The U.S. Centers for Disease Control and Prevention on Friday updated its public health guidance about how COVID-19 spreads, acknowledging that the virus is airborne. It's an important change from previous guidance, which claimed the virus was spread by "close contact, not airborne transmission."
The CDC now says viral transmission of SARS-CoV-2 occurs by inhaling "very fine respiratory droplets and aerosol particles," by coming into contact with sprayed droplets, or by touching your face with hands that have been contaminated with "virus-containing respiratory fluids."
"People release respiratory fluids during exhalation (e.g., quiet breathing, speaking, singing, exercise, coughing, sneezing) in the form of droplets across a spectrum of sizes," the CDC guidance states. "These droplets carry virus and transmit infection."
Large droplets, say from coughing or sneezing, settle out of the air and can be deposited on surfaces within seconds or minutes. The smallest droplets, however, can rapidly dry and form aerosol particles that linger in the air for minutes or hours. Breathing in those particles can lead to viral infection, which can occur even if you are more than six feet separated from an infected individual.
The places most at-risk of airborne viral transmission are enclosed spaces with inadequate ventilation or air handling (nursing homes, for example). Exercising, shouting, or singing are activities that increase the exhalation of respiratory fluids that can linger in the air. If an individual is exposed to conditions where these factors are present for a period longer than 15 minutes, the risk that they may inhale infectious particles increases.
This new guidance better informs how COVID-19 spread might be mitigated successfully.
"Current evidence strongly suggests transmission from contaminated surfaces does not contribute substantially to new infections," the CDC states.
"[T]he available evidence continues to demonstrate that existing recommendations to prevent SARS-CoV-2 transmission remain effective. These include physical distancing, community use of well-fitting masks (e.g., barrier face coverings, procedure/surgical masks), adequate ventilation, and avoidance of crowded indoor spaces."
As the pandemic unfolded last year, infectious disease experts warned for months that both the C.D.C. and the World Health Organization were overlooking research that strongly suggested the coronavirus traveled aloft in small, airborne particles. Several scientists on Friday welcomed the agency's scrapping of the term "close contact," which they criticized as vague and said did not necessarily capture the nuances of aerosol transmission.
"C.D.C. has now caught up to the latest scientific evidence, and they've gotten rid of some old problematic terms and thinking about how transmission occurs," said Linsey Marr, an aerosol expert at Virginia Tech.
In addition to social distancing and mask-wearing, the CDC strongly urges people to continue to wash their hands and practice good hygiene.