How much virus does a person with COVID exhale? New research has answers

People infected with the highly transmissible Alpha, Delta and Omicron variants of SARS-CoV-2 spew out higher amounts of virus than those infected with other variants, according to a new study¹. Further, individuals who contract COVID-19 after vaccination, and even after a booster dose, still shed virus into the air.

The work was posted on the medRxiv preprint server on 29 July. It has not yet been peer reviewed.

“This research showed that all three of those variants that have won the infection race … come out of the body more efficiently when people talk or shout than the earliest strains of the coronavirus,” says John Volckens, a public-health engineer at Colorado State University in Fort Collins.

Study co-author Kristen Coleman, who researches emerging infectious diseases at the University of Maryland in College Park, says this means that people should be “pushing governments to invest in improving indoor air quality by improving ventilation and filtration systems”.

Breathe out

For the study, Coleman and her colleagues recruited 93 people between mid-2020 and early 2022 who were infected with SARS-CoV-2. Participants’ infections were caused by strains including the Alpha variant, which emerged in late 2020, and the later Delta and Omicron variants. All participants with the latter two strains had been fully vaccinated before catching the virus.

The infected people faced into a cone-shaped apparatus and sang and shouted — with inevitable coughs and sneezes in between — for 30 minutes, while an attached machine collected the particles they exhaled. The device, called a Gesundheit-II, separated out the fine ‘aerosol’ droplets measuring 5 micrometres or less in diameter, which can linger in the air and leak through cloth and surgical masks.

The team found that participants infected with the Alpha, Delta and Omicron variants emitted significantly more viral RNA when exhaling than did people infected with other variants. These include ancestral variants, such as the one first detected in Wuhan, China, and those not associated with increased transmissibility — such as Gamma, which arose in late 2020. For participants with Delta and Omicron, their fine aerosol contained on average five times the amount of virus that was detected in their larger, coarse aerosol.

The team also seeded cells in the laboratory with aerosol samples and found that four samples, each from a participant with either Delta or Omicron, infected the cells. Shed virus is not always infectious, says study co-author Jianyu Lai, an epidemiologist at the University of Maryland, and the samples’ ability to infect laboratory cells means that viral RNA in exhaled aerosols can spread the disease.

Malin Alsved, an aerosol technology scientist at Lund University in Sweden, says: “I’m bit concerned that they mix all the respiratory [aerosols] — they have breathing, talking, speaking, screaming, coughing, even sneezing in the sample.” Coleman responds that the team combined respiratory samples to mimic a real-life scenario such as being in a restaurant.

Going viral

The study also highlights the variation between individuals in the amounts of exhaled virus, which ranged from non-detectable levels to those associated with ‘superspreaders’. One Omicron-infected participant, for example, shed 1,000 times as much viral RNA through fine aerosol as the maximum level observed in those with Alpha or Delta. The researchers say that the root of these discrepancies remains a mystery but could be related to biological factors such as a person’s age. Behaviour might play a part, too: the study’s superspreader coughed more frequently than others.

If new variants are more prone to superspreading, that might drive them to dominate COVID-19 cases. The team notes that people infected with SARS-CoV-2 exhale much lower amounts of viral RNA than people infected with influenza, a comparable airborne disease. This suggests that SARS-CoV-2 could spin off variants that transmit even more virus.

“That is something to be concerned about,” says Alsved.