The Gesundheit II machine collects the breath exhaled from flu sufferers Credit: Donald Milton
Another study from PloS Pathogens this morning makes a surprising discovery about the quantity of influenza virus contained in large droplets versus tiny aerosol particles, and gauges the ability of surgical masks to reduce their spread.
The study is called:
Influenza Virus Aerosols in Human Exhaled Breath: Particle Size, Culturability, and Effect of Surgical Masks
Donald K. Milton, M. Patricia Fabian equal contributor, Benjamin J. Cowling, Michael L. Grantham, James J. McDevitt equal contributor
The relative importance of direct and indirect contact, large droplet spray, and aerosols as modes of influenza transmission is not known but is important in devising effective interventions. Surgical facemasks worn by patients are recommended by the CDC as a means of reducing the spread of influenza in healthcare facilities.
We sought to determine the total number of viral RNA copies present in exhaled breath and cough aerosols, whether the RNA copies in fine particle aerosols represent infectious virus, and whether surgical facemasks reduce the amount of virus shed into aerosols by people infected with seasonal influenza viruses.
We found that total viral copies detected by molecular methods were 8.8 times more numerous in fine (≤5 µm) than in coarse (>5 µm) aerosol particles and that the fine particles from cases with the highest total number of viral RNA copies contained infectious virus.
Surgical masks reduced the overall number of RNA copies by 3.4 fold. These results suggest an important role for aerosols in transmission of influenza virus and that surgical facemasks worn by infected persons are potentially an effective means of limiting the spread of influenza.
The surprise here is that large droplets - long believed the the primary delivery vessel of influenza viruses - actually contained far fewer viral copies than did fine (≤5 µm) aerosol particles.
And surgical masks, when worn by someone who is infected, do a credible job of protecting others from the virus.
From the University of Maryland we get a press release with more.
UMD study provides new clues to how flu virus spreads
Shows that using a surgical mask on flu patients can reduce the release of even the smallest droplets containing infectious virus
People may more likely be exposed to the flu through airborne virus than previously thought, according to new research from the University of Maryland School of Public Health. The study also found that when flu patients wear a surgical mask, the release of virus in even the smallest airborne droplets can be significantly reduced.
"People are generally surprised to learn that scientists don't know for sure how flu spreads," says Donald Milton, M.D., Dr.P.H., who directs the Maryland Institute for Applied Environmental Health and led the study of influenza virus aerosols published in the journal PLOS Pathogens on March 7, 2013.
"Our study provides new evidence that there is nearly nine times more influenza virus present the smallest airborne droplets in the breath exhaled from those infected with flu than in the larger droplets that would be expected to carry more virus," explains Dr. Milton. "This has important implications for how we prevent the spread of flu."
Routes of flu transmission include: 1) direct or indirect (e.g., doorknobs, keyboards) contact with an infected person, 2) contact via large droplet spray from a respiratory fluid (via coughs and sneezes), and 3) inhalation of fine airborne particles, which are generated by the release of smaller, virus-containing droplets via normal breathing and coughing. The relative importance of these modes of influenza transmission has not been well understood, but is critical in devising effective interventions to protect healthcare workers and vulnerable people, such as infants and the elderly.
The Centers for Disease Control recommends that persons with influenza wear surgical masks to prevent transmission to susceptible individuals. Yet, this recommendation has been supported so far by only one study of mask impact on the containment of large droplet spray during influenza infection. Maryland's study is the first to provide data showing that using a surgical mask can reduce the release of even the smallest droplets containing infectious virus. For this reason, health care facilities should put surgical masks on those suspected of having influenza, and individuals with influenza can protect their families by wearing a mask.
Dr. Milton and his research team, including scientists from Harvard and Boston University Schools of Public Health and the University of Hong Kong, collected the exhaled breath from 38 flu patients and tested both the coarse (≥ 5 µm) and fine (< 5 µm) particles for the number of viruses using molecular methods. They found that the fine particles had 8.8 times more virus than the coarse particles (larger but still airborne droplets). They also tested the airborne droplets for "culturable" virus and found that virus was not only abundant in some cases, but infectious. However, there was a big range of how many viruses people put into the air – some were undetectable while others put out over 100,000 every 30 minutes.
The researchers also tested the impact of wearing a surgical mask on the virus shedding into airborne droplets. Wearing a surgical mask significantly decreased the presence of virus in airborne droplets from exhaled breath. There was a 2.8 fold reduction in the amount of virus shed into the smallest droplets, and a 3.4 fold overall reduction in virus shed in both the coarse and fine and airborne particles.
Buried mid-way through this press release is the take-home message from this study:
Maryland's study is the first to provide data showing that using a surgical mask can reduce the release of even the smallest droplets containing infectious virus.
For this reason, health care facilities should put surgical masks on those suspected of having influenza, and individuals with influenza can protect their families by wearing a mask.
The high viral load detected in fine aerosol particles may also help explain how those infected may be able to spread the influenza virus up to 24 hours before overt symptoms (fever, cough, etc.) begin to appear.
The debate over the ability of surgical masks to protect the wearer against airborne viruses remains unresolved (see Influenza Transmission, PPEs & Super Emitters), but this study provides reassurance over their value in reducing the spread of respiratory infections from the wearer to others.