Most people with coronavirus have symptoms but asymptomatic cases may make up around 20 per cent of infections, a new study suggests.
Researchers analysed 79 studies with 6,616 participants to determine the proportion of Covid-19 sufferers who did not develop signs.
Published in PLOS Medicine, the research showed that there is a full spectrum of reactions to the strain including asymptomatic, mild, viral pneumonia, respiratory distress syndrome, and death.
Despite the study being limited by its inability to gauge the impact of false negatives, the researchers estimated that 20 per cent of infections remained asymptomatic during follow-up.
It comes as another team of scientist claimed that people with symptomless Covid-19 could be driving the spread of the virus.
In PLOS Medicine, 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.
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 per cent 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.”
Meanwhile experts said that people who are infected but have no symptoms may have “comparable potential” for spreading the virus as those who have symptoms – including fever, a new and persistent cough and a new loss or change of taste or smell.
The findings support the use of face coverings among the general public, the study’s lead author said.
A team of researchers examined people who were at a community facility designated for the isolation of patients with mild Covid-19 in South Korea.
They evaluated patients’ symptoms as well as the amount of virus they were carrying – or their viral load – in their nose and throats.
There were 213 patients without severe symptoms. Of these, 19 per cent were asymptomatic.
Researchers found that people without symptoms had comparable viral loads to symptomatic patients.
Their study, published in the journal Thorax, concludes: “Asymptomatic individuals were frequent among those infected with SARS-CoV-2, but harboured a comparable viral load compared with that of symptomatic patients and may thus act as a meaningful driving force for the community spread of Covid-19.”
In a linked podcast, lead author Professor Sung-Han Kim said the findings “support the possibility asymptomatic individuals are contributing to the ongoing community spread of Covid-19.
“The viral load was similar between asymptomatic individuals and symptomatic patients. This suggests that asymptomatic with a Sars-CoV-2 infection have a comparable potential for spreading the virus as symptomatic patients.
“Our data add further support to the general public use of face masks, regardless of the presence of symptoms, and suggest that the scope of SARS-CoV-2 testing should be expanded to include asymptomatic individuals in high-risk settings, such as nursing homes or healthcare facilities.”