Some people who contract Covid-19 never experience any symptoms, and there remains disagreement about what proportion of total infections these cases represent.
The full spectrum and distribution of the severity of symptoms are still not well understood.
Some infected people may experience severe infections resulting in viral pneumonia, respiratory distress syndrome, and death.
While others remain completely asymptomatic or develop mild, nonspecific symptoms.
The findings of this systematic review of publications early in the pandemic suggest that most SARS-CoV-2 infections are not asymptomatic throughout the course of infection
Researchers analysed 79 studies reporting empirical data on 6,616 people, 1,287 of whom were defined as asymptomatic, in order to determine the proportion of infected people who did not develop symptoms.
Despite the study, published in PLOS Medicine, being limited by its inability to gauge the impact of false negatives, the researchers estimated that 20% of infections remained asymptomatic during follow-up.
The researchers argue that accurate estimates of true asymptomatic and presymptomatic infections are critical to understanding transmission of the virus at the population level.
Diana Buitrago-Garcia, at the University of Bern, Switzerland, and colleagues, wrote: “The findings of this systematic review of publications early in the pandemic suggest that most SARS-CoV-2 infections are not asymptomatic throughout the course of infection.
“The contribution of presymptomatic and asymptomatic infections to overall SARS-CoV-2 transmission means that combination prevention measures, with enhanced hand and respiratory hygiene, testing and tracing, and isolation strategies and social distancing, will continue to be needed.”
Future research should include prospective longitudinal studies that document symptom status, the researchers indicate.
Improved accuracy of serological tests is also needed to reduce the number of false negatives, the study suggests.
As each person infected with the virus is initially asymptomatic, the proportion that will go on to develop symptoms is estimated to be around 80%, suggesting that presymptomatic transmission may significantly contribute to overall SARS-CoV-2 epidemics.
Dr Julian Tang, honorary associate professor in respiratory sciences, University of Leicester, said: “I would interpret this paper’s findings with some caution – and use their estimate of a 20% asymptomatic infection rate as just a lower limit.
“Although it addresses an important question, it has several limitations, which are mainly due to the limitations in the source material
“Many of the papers included in the review likely did not test for other respiratory viruses when they tested for SARS-COV-2.
“If patients were infected with other respiratory viruses as well as SARS-COV-2, then the Covid-19 symptoms may have been due to these other viruses (like flu, RSV, parainfluenza, adenovirus, human metapneumovirus, rhinovirus and seasonal coronaviruses).”
Dr Simon Clarke, associate professor of cellular microbiology at the University of Reading, said: “This extensive overview of multiple studies comes to the conclusion that most people who are infected with the coronavirus causing Covid-19 eventually develop symptoms.
“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.”
