With a pneumonic plague outbreak simmering on the island of Madagascar, the expected return of avian H7N9 in China this winter, unexplained sporadic cases of community-acquired MERS in Saudi Arabia - and concerns over a potentially nasty H3N2 flu season ahead - it may come as a bit of a surprise that our understanding of exactly how respiratory viruses are transmitted is still fairly limited.
We know (or think we know) the basic mechanics.An infected person coughs, sneezes, or simply exhales virus particles which (depending on their size) either float in the air for a variable amount of time or quickly fall out of the air (potentially contaminating surfaces).
But how long different viruses remain viable outside the body, what environmental conditions allow them to persist the longest, and what route (or routes) they take in infecting others are still subject to study and debate.While there are likely no one-size-fits-all answers to these questions, until we get a better handle on how these things work in the real world, our ability to control or prevent infectious transmission in health care environments, or in the community, will be limited.
Some of the many investigations we've looked at in the past include.
J. Virol: H9N2 Virus Isolated From Air Samples In LPMs In Jiangxi, China
mBio: Persistence Of Human Coronavirus 229E on Environmental Surfaces
NIOSH Video: Adventures In Toilet Plume Research
PLoS One: Concentration, Size Distribution, and Infectivity of Airborne Particles Carrying Swine Viruses
Bird Flu’s Airborne `Division'
CID Study: Airborne Norovirus In Healthcare Facilities
As you can see by the list above, respiratory viruses aren't just transmitted when someone sneezes. Flushing toilets in hospitals, de-feathering poultry or living near a live poultry market in China - even sweeping out mouse droppings from an old shed - can lead to human infection with respiratory viruses.
Add in different types of viruses, varying ambient temperatures, humidity and UV exposures, host variables (i.e. superspreaders), and modes of transmission (fomite, large droplet, aerosolized) and you begin to appreciate how complex this field of research really is.All of which brings us to an EID Journal report on an international conference on respiratory virus transmission and control which was held in Hong Kong this past summer. The authors - Benjamin J. Cowling, Tommy Tsan-Yuk Lam, Hui-Ling Yen, Leo L.M. Poon, and Malik Peiris - are a veritable who's who in the infectious disease world.
While this report outlines more questions than provides answers, it is a sobering reminder on the limits of our current understanding of viral transmission. I've only included some excerpts, so follow the link to read it in its entirety.
Evidence-Based Options for Controlling Respiratory Virus Transmission
Benjamin J. Cowling, Tommy Tsan-Yuk Lam, Hui-Ling Yen, Leo L.M. Poon, and Malik Peiris
Given the speed with which viruses transmitted by the respiratory route spread globally, epidemics caused by these viruses pose great threats to global public health. Recent examples include the global outbreak of severe acute respiratory syndrome caused by a coronavirus in 2003–04 (1), the rapid global spread of pandemic influenza A(H1N1) in 2009 (2), and the ongoing outbreaks of Middle Eastern respiratory syndrome (MERS) caused by another coronavirus (3). Human respiratory viruses such as influenza virus, respiratory syncytial virus (RSV), parainfluenza, adenovirus, and rhinovirus cause considerable illness each year (4,5).
Surprisingly, little is known about the mechanisms by which these viruses are transmitted; much of what is believed to be known is based on dogma. Such knowledge gaps include the relative importance of contact, fomite, and airborne (large droplet versus small droplet) spread; how environmental factors affect different modes of transmission; the aerobiology of virus transmission; the role of viral “quasi-species” and fitness landscape in transmission (6); and viral and host determinants of adaptation of animal viruses for transmission in humans (7,8).
In turn, these knowledge gaps compromise the effect and rational use of nonpharmaceutical interventions for infection prevention and control. Such nonpharmaceutical interventions may be the only available options for control of a newly emerged respiratory pathogen such as the severe acute respiratory syndrome coronavirus.
As part of its 130th anniversary celebration, the Li Ka Shing Faculty of Medicine of the University of Hong Kong has provided financial support for several international conferences in its priority research areas, including respiratory virus transmission and control. An international conference on this topic was held June 19–21, 2017, in Hong Kong, attended by >190 participants from 19 countries (http://transmission2017.med.hku.hk/). The Croucher Foundation, an independent private Hong Kong foundation, also provided financial support for this conference.
(Continue . . .)
During the next pandemic (or any epidemic outbreak) - the immediate goal will be to slow the spread of the virus and to limit its impact through the use of NPIs - Nonpharmaceutical Interventions (see Community Pandemic Mitigation's Primary Goal : Flattening The Curve).
The CDC’s Nonpharmaceutical Interventions (NPIs) web page defines NPIs as:
Nonpharmaceutical interventions (NPIs) are actions, apart from getting vaccinated and taking medicine, that people and communities can take to help slow the spread of illnesses like influenza (flu). NPIs are also known as community mitigation strategies.To be the most effective they can be, we need to have a better understanding of the mechanics of viral transmission, and the effectiveness of specific countermeasures.
While some may seem obvious (covering coughs, wash/sanitize hands, staying home when sick, etc.) others - like the specifics of social distancing, when and how long to close schools, and the use of masks in the community are subject to debate.All of which makes this sort of research of more than just academic interest.