This is my sixth post of Scientific Discovery, a weekly newsletter where I’ll share great new scientific research that you may have missed.

[I’ll apologise up front for any typos or anything odd in this post – I’ve had Covid since last week1 and it’s been a little rough, although I’m now feeling mostly recovered.]

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In previous posts, I’ve explained how flu vaccines can reduce your risk of heart attacks and strokes. Here, I explain another breakthrough – of vaccines that can prevent cancers.

How many cancer cases are caused by infections?

It’s common to think of cancers as simply very unfortunate afflictions that we can’t get rid of. Or perhaps as diseases that doctors can treat to reduce their symptoms but not prevent.

But over decades, researchers have found more and more specific agents that cause cancers – like pollutants, toxins, radiation and even bacteria and viruses. Knowing about these agents is enormously useful. It gives us an opportunity to block them directly and avoid them at large.

To understand them, let’s take a step back: how do cancers arise?

In almost every cell, the human genome has over 3 billion base pairs of DNA (of A, T, C and G), and within them are around 20,000 genes. In order for us to grow from a single cell into adults with tens of trillions of cells, our cells need to divide – a lot.

When cells divide, they replicate their genome, and this increases the risk that our DNA gets damaged. Enzymes that replicate DNA can get stuck; the wrong bases can be added; chemical bonds can form between the wrong bases, or they can break between the right ones.

In total, it’s estimated that 10,000 to 100,000 of these events occur per cell per day.

A miracle of evolution is that we usually have effective mechanisms that protect us against most of these: our cells have enzymes to repair DNA damage, they have ‘checkpoints’ they have to pass to survive and grow, and they can use signals to self-destruct otherwise.

Not all errors are corrected, of course, but fortunately most changes tend to be benign.

But you might have also picked up on another problem. Occasionally, this damage can affect the genes that encode these crucial points – they can damage the repair enzymes, checkpoints, and self-destruction signals themselves.

We have several layers of protections like these, but when more and more damage accumulates, it can overwhelm them, potentially leading to a spiral of uncontrolled growth that forms cancer.

Now with that background, you can see how various agents might affect our layers of protection.

Some of them affect our cell metabolism, causing them to grow or divide more than usual, which increases the chances that something goes wrong. Others damage our DNA directly or hamper our repair mechanisms.

One of these agents is the Human Papillomavirus (HPV). This virus survives and replicates by inserting itself into our DNA, making us make more copies of it. And some of its genes are really harmful – they can degrade one of the most protective proteins in our cells, p53. This protein is often called ‘the guardian of the genome’ because it triggers our cells to die when their DNA is damaged, before they can grow and cause more harm.

There are several different strains of HPV and they can infect many different types of surface-level cells. Over many years, after enough DNA damage has accumulated, some of these strains can lead to cancers – including cervical, penile, oral, mouth, throat, head and neck cancers.

In fact, HPV is one of the leading causes of cancers worldwide. Based on its prevalence and the chances that it leads to cancers, it’s estimated that its strains caused almost 700,000 new cases of cancer in just 2018 alone.

In total, it’s estimated that 13% of all new cancer cases worldwide are caused by pathogens we know can cause cancer; 4% are caused by HPV alone. Nearly all cases of cervical cancer are estimated to be caused by HPV.

How many new cancer cases in 2018 were caused by infectious pathogens? This chart shows estimates, for the pathogens that we know can cause cancers. *WHO Global Cancer Observatory, based on de Martel et al. (2020)*

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There are also other pathogens that we know cause cancers: Helicobacter pylori causes various stomach cancers, hepatitis B and C viruses cause various liver cancers, and there are some others too.

These cause cancer in an indirect way: hepatitis B infections lead to the body attacking its own liver, which releases lots of molecules called ‘reactive oxygen species’ and ‘nitric oxide reactive species’ that damage DNA, eventually leading to cancer. The bacterium Helicobacter pylori is thought to have a similar mechanism.

The thing about scientific knowledge like this is that once we understand how diseases develop, we can move past experimenting randomly through trial and error, and instead make real and reliable breakthroughs in preventing them.

The discovery that HPV caused these cancers led to the development of vaccines against it. These vaccines protect against multiple strains at a time, and they are very safe and very effective. Because of this, in some countries already, younger generations will be able to avoid the cancer entirely.

