As evidenced by the cruise ship quarantined in Yokohama, 2019-nCoV is highly contagious, with no unexpected selectivity. The lethality of the virus is still a question, which cannot be answered until the actual prevalence of subclinical and missed cases is tallied, which can only be accomplished when test kits are available and cheap.
Two factors may enhance the lethality of nCoV-2019 in China compared to the rest of the developed world, smoking, and genetics.
In China, 60% of doctors are smokers, with rates in the general population said to be similar. Quoting from (Wikipedia) Smoking in China.
Nearly 60% of male Chinese doctors are smokers, which is the highest proportion in the world. China does not have laws to punish health care facilities, medical workers and health officials who violate smoking bans, and is instead relying on the Chinese media to act as a watchdog.
Smoking is a social custom in the PRC, and giving cigarettes at any social interaction is a sign of respect and friendliness.
Now refer to (PLOS via NCBI) Effect of tobacco smoking on the risk of developing community acquired pneumonia: A systematic review and meta-analysis. From the abstract conclusion (square brackets mine):
Tobacco smoke exposure is significantly associated with the development of CAP [pneumonia acquired outside a hospital] in current smokers and ex-smokers. Adults aged > 65 years who are passive smokers are also at higher risk of CAP. For current smokers, a significant dose-response relationship is evident.
This is a meta-study. Prior studies with varying endpoints were analyzed via statistical methods to remove confounding factors, isolating the association of smoking with pneumonia. Community acquired is the term for an infectious disease acquired anywhere other than in a hospital.
This study does not examine lethality, probably because statistical techniques could not dig it out with sufficient confidence levels. To see the likely connection, consider the structure of the lung. Two parts of the lung come in direct contact with air:
- The bronchioles are air tubes, arranged like an upside down tree, decreasing in diameter the further in you go from the windpipe.
- The alveoli are expandable balloons, attached to the bronchioles like the leaves of a tree. This is where oxygen exchange occurs.
- Both are lined with respiratory epithelium, which is lined with fine motile hairs called cilia, and wet with mucus.
In a healthy nonsmoker, the cilia “beat” to expel inhaled particles back up the bronchioles to the throat, where they are expelled or swallowed, usually without notice.
In a nonsmoker, the innate immune system, which requires no prior knowledge of a virus, is active. (NCBI) Innate immunity and mucus structure and function. This is why, in most cases, an individual virus particle cannot cause infection. The number is small, but usually greater than one.
In a smoker, the actions of the cilia, and the innate immune system, are suppressed. Recovery from viral pneumonia is possible due to the adaptive immune system, which requires time to manufacture antibodies specific to the pathogen. It becomes a race in time, between viral load and antibody production.
So it is highly plausible, though missing study results, that a China where most people smoke, including doctors and staff, is likely to experience enhanced lethality due to 2019-nCoV.
By strict definition, pneumonia is an infection of the alveoli, not the bronchioles which connect. Broadening the definition, it includes bronchiolitis. This is considered clinically significant primarily with infants. There is an interesting exception, diffuse panbronchiolitis.
Diffuse panbronchiolitis is considered an Asian phenomenon, which means that the etiology (cause) and prevalence are not nailed down. It is clinically known in Japan and Korea, with suspicion of a genetic influence. The origins of these groups, like the rest of us, are lost in prehistory. Current theories of origin:
- Koreans have a large genetic representation of tribes from eastern Siberia.
- Japanese contain some of the above, as well as other groups.
In prehistory, the Han Chinese were in conflict and competition with every other adjacent group. While they remained culturally distinct, genetic isolation is unlikely. Literature suggests that the prevalence of this disease in China is unknown. A contributor to 2019-nCoV lethality may lurk within this vacuum.
3. Coincidence of Factors
The combination of a possible genetic factor, and smoking, may be more than additive. It may be multiplicative .
The prevalence of smoking in and near health care facilities may enhance nosocomial (in a hospital) transmission.