A major study of 30 million people found that 40 percent of people infected with the Covid virus had no symptoms.
It was previously thought that about 20 per cent of Covid carriers were asymptomatic, making the outbreak difficult to control even with mass testing.
But a new international review of nearly 100 studies found that 40.5 percent may actually be “silent carriers.”
The studies were conducted in the first year of the pandemic, when the original virus or alpha variant was dominant worldwide.
The review, led by researchers at Peking University in Beijing, found that asymptomatic cases are most common among pregnant women, with 54 percent showing no signs of illness. But the researchers did not speculate why this was the case.
The team wrote that the high rate of asymptomatic cases ‘highlights the potential transmission risks’ posed by so-called silent scatterers.
The researchers said they could only be identified through “extensive” testing and close contact tracing.
Asymptomatic infections are those who do not feel unwell despite being infected, which means that they are likely to spread the virus while continuing to interact with others.
The main symptoms of Covid are fever, cough and change in taste or smell, according to the NHS.
But experts warn that symptoms are becoming more widespread and milder and more cold-like with an increase in Omicron cases across the UK, which could further spread it.
Researchers at Peking University in Beijing identified 95 studies involving 29.7 million people who had been tested for the COVID-19 virus. Of those who continued to test positive, 40.5 percent (yellow bar) had no symptoms. The number was higher for pregnant women (red bar), air or cruise travelers (green bar), nursing home residents and staff (orange bar), while it was lower among health workers and hospital patients (blue bar).
The team found that 40.5 percent of those who tested positive for the virus had no symptoms. The graph shows the percentage of asymptomatic infections among the different groups. Groups in which the black square is to the right of the vertical line indicate a higher than average asymptomatic incidence. This has been seen among people from Europe, the United States, developed countries, travelers, residents and staff of nursing homes, pregnant women and young people
Reports from a study tracking COVID-19 symptoms in the UK suggest that Omicron cases cause cold-like health problems such as a runny nose and sneezing, in contrast to traditional signs of the virus earlier in the pandemic, such as a persistent cough.
Researchers searched databases of the scientific journals PubMed, EMBASE and ScienceDirect in February to identify studies related to asymptomatic infection.
They identified 95 studies involving 29.7 million people who were tested for Covid.
Their paper, published in the journal JAMA Network Open, found that 0.25 percent of all surveyed had no symptoms.
The rate of asymptomatic cases was higher among household resistance personnel and employees (4.5 percent), air or cruise travelers (2%) and pregnant women (2.34 percent).
main symptoms of covid
The main symptoms of coronavirus are:
- a high temperature – this means you feel hot when you touch your chest or back (you don’t need to take your temperature)
- new and persistent cough – this means a severe cough for more than an hour, or 3 or more coughing fits in 24 hours (if you usually have a cough, it may be worse than usual)
- Loss of or change in your sense of smell or taste – this means that you notice that you cannot smell or taste anything, smell or taste different than usual.
But among those who continued to be tested for the virus, 40.5 percent had no symptoms.
The figure was higher for pregnant women (54.1 percent), air or cruise travelers (52.9 percent) and residents and staff of care homes (47.5 percent).
The researchers said that more asymptomatic cases may be registered among residents of care homes because more studies have been done among the group, which means they were more likely to be tested.
And as more countries do improved testing, they said, studies will increasingly find that there are more asymptomatic cases among other groups.
The team wrote that the high rate among people on vacation indicates that their screening and isolation upon arrival at airports is ‘important to reduce transmission in the community, especially in countries where there is no local transmission’.
Fewer asymptomatic cases were observed among health care workers, hospital inpatients (30 percent) and close contacts of infected individuals (26.9 percent).
The researchers wrote that asymptomatic health workers can ‘contribute to the spread of disease in and out of hospitals’, so testing them is important for infection control.
The rate of asymptomatic cases varied between different parts of the world, with nearly half of those infected in the United States (46.3%) showing no symptoms, compared to 44.2% in Europe and 27.6% in Asia.
The rate of asymptomatic infection was higher in more developed and younger countries and in research papers published by June, the researchers said.
In studies looking at infections in people under the age of 20, 60.2 percent of cases were asymptomatic, while nearly half of people between the ages of 20 and 39 had no symptoms.
Asymptomatic people can infect others, the team wrote, as previous studies showed that the viral load – the amount of virus in a nose and throat swab – among asymptomatic people was similar to that found among infected people.
Other studies have shown that those without symptoms can fuel outbreaks indoors and elsewhere.
They wrote: “Asymptomatic infection should be considered as a source of Covid infection which plays an important role in the spread of the virus within the community as public life is gradually returning to normal.
“Management of asymptomatic carriers was essential to prevent outbreaks and transmission of staph within the community.”