Unlike other drugs, which we give to some people  when they're sick, we give vaccines to huge  

numbers of people while they're well. That's one  reason vaccines go through such extensive testing.

Vaccines work by simulating an infection in the  body. This isn't a real infection, but it teaches  

the immune system to recognize and neutralize  similar pathogens later. If the immune system  

can stop viruses from replicating, they no longer  pose a health risk to the vaccinated individual.

We've used this strategy to develop  dozens of vaccines over hundreds of years.

People have been immunizing themselves for  centuries, starting in India and China.  

By the early 1600s, people were deliberately  infecting children with tiny doses of smallpox,  

in a process called “variolation.”  Variolation was fatal about 2-3% of the time.  

But it made children immune to the disease,  which was normally deadly about 30% of the time.

In 1717, Lady Mary Montagu, wife of  the British ambassador to Turkey,  

introduced the technique to the British  medical establishment. She learned about  

variolation from Ottoman practitioners, and  then used it to immunize her own children.

Decades later, physician Edward  Jenner learned that British dairy  

workers had discovered an even safer  protective option against smallpox:  

injecting people with cowpox, a  related but less lethal disease  

that turned out to confer immunity. Jenner  tested the theory by injecting an eight-year-old  

boy with scrapings from a milkmaid's  cowpox blisters. Fortunately, it worked.

When the immune system detects a virus, it  makes antibodies to neutralize it. The goal  

is to block the virus from binding to  healthy cells, so it can't replicate.

Because pox viruses are related,  and use similar binding proteins,  

cowpox antibodies also ended up  protecting the patients from smallpox.  

And it was much safer to inject  patients with cowpox than smallpox.

We no longer immunize people by  giving them diseases. Instead,  

we use vaccines, which work  similarly but are much safer.

In the 1930s, researchers discovered  they could inactivate seasonal flu  

viruses using a formaldehyde solution.  Formaldehyde itself is toxic. But people  

injected with the inactivated virus particles  ended up developing protection from the flu.

To make a flu vaccine for the wider population,  researchers just needed a controlled way  

to generate lots of virus particles,  inactivate them, and then harvest them.

Based on some early experiments, researchers  turned to fertilized chicken eggs,  

where the viruses multiply exceptionally fast.

[If we want to license it, Getty has footage  of eggs being injected in 1965 and 2000]

The first flu vaccines were released in the  1940s. Even with recent advances in cell culture  

technology, about 80% of flu vaccines are still  made using chicken eggs—hundreds of millions of  

them, sourced from farms that governments  keep secret to protect against tampering.

We can also make vaccines using live viruses,  weakened enough so they can't actually cause  

the disease. Alternatively, we can use  non-infectious pieces of the viruses,  

or particles manufactured  to resemble the pathogens.

Scientists' latest strategy to fight  viruses is using messenger RNA—being  

deployed for the first time to fight SARS  CoV-2, the virus that causes Covid-19.

To make an mRNA vaccine, experts  start by sequencing the viral  

genome and finding the instructions  for how it binds to healthy cells.  

For SARS CoV-2, it turns out that it binds using  spike proteins that stud the virus's surface.

Then, scientists copy and package those  genetic instructions and inject them into  

healthy volunteers, so cells in their body  will start producing their own spike proteins  

(but not attached to any virus). That way,  patients create their own blueprint of a  

critical piece of the virus for their immune  systems to learn to identify and neutralize.

mRNA vaccines haven't been widely used before,  mostly because it's hard to keep artificial  

messenger RNA intact long enough to reach host  cells. But scientists have overcome that hurdle  

with new technology (particularly, synthesizing  better enzymes to flank the blueprint sequences)  

and now they're able to make vaccines incredibly  fast. For SARS-CoV-2, they also made changes to  

the RNA so it produced a very stable version of  the spike protein, one the immune system could  

easily recognize--the natural virus spike  kind of wobbles around in a confusing way..

Researchers were able to synthesize  RNA for the SARS-CoV-2 vaccine  

within a week of sequencing the  virus' genome, back in January 2020.  

That allowed them to start the first phase  of drug trials by March of last year.

Vaccines aren't miracle cures: they don't  make every individual immune to disease.  

But what matters is that they  work on a population level.

The key to a successful vaccination program is  immunizing enough people to develop so-called  

“herd immunity,” where most infected individuals  can't spread it to anyone else. This way,  

over time, fewer and fewer people get infected,  ideally until the disease is wiped out entirely.

Diseases still pose a risk as long  as there are some cases anywhere.

This year, we're witnessing the largest  international vaccine development effort ever.  

SARS-CoV-2 showed how quickly diseases  can spread in our globalized world.  

Now we're about to find out  if the vaccination techniques  

we've developed over centuries are  sufficient to meet the challenge:  

whether we can develop global herd immunity,  or if many countries will continue to struggle.

It's not just about emerging from this pandemic,  

but about creating a fast and effective  strategy to deal with future contagions.  

They may be inevitable, given how many viruses  seem poised to jump from animals to humans.

For now, and the upcoming decades,  

vaccines will likely be the key to ensuring our  collective wellbeing, and perhaps our survival.