Studies Stir New Debate About Influenza Virus Size, Transmission Risk, PPE Use

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By Kelly M. Pyrek

Editor's note: This is part 1 of a two-part series examining viral size, transmission of disease, and implications for respiratory protection worn by healthcare workers.

Clinicians and infection preventionists may need to rethink what they currently know about respiratory protection in light of several recent studies indicating that the influenza virus can be carried in smaller particles than previously thought.  Additionally, there is debate over airborne transmission and what kind of PPE healthcare workers should don in these situations where exposure is imminent. As Lindsley and Blachere, et al. (2010) explain, "Although influenza is known to be transmitted by infectious secretions, these secretions can be transferred from person to person in many different ways, and the relative importance of the different pathways is not known. The likelihood of the airborne transmission of influenza virus by infectious aerosols is particularly unclear, with some investigators concluding that airborne transmission is a key route, while others maintain that it rarely, if ever, occurs. The question of airborne transmission is especially important in healthcare facilities, where influenza patients tend to congregate during influenza season, because it directly impacts the infection control and personal protective measures that should be taken by healthcare workers. During the 2009 H1N1 pandemic, for example, a United States Institute of Medicine (IOM) panel recommended that healthcare workers in close contact with influenza patients wear respirators to avoid infectious aerosols. This recommendation was subsequently adopted by some health authorities such as the Centers for Disease Control and Prevention (CDC), but not by others, such as the World Health Organization (WHO). The IOM panel also noted that many questions about the airborne transmission of influenza are unresolved, and the issue remains controversial."

One recent study published in the Journal of Infectious Diseases suggests that patients with influenza can emit small virus-containing particles into the surrounding air during routine patient care, potentially exposing healthcare providers to influenza. Published in The, the findings raise the possibility that current influenza infection control recommendations may not always be adequate to protect providers from influenza during routine patient care in hospitals.

Werner E. Bischoff, MD, PhD, and colleagues from the Wake Forest School of Medicine in North Carolina screened 94 patients for flu-like symptoms during the 2010-2011 influenza season. Study participants had been admitted to the emergency department (52 patients) or an inpatient care unit (42 patients) of Wake Forest Baptist Medical Center, where vaccination for influenza is mandatory for healthcare providers. Nasopharyngeal swabs were collected from each patient. Samples were analyzed by rapid testing and by PCR analysis. Air samples were obtained by placing three six-stage air samplers from within 1 foot, 3 feet, and 6 feet of patients. No aerosol-generating procedures—such as bronchoscopy, sputum induction, intubation, or cardiopulmonary resuscitation—were conducted while air sampling took place. During air sampling, the number of patients’ coughs and sneezes were counted and assessed for severity. Patients also completed a questionnaire at admission to report symptoms and the number of days they were sick.

Of the 94 patients enrolled, 61 patients (65 percent) tested positive for influenza virus. Twenty-six (43 percent) released influenza virus into the air. Five patients (19 percent) emitted up to 32 times more virus than others. This group of patients with influenza, described by the researchers as “super-emitters,” suggested that some patients may be more likely to transmit influenza than others. High concentration of influenza virus released into the air was associated with high viral loads in nasopharyngeal samples. Patients who emitted more virus also reported greater severity of illness.

The current belief is that influenza virus is spread primarily by large particles traveling up to a maximum of 3 feet to 6 feet from an infected person. Recommended precautions for health providers focus on preventing transmission by large droplets and following special instructions during aerosol-generating procedures. In this study, Bischoff and his team discovered that the majority of influenza virus in the air samples analyzed was found in small particles during non-aerosol-generating activities up to a 6-foot distance from the patient’s head, and that concentrations of virus decreased with distance. The study addressed only the presence of influenza-containing particles near patients during routine care, not the actual transmission of influenza infection to others.

As Bischoff, et al. (2013) explain, "Influenza virus can be transmitted by air. Breathing, talking, coughing, and sneezing release influenza virus into air, with sizes ranging from submicron particles (during breathing) to large droplets (during coughing/sneezing). The Centers for Disease Control and Prevention (CDC), the Institute of Medicine, the European Centre for Disease Control and Control, and the World Health Organization (WHO) have expressed lack of knowledge and the urgent need for research in influenza virus transmission routes. CDC and WHO state that influenza virus transmission primarily occurs by large-particle respiratory droplets traveling within a short distance of the source and that such particles are blocked during encounters between patients and healthcare professionals (HCPs) by face masks worn by HCP." Fit-tested respirators are only required during aerosol-generating procedures such as bronchoscopy. During routine, non-aerosol-generating patient care, the current precautions recommend that providers wear a non-fitted face mask.