In England, for example, all teenagers in schools are invited to get vaccinated against it – all girls have been offered it since 2008 and all boys since 2019, and there’s a universal screening programme to detect people who have developed signs of the disease, so they can be treated.

Below you can see how this has affected their rates of cervical cancer later on, compared to people born earlier, who were not offered HPV vaccines, because they didn’t exist until 2006.

This shows the number of new cases of invasive cervical cancer and cervical carcinoma per 100,000 women, in different birth cohorts in England. The red lines show the youngest cohort in this study, born between 1995 and 2000, while the dark blue lines show the oldest cohort, born between 1989 and 1990. You can see that, as they got older, some developed cervical cancers or carcinomas. But in each younger cohort, with higher rates of vaccination, far fewer developed those cancers. *Falcaro et al. (2021)*

There are also vaccines against hepatitis B viruses: they too are highly effective, reducing the chances of hepatocellular carcinomas (a type of liver cancer) by around 70%.

Not everyone is offered these vaccines. Many countries don’t offer HPV vaccines routinely, including where I grew up, in Hong Kong, even though I was born recently enough to receive them when I was young.

Most people who are unvaccinated get infected by HPV when they are teenagers, meaning that HPV vaccination is most effective when given to young people.2 A portion who are unvaccinated, mainly women, go on to develop cancer, as you can see above.

For myself, personally, that might be a big deal, or it might not. But knowing this – that today we have the ability to change things so that our children, and other people’s children, won’t have to suffer from some of the diseases that we face – that feels like enough for me.

Already, some cancers aren’t simply unfortunate afflictions that we can’t do anything about. We can sometimes prevent them entirely, and sometimes, after decades of research, that can be done with something as simple as a little needle.

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More links

I’m still a little too tired to write any longer summaries this week, but here are lots of interesting things I’ve been reading and listening to.

Did you know that some rare cancers are infectious and can be transmitted between organisms? It is usually very difficult for cancers to develop this ability, but some have. So far, scientists have discovered several of these ‘transmissible cancers’ in dogs. This new review explains what we know about them.
Although I am a big fan of her films, I did not know that Marilyn Monroe worked at a drone aircraft factory during World War Two before she became an actress. This Ridiculous History podcast episode is about surprising careers that celebrities had before they became famous.
What’s the lifetime risk of prion disease? The ‘1 in a million’ figure that is often quoted actually refers to the annual incidence (the risk of developing it in a given year). This blogpost by a great researcher in the field explains methods behind better estimates, which puts it at around 1 in 61,000 people over their lifetimes. I hope we find treatments and preventions for this soon.
I find it disturbing when governments get directly involved in fertility policy, particularly as they have often involved sterilization. Some countries have recently attempted to increase birthrates, but not very successfully. This episode of Beneath the Surface explores how these policies have turned out around the world.
The U-shaped relationship between income and birthweight. The poorest and richest people tend to have children with lower birthweights. (In richer people this effect is due to the parents tending to be older and having more twins through IVF, etc. But despite their lower birthweights, they have much higher rates of survival than others.)
Belief in conspiracy theories hasn’t increased over time – a cool study looking at this from multiple different perspectives.
How do bird feathers get their colours? An episode from the Science of Birds podcast.
The two new papers on the origins of Covid-19, here and here. Lots of evidence from lots of different angles; the low cost of genome sequencing technology today means that scientists can narrow down the emergence of epidemics that would have been more difficult to understand in the past.
A new self-test for HIV that will be made available for under $1 USD. Very good news for a lot of people.

And that’s all. I hope you are having a good week and that you have a great weekend :)

If you enjoyed this and haven’t subscribed already, go ahead and click that button, and then if you’re feeling generous, share it with your friends.

See you next time.

– Saloni

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[Correction: in the original version of this post, I didn't specify that it was hepatitis B and C viruses that cause liver cancer, which made it sound like all hepatitis virus strains carried those risks.]

1

It turns out I actually had Covid when I tweeted this, which I’ll take as a sign that the universe is laughing at me.

2

You could still get vaccinated against HPV later on, as I have – since there are multiple HPV strains it’s likely to still be beneficial when you’re older, although not as much as if you received it while young. If you’re interested, I recommend this blogpost which has much more details on how the HPV vaccine works.

Scientific Discovery

#6: Vaccines against cancers