I have two problems with the title of this video.
First, things don’t really cause cancer.
Like, imagine the most carcinogenic thing you can.
Hopefully you are currently imagining smoking a cigarette right now,
because that is the most cancer-causing thing humans do.
Most people who smoke cigarettes for their whole lives will never get lung cancer.
Now, their risk is DRAMATICALLY increased, and also there are a ton of other
bad health outcomes so, like, don’t smoke cigarettes!
But in the strictest sense, things do not cause cancer.
Our bodies are too smart for that.
Things increase our risk of cancer.
Second, since being diagnosed with a cancer myself,
I’ve been working hard not to refer to cancers as a single disease.
We really shouldn’t be saying cancer, we should be saying “cancers.”
So the title should probably be more like, “Does infection with the SARS-COV-2
virus increase subsequent risk of malignant carcinomas.”
Or, if we wanted to be brief, “Is SARS-COV-2 oncogenic.”
But fewer of you would have clicked on that
and this is super important information, so we’re doing what we’re doing!
Why is it important?
Well, because regardless of how you ask the question,
it’s starting to look like the answer is “yeah.
Yeah, COVID might increase your chances of getting cancer.”
But before you break out into a cold sweat,
there is a lot of complexity that you should know about that simple answer.
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Just to be super up-front about why I want to talk about this,
it’s because as soon as “COVID might cause cancer” becomes a common idea,
it’s going to be followed by an absolute buttload of misinformation.
So what I want to do is less establish whether COVID for sure causes cancer,
and more show how we’re going to be able to recognize
that this is the case and how much it’s going to matter.
Even though I just said nothing “causes” cancer,
some viruses tiptoe right up to that line.
Human papillomavirus, or HPV,
has a strong link to cervical cancer and other cancers.
The HPV vaccine has dramatically reduced the rates of cervical cancers
in places where it is commonly administered.
So this would definitely not be the first time a virus caused a cancer.
That’s actually extremely common.
Worldwide, around 10% of cancers would not have occurred
without a virus kicking them off.
The cancer I was diagnosed with, Hodgkin lymphoma, is caused, in part, by
infection with Epstein-Barr virus, which is the virus that causes mononucleosis.
A lot of you might have just gotten worried because oh man, mono?
I’ve had mono, I must be doomed.
But more than 90% of Americans are infected with Epstein-Barr virus
at some point in their lives, and only 0.2% will get Hodgkin lymphoma.
Our statistical and biochemical understandings are sophisticated enough
that we were able to first start to understand that link back in the 1970s,
which is pretty amazing!
But the risk of getting Hodgkin lymphoma doesn’t peak
until four or five years after initial infection.
And with HPV, it can be fifteen or twenty years
between initial infection and the onset of cancer.
At the moment, no one is ten or twenty years out from a SARS-CoV-2 infection
because it’s only been out there in the world for four years.
And that’s a problem, because if there is a connection between COVID-19
and any kind of cancer, we’d have to watch people get sick with COVID,
and then develop a particular kind of cancer.
That’s a waiting game, which is no fun for anyone – the scientists or the patients.
But it’s important to say, so far…
we haven’t seen an increase in cancers that would be easily explained by COVID.
If the effect were super fast and super big,
we might be able to have seen it by now.
But we also can’t just look at people who got a serious COVID infection
and see if they were more likely to get cancer.
Because, look, a person who gets serious COVID might very well
develop cancer afterward because likely the biggest risk factor for a serious case
of COVID and the biggest risk factor for cancer are the same thing: age.
Lots of things could increase both the risk of a serious case of COVID
and the risk of cancer, and it’s very hard to tease out all of those details!
But there is one thing we can do to try to peer into the future
and see if there’s a causal connection between COVID and cancer.
There is some limited evidence for genetic risk factors
that increase your susceptibility to COVID.
And those genetic risk factors, if they exist,
would not increase your risk of getting cancer.
They would only increase your susceptibility to COVID infection.
So! You can look at a very large population and see that folks with those
suspected genetic risk factors for COVID have a higher chance of getting COVID.
And then you can look at that same population and see if people
with those same risk factors have gotten more cancers in the last few years.
This type of study is called a Mendelian randomization study,
after that guy with the pea plants, and it’s sort of a shortcut
to figuring out causal links between risk factors and disease outcomes.
And we did find one that claimed to have found a causal link between COVID
and cancer, but it wasn’t…like, a great study.
The study calculated the odds ratio of the increased risk of cancer.
And this is a measurement that tells you how likely a certain exposure,
in this case COVID, is to be related to a certain outcome, in this case, a cancer.
An odds ratio of one means it’s basically 50/50,
no association between the two things.
Odds ratios can go below one,
that means that something decreases the risk of something else.
On the other hand, the higher they go above one, the higher the risk.
This paper only reported one odds ratio above 2.
And that was for stomach cancer – the odds ratio for an association between
infection with the virus and stomach cancer was about 2.9.
And even that is kinda low, like, the odds ratio
for cigarettes and lung cancer is around 8 at minimum,
and that’s for people who’ve ever touched a cigarette, not heavy smokers.
So these odds ratios just… aren’t enough to get worked up about yet.
There is another way to look at this, though, and one that is actually easier to
investigate at this early stage where we don’t have many years of cancer data.
