Friday, January 30, 2015

Flu and death - a short introduction.

I think the issue surrounding H3N2 here, is whether the current state of affairs represent the logical outcome, given contemporary scientific knowledge on influenza virology and epidemiology. I believe that, in the current form, it is probably correct that reinforcement of education in personal hygiene and mask-wearing behavior be done, and citizens be taught of the vulnerable group of people towards these viruses (the extremes of age, those with chronic illness, organ failure and so on.)

I would like to disclaim first, that by profession I am not a microbiologist nor have I undergone any sort of post-graduate training in microbiology. So here, I approach the problem as an amateur, a person with interest in science.

Influenza and H3N2

The influenza A viruses are classified into subtypes according to surface glycoproteins: haemagluttinin, and neuraminidase. Currently, there are 18 H subtypes, and 11 N subtypes, and these are largely found infecting other animals. In the past 100 years, though, there are only three influenza A virus subtypes that occured in human, H1N1, H2N2, and H3N2 (IIRC). Most epidemics occurred when an originally non-human type virus changes enough to settle on the human respiratory tract; the most important element here is that it needs to stick properly to, and stay on the respiratory tract. 

The ability to stick properly is dependent on the virus having "receptor binding sites" on the surface and the ability for it to bind to human-type alpha-2,6-linked sialic acid (as opposed to alpha-2,3-linked sialic acid) containing receptors, located on the surface of the upper respiratory tract. Specific mutations in the receptor binding sites are nowadays used to delineate or predict the ability of a virus strain to bind on human upper respiratory tract.

It has been seen that influenza viruses, and especially for strains of subtype A(H3N2), have continual evolutionary changes (which we call "antigenic drift" because it is a continual, slow, and gradual process). It is not difficult to understand why this change is forced upon: the human adaptive immune system is very good at combating an infection if it is known to the system; and moreover, we now use vaccines to make sure that viruses with old antigen won't work anymore. So there is a clear selection pressure on the influenza virus. And for the record, after forty-odd years of history, A(H3N2) is still alive and well.

I honestly do not think that this A(H3N2) is so different from previous ones that infected many of us before - you can probably find some distinct genetic features, and you can also probably see a statistical significant increase in death and infection, even after vaccine failures has been controlled for -- after all, vaccine is only a contributing factor, and the immune system as a whole is more important, but I think the difference is a quantitative one not a qualitative one. 

It is notable, though the reason being unclear, strains of A(H3N2) are known to cause increased mortality (compared with other subtypes of flu viruses such as H1N1). The comment quoted in the article saying that the virus targets younger people probably reflects increased admission of young people into ICU, which implies that the virulence (ability to infect and kill) of the virus has been higher than before.


I think one should always look at mortality number with the background of those who died in mind. The thing is, who is at risk. For example, while SARS killed 299 in 2003, in the same year, tuberculosis killed 275, and even in 2014, there has been 184 tuberculosis death. So is tuberculosis approximately 92% as severe as SARS in 2003? I don't think so, and I'd assume that you won't believe in that stupid percentage trick as well. YES - tuberculosis is a big problem in Hong Kong but it isn't as much a problem as SARS.

I guess the determinant here is, whether it is something that leads to a lot of death in previously healthy people. If it does, it is a big problem - something should be done immediately to stop its spread. But if it only targets certain group of people, well, then, the problem is easier to tackle, because there aren't as many people who are at risk. And then, another factor plays in - does the infection have a period of time in which the infected person is not quite unwell, but already quite infectious?

Influenza is typically infectious around 1 day before, and up to 4-7 days after initial symptom appear. and to make it better, if you wear a mask in the public area, and keep it off at work, chances are, you probably won't get infected by it.

The bottom line

Influenza A(H3N2), in its current form, had a much lower mortality rate than other dreaded respiratory infections, but obviously is worse off compared with previous years.

There aren't much easy ways of preventing it other than asking people to do their personal hygiene properly and using the vaccine  (despite the inefficacy - 20% protection is still way better than no protection). Using the mask is good, but is probably limited to those with reasonable civil education/moral sense, which, lately become a rare occurrence in Hong Kong.