptoms, and asymptomatic, researchers periodically searched the published medical literature. They also searched for unpublished manuscripts using the bioRxiv and medRxiv services. In addition, they looked for news reports using Google and monitored relevant information shared on Twitter.
"Since February 2020, there have been reports of persons who were infected with SARS-CoV-2 but did not develop symptoms of Covid-19. In some cases, the viral load of such asymptomatic persons has been equal to that of symptomatic persons, suggesting similar potential for viral transmission. The prevalence of asymptomatic SARS-CoV-2 infection, however, has remained uncertain. We sought to review and synthesize the available evidence on asymptomatic SARS-CoV-2 infection," says the team.
In the largest cohort, the review found that in Iceland, out 13,080 persons who volunteered for screening, 100 tested positive for Covid-19. Among those with positive results, 43 (43%) had no symptoms of coronavirus at the time of testing. In Vo', 2,812 residents were tested during the first sampling effort and 2,343 during the second. This represented 85.9% and 71.5%, respectively, of the entire population. In the first group, 30 (41.1%) of 73 persons who tested positive for SARS-CoV-2 had no symptoms. In the second, 13 (44.8%) of 29 who tested positive were asymptomatic.
According to researchers, in the roughly two-week period between the sampling efforts, none of the asymptomatic persons developed any symptoms of Covid-19. Besides, through contact tracing, officials confirmed that several new cases of SARS-CoV-2 infection that appeared during the second sampling had been caused by exposure to asymptomatic persons.
The review also talks about 4,611 residents of Indiana who were tested from April 25 through May 1 by the Indiana State Department of Health and the Indiana University Richard M. Fairbanks School of Public Health for Covid-19. In total, 78 (1.7%) tested positive and 35 (44.8%) of these persons were asymptomatic.
The study looked at multiple different settings. At a Boston homeless shelter, among 408 occupants, 147 (36%) tested positive for SARS-CoV-2, of whom 129 (87.8%) were asymptomatic. At a Los Angeles homeless shelter, 43 (24.2%) of 178 completed tests were positive for SARS-CoV-2 and 27 (63.8%) of the persons who tested positive were asymptomatic. In two New York City hospitals, among 214 patients, 33 (15.4%) tested positive, 29 (87.9%) of whom were asymptomatic. On USS Theodore Roosevelt, 60% of those with positive results were asymptomatic. An internal US Navy document stated, “Results of out-testing portions of the crew following 14 days of quarantine leads us to reevaluate our assessment of how the virus can remain active in an asymptomatic host.”
From March 20-25, 783 Greek citizens were repatriated from Spain, Turkey, and the UK on 7 flights. Forty (5.1%) tested positive for Covid-19. At the time of testing, 39 (97.5%) were asymptomatic. At follow-up about 2 weeks later, 35 (87.5%) had remained asymptomatic.
Among other settings, the analysis reviewed reports on inmates in Arkansas, North Carolina, Ohio, and Virginia. It says as of April 25, SARS-CoV-2 test results that include data on symptom status were available for 4693 inmates in the state prison systems of Arkansas, North Carolina, Ohio, and Virginia. Among these inmates, 3277 (69.8%) tested positive, of whom 3146 (96%) had no symptoms at the time of testing.
The researchers say that based on three cohorts with representative samples — Iceland and Indiana, with data gathered through random selection of participants, and Vo', with data for nearly all residents — the asymptomatic infection rate may be as high as 40% to 45%. "A conservative estimate would be 30% or higher to account for the presymptomatic admixture that has thus far not been adequately quantified. In any case, these high rates are not aligned with current testing programs that have predominantly focused on symptomatic cases. Beyond expanding testing to those without symptoms or known exposure, our inability to recognize carriers might make necessary the broad adoption of preventive strategies, such as masks," says the team.
The Vo' cohort seems to confirm that asymptomatic persons can indeed transmit SARS-CoV-2 to others, and the experience aboard the USS Theodore Roosevelt suggests that they might be able to transmit the virus to others for longer than 14 days, says the study. "These worrisome findings could explain, in part, the rapid spread of the virus around the globe. Persons who do not feel or look ill are likely to have far more interaction with others than those who have symptoms. If asymptomatic transmission is indeed common, testing only those with symptoms would seem to be folly," say experts.
Researchers further say the finding that 54% of the 76 asymptomatic persons on the Diamond Princess who were examined by computed tomography appeared to have significant subclinical abnormalities in their lungs is "disturbing". They recommend further research to confirm "this potentially important finding", taking into account possible confounding factors, including the age of passengers aboard the Diamond Princess. "If confirmed, this finding suggests that the absence of symptoms might not necessarily mean the absence of harm. The subclinical nature of the finding raises the possibility that SARS-CoV-2 infection causes subtle deficits in lung function that might not be immediately apparent," they add.
The team also questions if the relatively high proportion (60.5%) of asymptomatic cases on the USS Theodore Roosevelt — whose crew members, presumably, are mostly in their 20s and 30s — suggest that asymptomatic infection is more likely in younger persons. "Perhaps, but it must be noted that the proportion of asymptomatic infection (47.8%) on the Charles de Gaulle aircraft carrier seems to be only marginally higher than average," they say.
The 96% rate of asymptomatic infection among thousands of inmates in four state prison systems is ‘remarkable,’ says the team. "Without any longitudinal data, we cannot estimate the number of presymptomatic cases," they add. Closed cohorts, such as cruise ships, aircraft carriers, and correctional facilities, offer both advantages and disadvantages. According to the researchers, since the likelihood of viral exposure is so much greater than in other settings, the ‘treatment’ that participants receive "may be close to uniform".
"As a result, we may learn more about the average incidence of asymptomatic infection. But the confined environment — which ensures frequent, overlapping interaction between participants — makes it challenging to accurately trace contacts and elucidate the chain of viral transmission," the findings state.
Based on their analysis, the researchers recommend that because of the high risk for silent spread by asymptomatic persons, testing programs must be substantially broadened to include persons who do not have symptoms of Covid-19. To supplement conventional diagnostic testing, which is constrained by capacity, cost, and its one-off nature, innovative tactics for public health surveillance, such as crowdsourcing digital wearable data and monitoring sewage sludge, might be helpful, they add.
"The early data that we have assembled on the prevalence of asymptomatic SARS-CoV-2 infection suggest that this is a significant factor in the rapid progression of the COVID-19 pandemic. Medical practice and public health measures should be modified to address this challenge," says the team.
"In countries like the US that have been hardest hit by the SARS-CoV-2 pandemic, it has been apparent for some time that the amount of testing must be significantly and rapidly increased — perhaps by an order of magnitude or more. With this new knowledge that a large proportion of those infected with SARS-CoV-2 have no symptoms, the urgency for more testing becomes even greater," the team cautions.