Basically, we can pull the virus apart and ask whether it’s doing anything likely
to tip our cells toward cancer, which doesn’t for sure answer the question,
but it lets us keep an eye out.
So, the first question is, why do viruses cause cancer at all?
And the answer is… because it helps them.
Some viruses are pretty brute force.
It’s the traditional virus story.
They turn your cells into virus factories until your cell realizes something is wrong
and blows itself up, and then more virus particles get released into your body
to infect more cells.
But other viruses have evolved to keep some of your cells alive
as little viral reservoirs. This is what HPV does.
Your cells have all kinds of ways to check for damage and then send out
a signal for the cell to destroy itself if things look weird.
But that would be bad for the virus, so HPV has evolved ways to dampen the cell’s
ability to check for DNA damage and to destroy itself.
That means a longer-lived, more active reservoir for viruses.
But one of the main traits of a cancer is that it is a group of cells
in your body that are worse at repairing DNA
and good at conveniently ignoring orders to destroy themselves.
So the virus has evolved in order to do what is good for the virus, but that also just
happens to take a cell a few steps closer to potentially becoming cancerous.
Which increases your individual risk,
and also increases the total number of cancers in a population of people.
The question is, then, does the SARS-CoV-2 virus
mess with cells in this way and, yeah, it’s starting to look like it does.
One of the most important tumor suppressing genes in your body is p53.
It does exactly what we’ve been talking about: it helps your cells check DNA
for damage and can signal a cell to destroy itself if things are looking weird.
People who inherit only one functional copy of this gene from their parents
are extremely likely to get multiple cancers early in their lives.
Researchers have observed that SARS-CoV-2
has been shown to lower the amount of p53 in a person’s cells.
In one 2022 report, researchers looked at blood samples from COVID patients
with mild, moderate, severe, or critical infections, including some with long COVID.
In patients with mild or moderate infections, their blood had lower levels of p53
16 weeks after infection, but it bounced back by 24 weeks.
But patients with severe or critical infections didn't have that bounceback.
Their p53 levels were still low after 24 weeks.
And it’s not just low, it’s actually worryingly low - the researchers specified
that if the reduction in p53 function continues,
it could be just as bad as a mutation that causes it to lose function.
Less P53 means that the cells are less able to fight the COVID infection,
but it also means that they’ve been pushed a little further down the path
to potentially becoming a cancer.
A brief dip in p53 levels probably isn’t anything to worry about.
But the longer you go without, the more the risk of cancer increases.
But while p53 is a big deal, there’s plenty more that might be going on here too.
For example, the SARS-CoV-2 virus interferes with something called
the RAAS system, which regulates blood pressure,
and results in increased activity of a protein called angiotensin II.
Angiotensin II is itself associated with more aggressive cases
of a certain type of lung cancer.
But it also can switch on all kinds of pro-growth signals
that cancer cells slurp up like candy.
On top of that, infection with the virus appears to disrupt normal cell division,
causing mutations and pushing cells to divide
even when normal checks and balances would stop them.
Based on the molecular pathways it alters and the organs it affects,
researchers suggest we should keep a particular eye out
for lung, colorectal, pancreatic, breast, and oral cancers.
At this point, it is looking like it is possible that the SARS-CoV-2 virus
increases the risk of certain cancers.
But there are much more important, bigger questions that remain unanswered,
like, what’s actually going to happen, when, and to whom?
We live every day with almost everyone in this country getting a cancer-causing
virus, Epstein-Barr, and we simply don’t think about that.
I mean, I don’t.
Some people do, though, and a vaccine for that virus could save thousands of lives
a year and there are people working on one!
Which is great, because Epstein-Barr also increases
a person’s risk of multiple sclerosis by around 32 times.
One last thing here.
You may have heard that rates of some cancers among young people,
especially colorectal cancer, have been recently increasing.
And indeed, rates of colorectal cancer in people between the ages of
35 and 45 have been increasing substantially, basically doubling since 1995.
This is a long-term trend, but it only really hit the news in the last few years.
We’ve seen people wondering online if this increase is because of COVID
and just wanted to add at the end here that this trend significantly pre-dates
the emergence of the SARS COV-2 virus
and the causes of the increase remain unclear.
Some of it is probably just because people getting
more face time with their doctors.
In fact, right before the pandemic began,
a report came out that showed people were spending more time with doctors...
and when you spend more time with doctors,
you find more diseases and disorders, including cancer.
COVID is, in some ways, the most studied virus in history,
but the reality is that we will need more time and more research to understand
the significance of the link between COVID and cancers.
We’ve already been able to see rates of cervical cancer decrease
after people started getting the HPV vaccine, but that took FOREVER.
As with many things in science, the more we know, the more questions we find.
And we have not found a DRAMATIC link between COVID and Cancer…
but we have found a potential link.
It’s not cigarettes, but is it nothing?
Let’s put it somewhere between cigarettes and nothing.
And drilling deep on this virus has been extremely effective in helping us better
understand cellular and viral phenomena, but it might also help us understand
our bodies more broadly, and the diseases that befall them.
Knowing more is always good,
even if what we find is not as comforting as we might like.
This video was brought to you by our patrons on Patreon.
They are an amazing group of people who help us make this show.
About, like, 1.5 out of every 10,000 people who watch SciShow
actually give us a little bit of money, and they are the whole reason we can do this.
Big thank you to all of you, and also thanks to everyone for watching.
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