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How many COVID-19-infected people are asymptomatic?

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One of the priorities in characterising the COVID-19 pandemic is estimating how many people are asymptomatic transmitters of the virus. These covert infections are different to unreported cases due to inefficiencies in the local authorities. Instead, they refer to people who are incubating and potentially spreading the virus without knowing it.

Asymptomatic rate estimates

Some of the first estimates suggest that this undetected pool of virus carriers can make up as much as half of all infections. A preprint published on MedRxiv provided a comprehensive assessment of the epidemiological characteristics of COVID-19 patients in Wuhan. It is estimated that at least 59% of the infected cases were out there in the community and potentially infecting others, without showing symptoms.

Another study published on the International Journal of Infectious Disease calculated the asymptomatic rate in Japanese nationals that were evacuated from China. It showed that 4 out of 13 evacuees (31%) never developed symptoms.

On the Diamond Princess Cruise ship, passengers were repeatedly tested and monitored after a COVID-19 outbreak occurred. A lower asymptomatic estimate of 17.9%  was reported. However, this population was made up of a disproportionally large number of elderly people, who tend to develop more severe symptoms. The leading author of the report pointed out that the actual rate of asymptomatic infections in a general population may be much higher.

Notably, a Chinese study involving 2143 paediatric patients with COVID-19 (median age of 7 years old), 4.4%, 50.9% and 38.8% were asymptomatic, mild or moderate, respectively. This means that more than half of the infected children may have been easily missed and contribute to spreading the virus. 

In a small town of Vò, in northern Italy, a pilot study involving all its 3300 residents was conducted since March 6. All residents were tested for COVID-19, regardless of whether they showed symptoms or not. The researchers found that at the time of the first diagnosed case, about 3% of the population had already been infected. However, most of them were not showing any symptoms at all.

Considering all preliminary findings, the asymptomatic rate ranges from at least 18% to 59%, likely averaging to 40% or higher, as some of the estimates were on the conservative side.

What would happen if asymptomatic people can be identified?

First, can people with mild or no symptoms infect others? A German study and a Chinese study both detected high viral loads on early days during the disease course in people with mild COVID-19 symptoms. The most alarming finding is that an asymptomatic patient had similar levels of viral load compared to symptomatic patients. These results suggest that it is likely COVID-19 can spread via a process known as viral shedding by people with mild or no symptoms. Therefore, identifying asymptomatic patients could be critical in controlling onward transmission.

In fact, the Italian town Vò successfully prevented further outbreaks by testing all of its inhabitants. At least six asymptomatic people were identified and isolated when everyone was tested on March 6. As of March 19, there had been no new cases for days. Therefore, mass testing appeared to be effective in controlling the pandemic in small towns like Vò. Unfortunately, the model is unlikely to be repeated in larger cities due to economic and logistic restraints.

Bottom line

Although researchers are still learning about the dynamics of how COVID-19 spreads, preliminary data confirms the nightmare that they feared the most: that a large proportion of the infected people present mild or no symptoms at all. These people seem to be as contagious and could be a core reason of the rapid spread of COVID-19. 

If these preliminary conclusions depict an accurate picture of the COVID-19 spread, there is an urgent need to ramp up preventative measures in public spaces and allow more testings. Recently, Australia has modified the criteria for COVID-19 testing to include those who develop symptoms even without close contact with confirmed cases. However, this is most likely still insufficient given the high proportion of asymptomatic cases.