Tuesday, November 28, 2017

Sci. Ttl. Enviro.: Cold-Dry Days Favor H7N9 Transmission

Credit FAO


As the epi chart at the top of this blog shows, while H7N9 declines sharply over the summer months in China, it surges again during the late fall and early winter. Some years, that surge comes as early as October, while other years it doesn't appear until January.

While it is no secret that influenza (and many other respiratory) viruses are more common during the winter months, influenza spreads year round in the tropics, and as we saw during the opening months of the 2009 pandemic, a novel flu can do pretty well during the summer as well.
There is still obviously a lot we don't yet understand about how influenza spreads, either via droplets, aerosols, or fomites. 
Over the years we've looked at a number of studies (on a variety of viruses) attempting to find what environmental conditions favor transmission, and what indoor environmental changes might reduce transmission, including:
Study: Survival Of Aerosolized Coronavirus In The Ambient Air
B&E: Assessing The Airborne Spread Of Avian Influenza From LPMs

NIH Study: Climate & Influenza Transmission

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.

Essentially, these researchers inoculated droplets of simulated respiratory fluids (containing salts & proteins) with influenza viruses, and tested their survivability at different humidity levels.
  • At low humidity (< 50%) the droplets evaporated quickly, and the virus survived well in a dry environment.
  • At high humidity (near 100%), the droplets were stable, and the virus survived as well.
But at humidity levels in-between, the droplets slowly dried out, increasing the concentration of salts and proteins to which the viruses were exposed, decreasing their survival rate.

The following year, in PLoS One: High Humidity Reduces Flu’s Infectivity, we saw a study that suggested that maintaining a higher indoor humidity level during flu season may curb influenza transmission.
As a side note, the Chinese have long boiled vinegar in their homes to ward off respiratory ailments, such as influenza. The noxious odor was supposed to `purify' the air inside the home.
While the use of vinegar was unlikely to have any beneficial effect, vinegar is 95% water, and boiling it undoubtedly raised the humidity inside their homes.  A spike in the sale of vinegar in China in 2002-2003 was credited as an early sign of their growing (SARS) epidemic.

Since different viruses may favor one environment over another, it is important to determine how a potential pandemic threat like H7N9 reacts to ambient temperature and humidity.

Over the past 18 months we've seen some concerns that H7N9 has become more `heat tolerant' (see June 2016  HK CHP Statement On Recent Mainland H7N9 Cases), and indeed in 2017 we saw human cases extend far later into the summer than in previous years.
But we are now 10 weeks since the last case (Sept 15th) reported by the Chinese government. As that lull isn't expected to last much longer, based on the following study recently published by the Journal Science of The Total Environment, we would do well to be keeping a close eye on the weather reports coming out of China.

Although the full study is behind a payway, we get a pretty good synopsis from the abstract. 

Independent and interactive effects of ambient temperature and absolute humidity on the risks of avian influenza A(H7N9) infection in China
Tao Liua, 1, Min Kangb, 1, Bing Zhanga, Jianpeng Xiaoa, Hualiang Lina, Yongqian Zhaoa, Zhao Huanga, Xiaojie Wanga, Yonghui Zhangb, Jianfeng Heb, Wenjun Maa, , 



  • The emergence of H7N9 virus poses a pandemic threat to human beings.•
  • Temperature and absolute humidity were negatively associated with H7N9 infection.•
  • The risks of H7N9 infection were significantly higher in cold-dry days.•
  • The risky windows of H7N9 were different in the northern and southern areas.•
  • Meteorological factors should be integrated into the establishment of preventive actions and precautionary measures.

The emergence of avian influenza A(H7N9) virus poses a pandemic threat to human beings. It was proposed that meteorological factors might be important environmental factors favoring the occurrence of H7N9 infection, but evidence is still inadequate.

In this study, we aimed to investigate the independent and interactive effects of ambient temperature (TM) and absolute humidity (AH) on H7N9 infection risks in China. The individual information of all reported H7N9 cases and daily meteorological data in five provinces/municipality (Zhejiang, Jiangsu, Shanghai, Fujian, and Guangdong) in China during 2013–2016 were collected. 

We employed a case-crossover study design, in which the 7–10 days before the onset date of each H7N9 case was defined as the hazard period, and 4 weeks before the hazard period was taken as the control period. The average levels of meteorological factors were calculated during the hazard and control periods. A Cox regression model was used to estimate the independent and interactive effects of TM and vapor pressure (VP), an indicator of AH, on H7N9 infection risks. 

A total of 738 H7N9 cases were included in the present study. Significantly nonlinear negative associations of TM and VP with H7N9 infection risks were observed in all cases, and in cases from northern and southern regions. There were significant interactive effects between TM and VP on H7N9 infection risks, and the risks of H7N9 infection were higher in cold-dry days than other days. We further observed different risky windows of H7N9 infection in the northern (TM: 0–18 °C, VP: 3 13 mb) and southern areas (TM: 7–21 °C, VP: 3–17 mb). 

We concluded that ambient temperature and absolute humidity had significant independent and interactive effects on H7N9 infection risks in China, and the risks of H7N9 infection were higher in cold-dry days. The risky windows of H7N9 infection were different in the northern and southern areas.

And, for reference, the weather this week in Beijing is forecast to be: