It is the first longitudinal, direct measure of the number of SARS-CoV-2 viral copies exhaled per minute over the course of the infection — from the first sign of symptoms until 20 days after.

On day eight, exhaled levels of virus drop steeply, down to near the limit of detection — an average of two copies exhaled per minute.

Northwestern investigators tested breath samples — collected multiple times daily from 44 individuals — over the entire course of infection to determine when a person is most infectious.

The study will be published in eLife and has been posted as a pre-print.

Mild and moderately symptomatic patients with COVID still exhale large amounts of virus, though severely symptomatic cases exhale higher levels on average, the study reports.

Vaccinated and unvaccinated patients exhale similar levels of virus over the course of infection, the research shows.

The amount of virus being exhaled while infected was the same no matter which variant a person was infected with — people infected with Alpha exhaled just as much as those infected with Omicron, the study reports.

“An important question in understanding transmission of SARS-CoV-2 is how much virus a patient is exhaling into the environment over the course of their infection and for how long,” said senior author Christina Zelano, assistant professor of neurology at Northwestern University Feinberg School of Medicine. “During COVID-19 infection, at what point are you exhaling a lot of virus, and when do you stop breathing it out?”