Because most philosophies that frown on reproduction don't survive.

Saturday, May 16, 2020

Understanding the COVID-19 Outbreak: Part 3

This is the third part in a series. You can find the first part here and the second part here.

What Is A Pandemic Anyway?

My father-in-law tells a story about the day in high school that the football coach had to take over teaching Earth Science. He went up to the chalk board and began to write definitions. First up. "Lava flow" He considered a moment, then wrote the definition "Flow of lava".

I had a similarly recursive feeling when trying to look up definitions of "epidemic" and "pandemic" a while back in order to address the question of whether the coronavirus was really a pandemic. What is an epidemic? A disease that affects a large number of people in a region. What's a pandemic? An epidemic that spreads throughout the world. The CDC expresses it most succinctly of all: "A pandemic is a global outbreak of disease."
And yet words like "pandemic" and "epidemic" and "plague" carry a set of implications that cause many people to engage their mental yardsticks. I've heard many people express sentiments to the effect of, "If this is a real pandemic, why aren't the bodies piling up like in the Black Death?" The Black Death is a sort of core myth of Western culture at this point for epidemic, complete with the Monty Python image of people trundling along with carts calling "Bring out your dead!" It's a terrible, terrible disease (which is still out there and occurs in about 5,000 people a year globally, though it can now be treated with anti-biotics) but it is only one of a huge number of infectious diseases which used to account for the majority of human deaths.

If you look up the leading causes of death in the US at this time in history, you get the following list:
Heart disease: 647,457
Cancer: 599,108
Accidents (unintentional injuries): 169,936
Chronic lower respiratory diseases: 160,201
Stroke (cerebrovascular diseases): 146,383
Alzheimer’s disease: 121,404
Diabetes: 83,564
Influenza and pneumonia: 55,672
Nephritis, nephrotic syndrome, and nephrosis: 50,633
Intentional self-harm (suicide): 47,173
Influenza and pneumonia are the only infectious diseases on the list, and they're ranked 8th out of 10. It didn't used to be this way. A New England Journal of Medicine Piece provides this comparison of what people died of in 1900 vs 2010.
Influenza and Pneumonia topped the list, followed by Tuberculosis. All infectious diseases of different sorts. Why is influenza less of a killer now? For the same reason that tuberculosis and pneumonia are, to an extent. The influenza virus is not always the primary killer. Often it weakens the body and immune system which allows bacterial pneumonia to take over and finish the patient off. With modern antibiotics we can treat those bacterial pneumonias thus lessening the death rate of influenza. Additionally, we have the influenza vaccines which build the body's resistance to the virus strains themselves and we have antiviral medications which can help to some extent.

But influenza and other infectious diseases were fully capable of killing lots of people, and that was just in a normal time when there wasn't an epidemic on the loose.

What makes an epidemic different from run of the mill "endemic" disease? It's not necessarily that that disease is spectacularly deadly. A disease is epidemic when it is spreading through the population rapidly and infecting a large number of them. In theory this could happen with some fairly minor disease, so long as it was quite contagious and not many people have antibodies that protected them from it. The difference is just that if a minor sniffle spread rapidly through the whole population, we wouldn't notice it a great deal.

The point about people not having antibodies that protect them from the disease is key. Diseases are, after all, essentially parasites. A virus can't reproduce all on its own. It needs to take over the cell of another creature and commandeer that cell to manufacture copies of itself. Once the body recognizes a virus for what it is, a hostile invader, the body develops antibodies that destroy the virus. So imagine that I have a virus that's colonized my body and is using my cells to crank out copies of itself. I'm coughing out micro-droplets of water vapor which contain those viruses. But all the people I come in contact with are people who have already come in contact with that type of virus, and as soon as one of the viruses comes into one of these bodies, the antibodies destroy it. So even though the viruses that have taken over my body are cranking out copies of themselves, they can't successfully colonize any other bodies. After a couple weeks, my body too develops good enough defenses to start attacking the virus, and so my body wipes out the infestation of viruses in my body and it's the end of the line for that strain of viruses because they weren't able to colonize the bodies of any other people I came into contact with.

