Avian Flu Diary: mSphere: Environmental Persistence of Influenza Viruses Is Dependent upon Virus Type and Host Origin

Photo Credit PHIL

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One of the potential Influenza NPIs (Non-Pharmaceutical Interventions) we've discussed in the past is the notion that office buildings, hospitals, schools, and even homes might be able to reduce the spread of pandemic flu by adjusting some of their environmental factors; specifically temperature and/or Relative Humidity (RH).

Over the past decade we've looked at research indicating that influenza viruses survive longer in the environment when temperature and humidity fall within certain ranges.

In 2008 researchers Jeffrey Shaman and Melvin Kohn established a correlation between the AH (Absolute Humidity) and the survival, and transmission of the influenza virus (see It's Not So Much The Heat, It's The Humidity).

A 2012 study (see Influenza Virus Survival At Opposite Ends Of The Humidity Spectrum) found both extremely low and extremely high humidity were conducive to flu transmission – at least when it resides in mucus and respiratory fluids like those found in your nose, throat, or lungs.

In 2013, in NIH Study: Climate & Influenza Transmission, we looked at a PLoS One study called High Humidity Leads to Loss of Infectious Influenza Virus from Simulated Coughs that concluded that `. . . maintaining indoor relative humidity at levels greater than 40% can significantly reduce the infectious capacity of aerosolized flu virus.'

In 2018's bioRxIv: Humidity As A Non-pharmaceutical Intervention For Influenza A, researchers reported finding: An increase in average AH from 6.33 mb in control rooms to 9.89 mb in humidified rooms (RH ~42-45 %) was associated with a significant decrease in influenza A virus presence in fomite and air samples in humidified rooms compared to control rooms.

None of this is exactly a new idea, as the Chinese have boiled vinegar for centuries in their homes to `disinfect the air' during epidemics (including SARS). While it is doubtful that vinegar adds and beneficial effect, it is 95% water, and boiling it undoubtedly raises the humidity inside their homes.

In contrast, hospitals are normally kept cool and dry in order to curb the growth of bacteria, but by doing so may be unintentionally providing an environment conducive to the spread of respiratory viruses like influenza, SARS & MERS.

How practical or effective changing indoor humidity would be in reducing influenza transmission is a big unknown. It also appears that there may be a `sweet spot' in the RH range, above or below which influenza viruses survive longer.

And it is far from clear that all flu viruses would react the same way to specific RH levels. What might suppress one influenza virus subtype might allow another to thrive.

All of which brings us to a lengthy and detailed study, published this week in the open-access journal mSphere, which finds considerable variation in RH tolerance among the 6 flu strains they studied, and between droplet and aerosolized particles.

The bottom line is, while RH appears to be a factor in the stability of seasonal IV and LPAI viruses, its impact varies between strains and host origins. And in general, aerosolized viruses at mid-range RH levels decayed slower than droplets.

While none of this negates the notion of managing indoor humidity levels during a pandemic, it does suggest that other concurrent measures - like increased air exchange rates, filtration, or UV germicidal irradiation - may be needed to have a major impact.

There's an immense amount of data to peruse, so you'll probably want to follow the link below to read the study in its entirety.

Environmental Persistence of Influenza Viruses Is Dependent upon Virus Type and Host Origin

Karen A. Kormuth, Kaisen Lin, Zhihong Qian, Michael M. Myerburg, Linsey C. Marr, Seema S. Lakdawala

Anice C. Lowen, Editor DOI: 10.1128/mSphere.00552-19

ABSTRACT

Highly transmissible influenza viruses (IV) must remain stable and infectious under a wide range of environmental conditions following release from the respiratory tract into the air. Understanding how expelled IV persist in the environment is critical to limiting the spread of these viruses. Little is known about how the stability of different IV in expelled aerosols is impacted by exposure to environmental stressors, such as relative humidity (RH).

Given that not all IV are equally capable of efficient airborne transmission in people, we anticipated that not all IV would respond uniformly to ambient RH. Therefore, we have examined the stability of human-pathogenic seasonal and avian IV in suspended aerosols and stationary droplets under a range of RH conditions.

H3N2 and influenza B virus (IBV) isolates are resistant to RH-dependent decay in aerosols in the presence of human airway surface liquid, but we observed strain-dependent variations in the longevities of H1N1, H3N2, and IBV in droplets.

Surprisingly, low-pathogenicity avian influenza H6N1 and H9N2 viruses, which cause sporadic infections in humans but are unable to transmit person to person, demonstrated a trend toward increased sensitivity at midrange to high-range RH.

Taken together, our observations suggest that the levels of vulnerability to decay at midrange RH differ with virus type and host origin.

(SNIP)

In this study, we examined the environmental persistence of six IV, including human-pathogenic seasonal and LPAI viruses (Table 1; see also Table S1 in the supplemental material), in response to a range of RH conditions. We explored the contributions of virus strain background, droplet composition via various propagation methods, RH, and duration of exposure on the stability of IV in aerosols and large droplets.

We found that human-pathogenic seasonal H3N2 and IBV were resistant to decay under most RH conditions for an extended period of time in aerosols containing ASL from HBE cells (HBE ASL).

However, the longevities of human-pathogenic seasonal IV stability in droplets differed between subtypes H1N1, H3N2, and IBV in an RH-dependent manner, suggesting a role for virus-specific factors in the environmental persistence of IV.

Surprisingly, we observed that LPAI IV were more vulnerable to decay at midrange RH than human-pathogenic seasonal IV.

Together, these results confirm that human-pathogenic seasonal IV can remain infectious under a range of RH conditions, in agreement with our previous study (15), but this work clearly demonstrates that RH may be an important factor affecting the stability of expelled IV over time. Overall, our results indicate that the levels of persistence of IV are not uniform and that virus-specific factors can impact the stability and longevity of IV in the environment.

(SNIP)
Here, we have clarified the relationship between RH and the stability of seasonal IV and LPAI strains resembling those that would be released into the environment from the airway of an infected person.

This report provides a distinction between IV strains and virus stability in response to propagation method and specific environmental parameters.

Aerosolized seasonal IV are highly resistant to RH-dependent decay, which suggests that removing them via increased air exchange rates, filtration, or UV germicidal irradiation may be critical to reducing the transmission of these viruses indoors.

We found that RH is important for the stability of IV on surfaces but also that infectious viruses have the potential to persist on surfaces for hours in physiological droplets, reinforcing the need for surface decontamination in high-risk environments.

(Continue . . . )

For more on environmental factors that may affect Influenza and Coronavirus viability, you may wish to revisit: