Saturday, February 05, 2022

What did COVID do to life expectancy?

[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]

According to the American Center for Disease Control and Prevention, in 2020 the COVID pandemic decreased life expectancy in the United States by about one year and a half. This finding was reported in countless media outlets, including the New York Times, Bloomberg, and Reuters. It was the largest one-year decline of life expectancy since World War II, when it dropped 2.9 years.

Does this mean that we all have to expect we’ll die one and a half years earlier? How can that be, if we haven’t even had COVID? And if that’s not what it means, then what did COVID do to life expectancy? That’s what we will talk about today.

First, let’s have a look at the numbers for the United States because that’s where the report made headlines. A group of researchers from Europe and the US published a paper in Nature’s Scientific Reports last year, in which they estimated how much total life-time was lost to COVID. They took the age at which people died from COVID and compared that with the age at which those people were expected to die. They found that on average each COVID death shortened a life by 14 years. So that’s the first number to take away. If you die from COVID in the US, you die on the average about 14 years earlier than expected.

At the time the study was done, in early 2021, about 300 thousand people had died in the US from COVID. We can then multiply the average of years lost per person with the number of deaths. This gives us an estimate for the total number of years lost, which is a little over 1.3 million.

Naively you could now say we calculate the average number of life-time lost per person by taking all those missing years and dividing them up over the entire population. The US currently has about 330 million inhabitants, so that makes 1.3 million years divided by 330 million people, which is about 1.5 days per person. Days. Not years.

Since the Nature study was done, the number of deaths in the US has risen to more than 800 thousand, so the lifetime lost per person has gone up by a factor two to three. And that’s the second number to take away, on the average each American has lost a few days of life, though if you’re still alive that’s in all fairness a rather meaningless number.

Clearly that can’t have been what the Center for Disease Control was referring to. Indeed, what the CDC did to get to the 1.5 years is something entirely different. They calculated what is called the period life expectancy at birth. For this you take the number of people who die in a certain age group in a certain year (for example 2020), and divide it by the total number of people in that age group. This period life expectancy therefore tells you something about the conditions of living in a certain period of time.

This means if you want to interpret the decrease of life expectancy which the CDC found, they assumed that from now on every year would be like 2020. If every year was like 2020, then people born today would lose 1.5 years of their life expectancy compared to 2019.

Of course no one actually thinks that every year from now on will be like 2020, to begin with because we now have vaccines. The number that the CDC calculated is a standard definition for life expectancy, which is formally entirely fine. It’s just that this standard definition is easy to misunderstand. The 1.5 years sound much more alarming than they really are.

More recently, in October 2021, another study appeared in the British Medical Journal which looked at the changes to life expectancy for 37 countries for which they could get good data. For this they compared the mortality during the pandemic with the trends in the period from 2005 to 19.

In the USA they found a decline of about 2 years, so a little more than the estimate from the CDC. For England and Wales, they found a decline in life expectancy of about 1 year. For Germany about 0.3 years. If you want to look up your country of residence, I’ll leave a link to the paper in the info below. The same paper also reported that in countries where life expectancy decreased, the years of lives lost to the covid pandemic in 2020 were more than five times higher than those lost to the seasonal influenza in 2015.

Keep in mind that this is the period life expectancy which really tells you something about the present conditions rather than about the future. When we go through a war or a pandemic, then this period life expectancy decreases, but when the bad times are over, the period life expectancy bounces back up again. That’s likely going to happen with COVID too. Indeed, in many countries the period life expectancy will probably bounce back to a value somewhat higher than before the pandemic, because the previous trend was rising.

Also, since people with pre-existing conditions are more likely to die from COVID, this will probably increase the average period life expectancy for the survivors. But before you think this is good news, keep in mind that this is a statistical value. It just means that the fractions of people with and without pre-existing conditions has shifted. It hasn’t actually changed anything about you. The changing weights between the groups don’t change the average life expectancy in either group. It’s like removing foxes from the race won’t make the turtles run any faster, but it will certainly shift the average over everybody in the race.

The Spanish flu pandemic from nineteen eighteen is an interesting example. In 1917, life expectancy in the USA was 54. During the pandemic, it dropped to 47.2, but in 1919 it went up to 55.3. In Germany it went from 40.1 in 1917, to 33 in 1918, and in 1919 went back up 48.4.

If this period life expectancy is so difficult to interpret, why do scientists use it to begin with? Well, it’s just the best number you can calculate from existing data. What you would actually want to know is how long it will take for the people born in a given year to die. The people born in a given time-span, for example a year, are called a cohort. The average of that distribution, when the people in the cohort die, is called the cohort life expectancy. Problem is, you can only calculate this when everyone in the cohort has died, so it’s really only interesting for people who are dead already.

This is why, if we want to talk about the living conditions today, we use instead the period life expectancy. This makes pragmatic sense because at least it’s something we know how to calculate, but one has to be careful with interpreting it. The period life expectancy is really a snapshot of the present conditions, and when conditions change rapidly, like with a pandemic, it isn’t a very useful indicator for what to actually expect.

Another common misunderstanding with life expectancy is what I briefly mentioned in an earlier video. What we normally, colloquially, but also in the media, refer to as “life expectancy” is strictly speaking the “life expectancy at birth”. But this life expectancy at birth doesn’t actually tell you when you can expect to die if you are any older than zero years which is probably the case for most of you.

If you look at the probability of dying at a certain age by present conditions, so the period life expectancy, then the distribution for a developed world country typically looks like this. So for every age, there is a certain probability of dying. The average age at which people die is here. But the age at which most people die is higher. In other words, the mode of the distribution isn’t the same as the mean.

But now, the older you get, the fewer years you have to average over to get the mean value of the remaining years. If you have survived the first year, you only have to average over the rest, so the mean value, which is how long you can expect to live on the average, shifts a bit to higher age. And the older you get, the less there is to average over and the further the mean shifts.

This means, counterintuitive as it sounds, the life expectancy at a given age increases with age. And this does not take into account that living conditions may further improve your expectations. The consequence is that if you have reached what was your life expectancy at birth, you can currently expect to live another eight years or so. That’s certainly something you should take into account in your retirement plans!

This shift of the mean of the curve with age is the origin of another misconception, namely that people died young in the middle age. Since this is history we can actually use the cohort life expectancy. For example, when Isaac Newton was born, the life expectancy in England was just 36 years. But he died at age 84. Was he also a genius at outliving people?

No, the thing is that the average life expectancy is incredibly misleading when you’re talking about the middle age. Back then, infant mortality was very high, so that pulled down the mean value far below the mode. Indeed, Newton himself almost died as an infant! But once people made it past the first couple of years, they had a reasonable chance to get grey hair.

So, it’s really misleading to take the low life expectancy at birth in the middle age to mean that people died young. This also makes clear that the biggest advances we have seen for the increases in life expectancy worldwide have actually been advances in decreasing infant mortality.

Around the year 1800, globally, more than 40 percent of children died during the first 5 years of their life. Today it’s less than 4 percent though there are big differences between the developed and developing world.

If you want to get a reasonable assessment of your own life expectancy at your present age, you should look for something called life period table for your country of residence. For example, here you see the 2017 table for the USA. At birth, life expectancy for men is currently 76 years and for women 81. But if you check the table at these ages you see that men still have an expectancy of another 10 years to live and 9 years for women.

And when you’ve reached that age you still have a few more years to live. It looks like some kind of Zeno paradox in which you never reach death. But as in the Zeno paradox, an infinite sum of smaller and smaller number can well be finite and indeed, spoiler alert, we will all die. So use your time wisely and click the subscribe button to learn more fascinating stuff.

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