Half of all Covid cases are spread by people WITHOUT symptoms, experts warn

AT least half of all people with Covid caught the disease from somebody who wasn’t showing symptoms, a study has warned.

The silent spread of the coronavirus is one reason it is so difficult to control. 

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The research was led by the US Centers for Disease Control and Prevention in Atlanta.

People infected with the coronavirus may show no symptoms, and are called asymptomatic.

Presymptomatic people are those who are contagious to others in the days before their symptoms start, which is anything between two and 14 days.

Using modelling, the scientists estimated that asymptomatic people account for 24 per cent of all coronavirus transmission.

A further 35 per cent of transmission is the result of presymptomatic Covid carriers.

Together, it suggests that 59 per cent of all new cases – or six in ten – originate from exposure to a “silent” carrier. 

It could be even higher – as high as 67 per cent, the team found.

A range of different scenarios in which the “peak” day of infectiousness in the course of a person’s disease were modelled. 

For example, if a presymptomatic person was most contagious on the second day of infection, but did not get symptoms until day five, that gives them several days to unknowingly spread the virus around. 

Ovall the findings indicate “at least half” of all people get Covid get it from someone who is not showing signs of the virus. 

The rest are the result of people coming into close contact with someone who does have symptoms, which can include a cough, high temperature and loss of taste and smell.


Scientific research has estimated between 20 and 80 per cent of all Covid cases show no obvious signs.

More accurate guesses – based on a wealth of research as the pandemic has worn on – settle on a figure of around one in three people.

They are almost as contagious as the people who do have symptoms, at around 75 per cent.


The research team, led by Dr Jay Butler, said their findings stress the importance of sticking to Covid guidelines.

They wrote in their paper, published in the journal JAMA: “These findings suggest that measures such as wearing masks, hand hygiene, social distancing, and strategic testing of people who are not ill will be foundational to slowing the spread of Covid-19 until safe and effective vaccines are available and widely used.”

Contact tracing is also important for containing a virus that has high silent spread.

And “strategic testing” of at-risk people, such as healthcare workers, can help to find asymptomatic cases.

However, the contact tracing system in the UK has not been viewed as a success. After launching in May, it repeatedly showed dire performance statistics. 

Testing people quickly, reaching out to close contacts of cases and isolating them are crucial to keep ahead of the virus.

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