What determines how many infections are transmitted?
The number of people infected by someone with COVID-19 depends on:
- The viral shedding rate. This is a measure of how much virus an infected person releases. It is affected by the viral load, usually measured as the quantity of virus in a nose or throat swab, and may also be influenced by specific disease symptoms. For instance, coughing expels virus particles and someone who coughs a lot increases the chance of onward transmission.
- The duration of viral shedding. This is the length of time over which an infected person sheds virus and so is infectious to others.
- The contact rate. This is the number of contacts (per day or other time unit) an infected carrier has with others in the community. Unaware that they are infected, pre-symptomatic or asymptomatic carriers might shed virus while maintaining contact with others in their communities, thereby contributing to transmission. How much they contribute to the spread of infection will influence control strategies for COVID-19.
What is the role of pre-symptomatic carriers?
Studies of patients with COVID-19 indicate that viable virus can be detected in swabs up to six days before to the onset of symptoms. Estimates of the percentage of all infections transmitted by an individual prior to the appearance of symptoms will vary according to how quickly they are isolated. Some studies suggest a substantial contribution of pre-symptomatic carriers to transmission: 73% in Wuhan and 62% in Tianjin, China; and 48% in Singapore.
One modelling study calculated that, in the early stages of the epidemic in China, each pre-symptomatic carrier infected 0.9 other people on average, contributing 47% of all transmitted infections. The value of 0.9 secondary infections is close to the threshold of 1, above which pre-symptomatic carriers could sustain an outbreak of COVID-19 on their own. By contrast, during the SARS coronavirus outbreak in 2003, pre-symptomatic carriers contributed almost no secondary infections, indicating that COVID-19 needs a different containment strategy from SARS.
What is the role of asymptomatic carriers?
The role of asymptomatic carriers is difficult to assess because they are usually not tested at all during the course of infection, and therefore not counted. And they are not easily identified as a source of infection to others, even of close contacts who develop COVID-19.
It is, however, possible to assess the percentage of people who never develop symptoms from studies where everyone is tested during a COVID-19 outbreak, irrespective of who became ill and who they contacted. One such study found that 18% of infected passengers (who were mostly over the age of 60) on the Diamond Princess Cruise Ship did not develop symptoms. Another investigation in a nursing facility in Washington State, United States, found that among 48 infected residents, 24 were pre-symptomatic at the time of testing, but only three remained asymptomatic throughout the course of infection.
So far, few studies have attempted to investigate whether asymptomatic carriers are actually infectious to other people, and they have produced divergent results. One study in the town of Vo, Italy, found little difference in the viral load of symptomatic and asymptomatic carriers. Another study in Nanchang, China, found that mild cases of COVID-19 had a lower viral load than severe cases and shed virus for a shorter period of time, which raises the question of whether asymptomatic carriers also shed less virus. The modelling study which calculated that pre-symptomatic carriers in China generated 0.9 secondary infections also found that asymptomatic carriers generated only 0.1 secondary infections, contributing just 6% of all transmission events.
Although uncertainties remain, it is clear that infected people without symptoms can transmit coronavirus. Pre-symptomatic carriers might contribute more than asymptomatic carriers, but this needs further investigation. Nevertheless, in controlling the spread of COVID-19, there are benefits to physical and social distancing irrespective of the presence of symptoms.