The researchers add, "The size of airborne particles determines how influenza virus is transmitted. Large particles (diameter, =20 µm) have limited travel distance, while smaller particles (diameter, <5 µm) stay airborne longer and spread widely. We found that up to 89 percent of influenza virus–carrying particles were <4.7 µm in diameter. Notably, no aerosol-generating procedures were undertaken during air sampling. The predominance of small particles has been reported previously, with influenza virus detected in the exhaled breath of 4 of 12 subjects (33 percent) breathing normally. Although the majority of particles (>87 percent) were <1 µm in diameter, the sizes containing virus were not identified. The effect of coughing was studied in 47 influenza virus–positive patients. Thirty-eight (81 percent) released influenza virus, with 65 percent of RNA contained in particles <4 µm in diameter. The published data and our findings indicate that small particles carry the majority of influenza virus other than virus released during aerosol-generating procedures. We consider it unlikely that, during routine care, influenza virus is transmitted solely by droplet-sized particles."
Based on their findings, Bischoff and investigators are concerned that providers may still be exposed to infectious dosages of influenza virus up to 6 feet from patients with small wide-spreading particles potentially exceeding the current suggested exposure zones. These findings suggest that current infection control recommendations may need to be reevaluated, the study authors say.

Another recent study suggests that 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, MD, DrPH, 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 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 and Prevention (CDC) 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, healthcare facilities should put surgical masks on those suspected of having influenza, and individuals with influenza can protect their families by wearing a mask.

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. As Milton, et al. (2013) note, "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 researchers report that when study volunteers were not wearing surgical masks, they detected virus RNA in coarse particles exhaled by 43 percent and in fine particles exhaled by 92 percent of influenza patients. Milton, et al. (2013) say their findings contrast with a study by Johnson et al. (2009), who detected influenza virus RNA in cough generated large droplet spray from 100 percent of influenza patients over two brief sampling trials, and from 78 percent on each trial. "These discrepant findings are likely due to the very different collection techniques and particle sizes collected in these two studies," the researchers explain. "We used a specially designed aerosol sampler to collect particles from 0.05 to 50 µm in diameter. Johnson et al., by contrast, used simple deposition on petri dishes, and based on particle settling rates and collection times, that method would have been unlikely to collect particles with diameters of less than approximately 50 µm because smaller particles would have remained suspended in air and flowed around the petri dishes. We view results from Johnson et al and the present study as complementary. Together the studies show that surgical masks can limit the emission of large droplet spray and aerosol droplets larger than 5 µm. However, surgical masks are not as efficient at preventing release of very small particles. It is well known that surgical masks are not effective for preventing exposure to fine particles when worn as personal protection. We had hypothesized that when used as source control, exhaled droplets might be large enough prior to evaporation to be effectively captured, primarily through impaction. This appears to be true for virus carried in coarse particles. But the majority of virus in the exhaled aerosol appear to be in the fine fraction that is not well contained. Nevertheless, the overall 3.4 fold reduction in aerosol copy numbers we observed combined with a nearly complete elimination of large droplet spray demonstrated by Johnson et al. suggests that surgical masks worn by infected persons could have a clinically significant impact on transmission. For example if one hypothesized that all transmission were due to aerosol particles <50 µm, and estimated a reproductive number of 1.5 for influenza (i.e. each infection generates 1.5 new infections on average at the start of the epidemic) [19], then the use of surgical masks by every infected case could reduce the reproductive number below 1. Compliance, however, would be a major limitation resulting in lower efficacy in real-world practice."
Milton, et al. (2013) add, "While it is generally assumed that large droplets shed from the respiratory tract contain infectious virus, there are limited data that indicate that fine particle aerosols released from the human respiratory tract contain infectious virus. In one previous study by Lindsley et al. (2010), infectious virus was detected in 2 of 21 cough aerosol samples, once with a sampler that did not discriminate between coarse and fine particles and once in the coarse particle fraction of a second instrument. This observation, along with our observation that it was possible to recover culturable virus from the fine-particle fraction using our device demonstrates that humans generate infectious influenza aerosols in both coarse and fine particle fractions. This lends support to the hypothesis that aerosols may be a common pathway for influenza transmission among humans. However, a clear test of the hypothesis requires intervention studies that can interrupt only one mode of transmission without interfering with others."

In that aforementioned study, Lindsley and Blachere, et al. (2010) measured the amount and size of aerosol particles containing influenza virus that were produced by coughing. Subjects were recruited from patients presenting at a student health clinic with influenza-like symptoms. Nasopharyngeal swabs were collected from the volunteers and they were asked to cough three times into a spirometer. After each cough, the cough-generated aerosol was collected using a NIOSH two-stage bioaerosol cyclone sampler or an SKC BioSampler. The amount of influenza viral RNA contained in the samplers was analyzed using quantitative real-time reverse-transcription PCR (qPCR) targeting the matrix gene M1. For half of the subjects, viral plaque assays were performed on the nasopharyngeal swabs and cough aerosol samples to determine if viable virus was present. Fifty-eight subjects were tested, of whom 47 were positive for influenza virus by qPCR. Influenza viral RNA was detected in coughs from 38 of these subjects (81 percent). Thirty-five percent of the influenza RNA was contained in particles >4 µm in aerodynamic diameter, while 23 percent was in particles 1 to 4 µm and 42 percent in particles <1 µm. Viable influenza virus was detected in the cough aerosols from 2 of 21 subjects with influenza.

These results show that coughing by influenza patients emits aerosol particles containing influenza virus and that much of the viral RNA is contained within particles in the respirable size range. Lindsley and Blachere, et al. (2010) say their results support the idea that the airborne route may be a pathway for influenza transmission, especially in the immediate vicinity of an influenza patient. They say that additional research is needed on the viability of airborne influenza viruses and the risk of transmission.

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