How fast you can catch COVID-19 through the air
The winter season virus has struck – and COVID-19 is still part of everyday life. But unlike during the pandemic, we now know more about how the virus is spread through the air we breathe. Research results from Lund University show that it only takes a few minutes in the same room as an infected person to catch the virus.
Jessika Sellergren – Publicerad den 28 December 2024
The aerosol researchers at LTH are behind the study, in which the infectiousness of the virus has been mapped in some forty people with confirmed covid infection.
– Aerosols are small airborne particles, and exhaled aerosols from a single person can transmit covid infection to others within a few minutes under normal indoor conditions, says Malin Alsved, lead researcher for the study, whose results have been published in Scientific Reports.
There has previously been uncertainty about whether the amount of virus in the air is sufficient to cause infection.
– Our study shows that what you breathe in can cause infection in a very short time, and you don't need to get a shower of a sneeze straight at you, a meeting in the same room is enough, says Malin Alsved.
Minutes are enough to become infected
Based on measurements of exhaled air from people infected with COVID-19, as well as a calculation method for the spread and inhalation of aerosol particles in indoor air, Malin Alsved and her research colleagues have developed a model to calculate how long it takes for a person to become infected via the air.
– In a room with normal ventilation, a person with no immunity can inhale an infectious dose within minutes of the infectious person entering. The variation depends on how much virus is exhaled and the ventilation in the room, says Malin Alsved.
The time it takes to become infected is shorter if the sick person has already been in the room for a while so that an even concentration of virus has been established in the air.
– In small and medium-sized rooms, the virus spreads quickly in the air and then it does not help to keep your distance to reduce the risk of becoming infected yourself, says Malin Alsved.
The amount of virus is greatest at the beginning of the infection
During the first few days of the infection, just when the symptoms begin, the amount of virus in the exhaled air is highest.
– On the first and second days, we measured viruses in the exhaled air of 70 percent of people, on the third day in half and on the fourth day in a third, says Malin Alsved.
The breath samples from the 40 participants who had measurable virus aerosols were analysed at the Department of Clinical Microbiology at Sahlgrenska Academy in Gothenburg. There, the viral aerosols were placed on cells to find out if they were culturable - that is, infectious.
– This is the first time that it has been possible to calculate the infectivity of an aerosol sample that is linked to a specific source, in this case people's exhaled air, says Malin Alsved.
Most virus in the smallest aerosol particles
To further understand the tendency of viruses to spread in air, the researchers investigated which particle sizes contain the most viruses. The results show that viruses are present in all particles from half a micrometre to ten micrometres, but the main amount is found in the smaller particles in the size range between half a micrometre and three micrometres.
– Aerosol particles around one micrometre do not fall to the ground but follow air currents, and there is a high risk that they will be inhaled, says Malin Alsved.
Investigating other infectious diseases
Since the beginning of the pandemic, there have been discussions about how COVID-19 infections spread. Jakob Löndahl, professor in aerosol technology and a colleague of Malin Alsved, was one of those who got involved in the ‘new virus’ early on. He paused his research on winter-vomiting disease to investigate whether the infection from the then unknown virus was airborne.
Much new knowledge about airborne transmission has emerged since the pandemic, and researchers now want to see if it can be applied to infectious diseases other than COVID-19.
– The work on COVID-19 has raised many new questions. Do viral aerosols from other respiratory infections such as rhinoviruses, influenza and RSV spread in the same way as viruses from COVID-19? When is the risk of infection highest? How effective are our various protective measures? There are many questions we are working on now, says Jakob Löndahl.
The article Infectivity of exhaled SARS-CoV-2 aerosols is sufficient to transmit covid-19 within minutes in Scientific Reports was published in December 2023 but the results have not been publicly communicated before.
Malin Alsved
Malin Alsved is a researcher in Aerosol technology at LTH, Lund University.
Jakob Löndahl
Jakob Löndahl is professor in Aerosol technology at LTH, Lund University.
The virus aerosols were collected in a mobile lab
The virus aerosols were collected during the pandemic in a mobile laboratory where COVID-19 infected people and their household contacts breathed, talked and sang into a funnel. The exhaled air was captured with an instrument that converted aerosol particles into droplets in a petri dish. Most viruses were found during speaking and singing, with songs such as the MFF anthem, Twinkle Twinkle Little Star, Queen and Carola. Singing produced more virus in the air than breathing.
– When singing, the vocal cords vibrate more than when speaking, and it is likely that the aerosols come from the vibrations of the vocal cords, says Malin Alsved.
Reading tips
Want to know more about the aerosol researchers' work on airborne transmission from the pandemic onwards? Here are some reading tips: