J.I.D.: Asymptomatic Summertime Shedding Of Respiratory Viruses

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Like a lot of people during flu season, when I'm in public and someone sneezes or coughs, I try to hold my breath and walk away in hopes of avoiding the invisible torrent of virus laden particles spreading through the air.

While it makes sense to avoid close contact (if you can) with people who exhibit obvious signs of active colds or ILI (Influenza-like Illnesses) - colds, flus, and other respiratory viruses aren't so easily avoided.

The problem is, many hosts can carry - and shed - respiratory viruses asymptomatically.  You can be exposed, yet never see it coming. 

This isn't a new revelation, but as more studies are done, our understanding of how common asymptomatic shedding of respiratory viruses really is continues to grow.  

• In 2011, in EID Journal: Pre-Symptomatic Influenza Transmission, we saw evidence of presymptomatic spread of the H1N1 virus in three clusters in Japan, which also suggests that asymptomatic carriers ought to be able to spread the virus as well.
• In 2012, in The Very Common Cold, we looked at a study that tested college students over an 8 week period and found asymptomatic infections led symptomatic infections by a factor of 4 to 1. Researchers estimated that as many as 60.5% of the asymptomatic student population was infected at some time with rhinovirus over an eight week study period
• Also in 2010, in PLoS One: Influenza Viral Shedding & Asymptomatic Infections, we saw a small study that found 21% of adult secondary influenza cases were asymptomatic or subclinical, yet they shed roughly the same quantity of virus as those showing clinical signs of illness. 
• In 2014 the Lancet: Community Burden & Severity Of Seasonal And Pandemic Influenza found over five years that as many as 75% of those who showed serological evidence of infection reported no significant influenza symptoms.

All of this makes some pandemic or epidemic interventions - like fever screening at airports, school classrooms, or among hospital staff - far less effective than most people would hope or expect.

Today we have the abstract from a new study, published this week in the Journal of Infectious Diseases, that tested asymptomatic tourists to NYC between April and July of 2016 for respiratory virus shedding.  
  
The 2015-2016 flu season you may recall was one of the mildest we've seen in years (see modified FluView ILI Chart below), and had ended before this surveillance project began.

While this helps explain the low incidence (7.2%) of positive samples, depending upon the criteria used for being `asymptomatic', between 57.7-93.3% of those positive cases qualified as being without symptoms.

Asymptomatic summertime shedding of respiratory viruses

Jeffrey Shaman, PhD Haruka Morita, MPH Ruthie Birger, PhD Mary Boyle, MD Devon Comito, MS Benjamin Lane, BS Chanel Ligon, BA Hannah Smith, BA Rob Desalle, PhD Paul Planet, MD

The Journal of Infectious Diseases, jix685, https://doi.org/10.1093/infdis/jix685
Published: 02 January 2018

Abstract

To determine rates of both symptomatic and asymptomatic infection among ambulatory adults, we collected nasopharyngeal swabs, demographics, and survey information from 1477 adult visitors to a New York City tourist attraction during April-July 2016.

Multiplex PCR was used to identify specimens positive for common respiratory viruses. 7.2% of samples tested positive;  71.0% of positive samples detected rhinovirus and 21.5% detected coronavirus.

Influenza, respiratory syncytial virus, and parainfluenza were also detected. Depending on symptomatologic definition, 57.7-93.3% of positive samples were asymptomatic. These findings indicate that significant levels of asymptomatic respiratory viral shedding exist during summer among the ambulatory adult population.

Still a matter of some debate is just how efficiently asymptomatic carriers can transmit respiratory viruses to others. It seems likely they are less of a threat than symptomatic carriers, but how much less is unknown.

And it isn't just asymptomatic humans we need to be aware of, other animals - like pigs - can carry flu viruses without showing symptoms (see EID Journal: Flu In Healthy-Looking Pigs).

This is a topic we also explored in 2012's Asymptomatic Pigs: Revisited, where we looked at a pair of studies from Ohio State University - one that found a surprisingly large percentage of flu infected swine to be asymptomatic - and another that established just how closely linked the human and swine variant strains of influenza that summer really were.

Presumably other healthy looking hosts - like asymptomatic camels carrying  MERS-CoV - are also capable of spreading their viruses.  But actually proving that in the field is extraordinarily difficult.

If carriers of respiratory viruses like influenza, rhinoviruses, coronaviruses, etc. were always easy to spot and avoid, these viruses would probably not have become the ubiquitous and perennial threats they have.

Their ability to spread (often stealthily) puts us at a tremendous disadvantage, and helps explain why - when it comes to a pandemic - influenza is the threat we worry about most.

And today's study should remind us that practicing good flu hygiene - washing hands, covering coughs, staying home when sick , etc. - is something we should be doing year round - not just during flu season.