In a new study, researchers found that during the initial wave of the COVID-19 outbreak in New York City, only 1 in 5 to 1 in 7 cases of the virus was symptomatic.
But the non-symptomatic cases substantially contribute to community transmission, making up at least 50% of the driving force of SARS-CoV-2 infection.
The research was conducted by a team at the University of Chicago.
When the COVID-19 epidemic arrived in the U.S., the team noticed that it was very difficult to estimate what proportion of people infected with SARS-CoV-2 would go on to develop symptoms, partially due to the initial challenges with testing capacity.
There are a number of existing models that use epidemiological data to estimate undetected case numbers and transmission rates.
But this study is the first model to incorporate data about daily testing capacity and changes in testing rates over time to provide a more accurate picture of what proportion of SARS-CoV-2 infections are symptomatic in a large U.S. city.
The team showed that the proportion of individuals who are symptomatic for COVID-19 is somewhere between 13% and 18%.
And regardless of uncertainty in all other parameters, they can tell that more than 50% of the transmission happening in the community is from people without symptoms — those who are asymptomatic and pre-symptomatic.
The model provides additional support for the importance of following public health guidelines to reduce community transmission of the virus, whether individuals show symptoms.
The team says even if asymptomatic people aren’t transmitting the virus at high rates, they constitute something like 80% of all infections.
This proportion is quite surprising. It’s crucial that everyone — including individuals who don’t show symptoms — adhere to public health guidelines, such as mask-wearing and social distancing, and that mass testing is made easily accessible to all.
One author of the study is Rahul Subramanian, a Ph.D. student of epidemiology at UChicago.
The study is published in the Proceedings of the National Academy of Sciences.