Last week the CDC changed its guidance about the transmission of SARS-CoV-2, recognizing airborne spread as a key mode of infection.
The new guidance acknowledges that inhalation of aerosols — which are tiny, lightweight viral particles that can float and linger in the air for extended periods of time — is one way COVID-19 spreads. Even when an infectious person is more than 6 feet away, aerosols have the ability to travel and infect others.
Following a similar change made recently by the World Health Organization (WHO), the CDC guidance acknowledges three primary routes of SARS-CoV-2 transmission: inhalation into the lungs, deposition of droplets onto the face, or touching infected surfaces, also known as fomites.
Experts have welcomed this modification, as many have been calling for such an acknowledgement for months.
“One of the most important aspects of the recent CDC updates is that they finally remove the ambiguous, ‘close-contact’ terminology, which just confused the whole discussion,” said Alex Huffman, PhD, an expert in aerosols and bioaerosols at the University of Denver. As “close-contact” refers to a distance between people — and not a process of transmission — Huffman told MedPage Today that the previous guidance was unclear, because it did not distinguish that inhalation can occur at both short and long distances.
While surface transmission has been deemed less important, it’s still difficult to discern just how much of spread is truly driven by long-distance (aerosol) versus closer-contact (droplet) transmission. The important thing, experts told MedPage Today, is that public health measures adapt to meet the data about how the virus spreads, ensuring interventions are effective.
Evidence for Airborne Spread
People infected with COVID-19 emit viral particles during respiratory activities such as breathing, talking, yelling, coughing, sneezing, and singing. But each of these exhalations can produce different amounts and sizes of respiratory particles, based on how loudly someone speaks, the number of deep breaths they take, and the duration of the activity.
Larger, heavier respiratory droplets can travel up to 6 feet, but then they fall to the ground. Smaller, aerosolized particles, however, can get caught in air flows and move throughout a space, infecting people at further distances.
“It’s really the size of the droplet nuclei that defines an aerosol-borne infection versus a droplet-borne infection,” said Monica Gandhi, MD, MPH, an infectious disease expert at the University of California San Francisco. She added that in recognizing the airborne nature of COVID-19, we acknowledge that “it’s most effectively transmitted in closed, indoor spaces, and that ventilation is one of the most important things to mitigate the virus.”
When an infected individual emits aerosols, they are more likely to spread disease in poorly ventilated, indoor spaces. Outdoors, however, the virus can disperse more easily. Several epidemiological studies have demonstrated the potential for COVID-19 aerosols to infect others indoors.
When 61 singers attended a choir practice in Washington state at the start of the pandemic, a single person was infected with COVID-19. But the 2.5-hour practice resulted in an outbreak of 52 confirmed or strongly suspected cases of COVID-19, as well as two deaths. Researchers have hypothesized that loud singing for a long time may have produced a high, continuous stream of aerosols and caused the vast outbreak.
Researchers also identified potential airborne spread in a restaurant in China, among three families who sat at neighboring tables. They observed no close contact or surface transmission (except for a few family members sitting back-to-back), yet 10 people who tested positive were traced back to one original infectious person. Additional analyses found that in a room with low ventilation, air conditioner flow may have circulated infectious particles to adjacent tables.
Another restaurant study, this one from South Korea, showed a teen girl was infected by a fellow diner who was about 20 feet away — and their visits to the restaurant only overlapped by 5 minutes, strongly indicating aerosol transmission.
Aerosol transmission was also suspected at a squash court in Slovenia, resulting from possible floating viral particles in a hallway. Scientists calculated there was at least 10 minutes between the time one infected squash player left the athletic facility and the other players arrived — yet a handful of people likely caught COVID-19 from that infected player, whom they never even came in contact with.
Scientists saw evidence of the airborne spread of coronaviruses as far back as the first SARS outbreak in the early 2000s. In a study published by the New England Journal of Medicine in 2004, residents that lived on higher floors of an apartment building exposed to the SARS virus were at a greater risk of infection, as researchers hypothesized the airborne particles traveled up with rising warm air.
Proving just how many COVID-19 cases are attributed to aerosol transmission versus other modes is not clear, Huffman said. He stated that it is hard to unambiguously separate airborne spread and droplet spread.
Larger Respiratory Droplets
Since the start of the pandemic, the CDC and WHO have asserted that the primary route of transmission for SARS-CoV-2 is through larger respiratory droplets, which can travel short ranges between 3 to 6 feet before falling to the ground.
The CDC still states that respiratory fluids overall are the main mode of transmission, but now distinguishes between inhaling small aerosols or deposition of virus in exhaled “splashes or sprays” onto mucous membranes, like the mouth or nose. There is limited evidence about the significance of droplets versus aerosols — but it is known that larger particles can infect people at close distances.
“COVID is generally transmitted through bigger particles, like tiny little balls of spit, if you will, that people create when they speak or cough or sneeze,” said Adrian Popp, MD, an infectious disease physician at Northwell Health’s Huntington Hospital in New York. “Those droplets tend to fall on the floor or disappear from the surroundings in about 15 or 20 minutes or so.”
Ravina Kullar, PharmD, MPH, speaking on behalf of the Infectious Disease Society of America, said there is no actual number she is aware of describing the frequency of SARS-CoV-2 spread via droplets versus aerosols. But she said that droplet transmission occurs more often than aerosol transmission, and aerosol transmission occurs more often than fomite spread.
Low Risk of Spread Via Fomites
The CDC has determined that SARS-CoV-2 particles can, in fact, be transmitted on surfaces. While the risk of transmission is possible, however, the agency emphasizes that the risk is low compared to droplet or airborne transmission.
Findings from mathematical models that estimate infectiousness of fomites show that each contact with a contaminated surface has a 1 in 10,000 chance of infection. And the level of infectiousness rapidly decreases over time, with surface studies showing a 99% reduction in infectiousness is likely in indoor environments after 3 days, the CDC states.
Lisa Brosseau, ScD, a research consultant with the Center for Infectious Disease Research and Policy at the University of Minnesota, said that fomite spread is most relevant for individuals who are in close, hands-on contact with COVID-19 patients. In healthcare settings, for example, providers may touch surfaces that were directly sneezed or coughed on by infectious individuals, and then touch their own eyes, nose, or mouth. In these cases, hand hygiene (as well as appropriate PPE) is vital to mitigate risk of infection.
Infection Control Implications
The CDC’s acknowledgement of aerosol transmission may impact how we think about public health measures, Gandhi said. The definitions of aerosol versus droplet spread is not always clear-cut, and transmission is likely a mix.
“Unlike something that is black and white, the definition of aerosolization and droplets likely needs a major overhaul,” she said. Then, public health experts can provide tailored precaution measures based on the multiple ways a pathogen might spread.
“It’s the three mitigation strategies — distance, ventilation, and masking — that I think we have to put together in an individualized way for each pathogen,” Gandhi said.
Gandhi said wearing a tight-fitting mask in poorly ventilated spaces, as well as spending time outdoors — where ventilation is about as good as it gets — have proven to be critical infection prevention approaches.
“Ventilation is emerging as one of the most important strategies to mitigate the pandemic,” she said.