With an endemic virus, a common virus that's been around for a while in the population, a lot of the people the virus comes in contact with are already able to fight the virus off, and so spread is slow. We can see how that is important when we look at times when endemic diseases became epidemic diseases because they came in contact with a new population. After Europeans reached the new world, diseases that were common in Europe (measles, smallpox, influenza, etc.) but which had not before been present in the Americas suddenly turned into epidemics. Measles was not an epidemic disease in Europe. It has highly contagious, but lots of people had had it before and were immune. Even children who hadn't had it before received some antibodies from their mothers during pregnancy and nursing, giving their bodies a head start when they encountered the virus for the first time. But in the Americas, measles was totally unknown. The result was that what in Europe was a bad but survivable childhood disease became in the Americas a population decimating epidemic. Much the same happened with other diseases that had been common in Europe but not in the Americas.

That there were so many diseases that were common in Europe but not in the Americas is believed to be due to the fact that Europeans had domesticated many more animals and lived in close contact with them. Many viruses first jump to the human population via a mutation of some animal virus. For instance, the flu virus which caused the deadly 1918 Spanish Flu pandemic is believed to have jumped from swine to humans in Kansas in the spring of 1918. If you want to read lots of interesting discussion about virus mutation, immunities, and pandemics, I'd recommend John Barry's The Great Influenza which I've been reading and finding fascinating.

When a new strain of an existing family of viruses (such as the 1918 Spanish Flu strain of the influenza virus) appears via mutation and/or jumping from some animal species to humans, if it is different enough from other strains that the immune system does not recognize it, the virus can spread rapidly in much the way that these new diseases did in the new world. But even the case of a "novel" virus (such as the novel coronavirus that we're dealing with now) there can still be degrees of susceptibility. The 1918 influenza pandemic was a novel strain which was significantly more deadly than normal seasonal flu. Seasonal flu kills around 0.1% of those who get it. In 1918, the fatality rate was more like 2.5%, twenty-five times higher than "normal" flu strains. Moreover, while seasonal influenza mostly kills those we'd think of as medically fragile (infants and old/infirm people) the 1918 flu hit people in their 20s and 30s particularly hard. And while all lives are of equal worth, it's not unreasonable that people would consider particularly tragic a disease which suddenly cuts down healthy young people who appeared to have their whole lives before them. But that 2.5% fatality rate was not consistent everywhere. The fatality rate was much higher among isolated populations that had seldom been exposed to other strains of flu. For instance, Inuit villages in Alaska and Canada in many cases experienced much higher death rates, sometimes near 100%. The same was the case for Pacific Islander populations, Australian Aborigines, etc. It's believed that the reason for this is that populations which had experienced fewer outbreaks of other flu stains were even more defenseless against the virus that populations where people's immune systems had at least identified and fought off other flu strains. Even though this particular stain was novel enough that everyone was susceptible to it, the familiarity with other flu strains somehow gave the immune system enough of an edge that it was able to build its defenses more quickly.

Although SARS-CoV-2 is a "novel virus", it may be that exposure to other types of corona viruses is one of the things which causes some people to suffer little or not sickness from the virus, developing antibodies that can fight off the virus without the body suffering much of any damage first, while other people experience a long, difficult, or even fatal respiratory disease.

So is COVID-19 a pandemic? Well, it is a rapid outbreak of a disease which has spread across the world. It first appeared in China and has now infected significant numbers of people in Iran, throughout Europe, the US, Canada, Mexico, Brazil, Ecuador, Turkey, Israel, India, the list goes on.

But as I said earlier, we could imagine a situation in which a disease was new enough that it swept across the world but in which the disease was not really that serious. What are we to say to the people who argue that COVID-19 isn't really killing many people, perhaps no more than an ordinary flu season?

I'd meant to get into the numbers question in this installment, but talking about the nature of pandemics took me longer than expected so I'll address numbers in the next installment, and at I'll also include a section of a the closely related topics: "lockdowns" and whether they have slowed the virus at all.


Click here for Part 4.

1 comment:

Jamie said...

I keep refreshing in hopes of finding Part 4.