According to a large study of 30million people, Covid is not symptomatic in 40% of cases. 

It had been thought previously that about 20% of Covid-carriers were asymptomatic. This has made it hard to control outbreaks, even using mass testing.

But a new international review of 95 studies found 40.5 per cent may actually be ‘silent carriers’. 

Studies were performed in the first year after the pandemic. At that time, either the Alpha virus or original virus was dominant around the world. 

Peking University, Beijing researchers conducted the review. They found that pregnant women are at highest risk for asymptomatic diseases. 54% showed no symptoms. Researchers did not explain why.

The team wrote that the high rate of asymptomatic cases ‘highlights the potential transmission risk’ the so-called silent spreaders pose.

They can only be identified through ‘extensive’ testing and close contact tracing, the researchers said.

Asymptomatic infection is a condition in which a person does not feel ill despite having been infected. They are more likely to transmit the virus to other people if they keep in touch with them.

Covid’s main symptoms are a fever, cough and change to taste or smell, according to the NHS. 

However, experts caution that Omicron is spreading faster than expected and the symptoms have become milder and more cold-like. 

Researchers at Peking University in Beijing identified 95 studies which included 29.7million people who had been tested for Covid. Among those who went on to test positive, 40.5 per cent had no symptoms (yellow bar). And the figure was higher for pregnant women (red bar), air or cruise travellers (green bar) and care home residents and staff (orange bar), while it was lower among health workers and hospital patients (blue bar)

Peking University researchers in Beijing discovered 95 studies that covered 29.7 million individuals who were tested for Covid. Only 40.5 percent of those tested positive had symptoms (yellow bars). The figure was even higher among pregnant women, air and cruise travelers (green bars), and residents and staff of care homes (orange bar), while the number was lower for hospital patients and health workers (blue bar).

The team found 40.5 per cent of those who test positive for the virus have no symptoms. The graph shows the percentage of asymptomatic infections among different groups. Groups where the black square is to the right of the vertical line signal a higher than average rate of asymptomatic infection. This was seen among people from Europe, the US, developed nations, travellers, care home residents and staff, pregnant women and young people

Researchers found that 40.5 percent have not experienced symptoms in those tested positive. This graph displays the proportion of people who have asymptomatic infection in different age groups. A higher rate of asymptomatic disease is indicated by groups where the black square lies to the right side of the vertical lines. These were seen in European, American, and other developed countries, as well as among travellers, staff at care homes, young women, and residents of hospitals.

The UK’s Covidsymptom Tracking Study has reported that Omicron cases can trigger cold-like health issues like runny eyes and sneezing. These symptoms are not the traditional signs of the disease from earlier periods like persistent coughs.

In February, researchers searched the scientific journals databases PubMed (EMBASE), ScienceDirect and ScienceDirect to find reports of asymptomatic diseases.

There were 95 studies, which contained 29.7 million people who had been tested positive for Covid.

The paper was published in JAMA Network Open. It found that 0.25 percent of those who were swabbed did not have symptoms.

Asymptomatic cases were more common among staff, air and cruise travelers (2%) and pregnant women (2.34%).

Main Covid symptoms

Coronavirus manifests itself as the following: 

  • a high temperature – this means you feel hot to touch on your chest or back (you do not need to measure your temperature) 
  • a new, continuous cough – this means coughing a lot for more than an hour, or 3 or more coughing episodes in 24 hours (if you usually have a cough, it may be worse than usual)
  • a loss or change to your sense of smell or taste – this means you’ve noticed you cannot smell or taste anything, or things smell or taste different to normal 

Source: NHS

However, only 40.5 percent of those who tested positive had symptoms.

The figure was even higher for pregnancies (54.1%), cruise or air travelers (52.9%) and residents of care homes and staff (47.5%).

According to the researchers, more care home residents could have symptomatic conditions because they had been subjected to more research. This would make them more likely for testing.

Studies will show that there is more cases of asymptomatic disease in the countries where enhanced testing is conducted, as they are reported by them. 

The high rate among holidaying people suggests screening and quarantining them when they arrive at airports is ‘important for reducing community transmissions, especially in countries without local transmission,’ the team wrote.

Healthcare workers, hospital patients and close friends of infected persons were less likely to be symptomatic (26.9%),

Health staff without symptoms could ‘contribute to disease spread in and out of hospitals’ so testing them is important for infection control, the researchers wrote.

There was a wide range of symptoms in different regions of the world. For example, nearly half of Americans infected with the disease did not have any symptoms (46.3%), 44.2 % in Europe, and 27.3 % in Asia.

According to the researchers, the rate of asymptomatic infected individuals was higher among more advanced countries and younger people in papers that were published between June and July.

Studying infections in children under 20 years old, 60.2 percentage of patients had symptoms, while almost half of people aged between 20 and 39 did not experience any symptoms.

The team wrote that people without symptoms can infect others, with previous studies showing that the viral load — the amount of virus in a person’s nose and throat swab — among asymptomatic people was similar to those among infected people.

Other studies also show that people without symptoms may fuel an outbreak in their home or other environments.

They wrote: ‘Asymptomatic infections should be considered a source of Covid infections that play an important role in the spread of the virus within community as public life gradually returns to normal.

‘The management of asymptomatic carriers was essential for preventing cluster outbreaks and transmission within a community.’