Just one in five people who catch the coronavirus never go on to show symptoms, a study has found. This figure is far lower than some previous estimates of how many people are asymptomatic, however experts told Newsweek the findings should not affect whether people follow guidelines to prevent the germ from spreading.
Over the course of the pandemic, our understanding of how many people infected with the virus are asymptomatic has evolved as more data has emerged. For example, earlier this month Dr. Anthony Fauci said around 40 to 45 percent of people infected were asymptomatic, medscape reports. However, in July he said this figure was between 20 and 40 percent.
It is important to understand the extent of asymptomatic cases to ensure measures to stop transmission are effective.
The authors of the latest paper published in the journal PLOS Medicine conducted a systematic review of 94 existing studies on the coronavirus in which participants were confirmed to have been infected with the virus and had their symptoms tracked over time.
A participant was deemed asymptomatic if they tested positive for the coronavirus but never showed symptoms. Presymptomatic patients tested positive without any symptoms, but later showed symptoms.
An estimated 20 percent of the participants caught the coronavirus but did not develop symptoms. In a subset of seven studies with a slightly different criteria for including participants, 31 percent of patients were found to be asymptomatic.
The authors also found that asymptomatic people were less likely to pass the virus on to individuals they had come into contact with than those who were symptomatic, according to the study.
Acknowledging the limitations of the research, the authors said most of the papers included were not designed to estimate the proportion of asymptomatic infections. The team also did not consider the effect of false negative test results, so the number of asymptomatic patients could be underestimated, they said.
Study co-author Nicola Law, an epidemiologist at the University of Bern, Switzerland, told Newsweek the review found that a minority of people are “truly asymptomatic” and “probably account for a relatively small proportion of all SARS-CoV-2 transmission.”
She said measures to prevent transmission, such as face masks, social distancing, and widespread testing and contact tracing, still need to be followed as “most people will go on to develop symptoms and there is a substantial amount of transmission during the presymptomatic phase.”
Experts who did not work on the paper had a mixed view of the findings. Danny Oran, a behavioral scientist at the Digital Medicine Group at Scripps Research Translational Institute who co-authored a study on asymptomatic spread earlier this year, told Newsweek the paper failed to include important recent data on the topic of asymptomatic infection as it was completed in June.
Oran said the team “may very well have reached erroneous conclusions” by giving too much weight to studies from China, where the surveillance of symptoms may have been significantly more intensive than other places.
The summary of studies co-authored by Oran found 32 percent of cases in England were likely asymptomatic, and 33 percent in Spain.
Jason Kindrachuk, assistant professor in the Department of Medical Microbiology and Infectious Diseases at Canada’s University of Manitoba, told Newsweek the study is a “good blueprint” for more focused studies on this topic. “While it has been assumed that asymptomatic infections made up a smaller proportion of cases than symptomatic or presymptomatic infections, this publication provides solid evidence for this,” he said.
However, echoing Oran, Kindrachuk said the study was limited because of the large proportion of studies from China, which is likely due to how quickly data was released from the county early in the pandemic.
Simon Clarke, associate professor of cellular microbiology at the University of Reading, U.K., who also did not work on the study, described the review as “extensive” in a statement: “Much has been made of the number of people with the virus, but without symptoms. While this is very interesting, it remains the case that on any given day, the majority of people with the virus will not be displaying any symptoms and these findings should not in any way detract from current infection control advice.”
Julian Tang, honorary associate professor in respiratory sciences at the U.K’s University of Leicester, said he would “interpret this paper’s findings with some caution—and use their estimate of a 20 percent asymptomatic infection rate as just a lower limit.”
Tang said the study had several limitations, mainly due to the source material, including that many of the papers included did not test for other respiratory viruses when they tested for SARS-CoV-2. If patients were infected with other respiratory viruses, like the flu, that may have explained their symptoms rather than the coronavirus.
Tang went on that most of the papers included were based on hospitalized patients. “Clearly such patients are more likely to have symptoms as this is why they attended hospital,” he said in a statement.