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Tantalizing Steps Toward a COVID-19 Vaccine. And How Does an mRNA Vaccine Work?

On Monday a company reported promising first signs for a vaccine against COVID-19. What made this news exciting was that, so far, the candidate vaccine looks safe, and taking two shots of it seems to give people as strong an immune response as found in people who have recovered from COVID-19. While this is encouraging news, the vaccine was tested on a very small number of people—only eight volunteers. There is still a long way to go before the vaccine could be ready for a large number of people to use.

Normally, developing a vaccine takes many years. Vaccines need to be tested in large numbers of volunteers, usually in three phases of experiments where each phase includes more and more people who take the proposed vaccine, to prove that it is safe and that it actually works to protect people against disease.

With COVID-19 disrupting lives in a way that hasn’t been seen for more than a century, researchers have plenty of motivation to develop a vaccine as quickly as possible, and experts like Anthony Fauci, the doctor who leads the National Institute of Allergy and Infectious Diseases, have estimated that a vaccine for COVID-19 could be ready in as little as one year.

Scientists have proposed dozens of candidate vaccines for COVID-19 in just the few months the virus has been around, and a small number of vaccine candidates are already being tested in people.

So how do vaccines work?

Normally, when someone becomes sick with a germ such as a virus, the body’s immune system recognizes the intruder and in a few days develops an army of tools to fight the invader. White blood cells fight infections by swallowing up and digesting germs, killing infected cells, and by making antibodies. Antibodies are large, Y-shaped proteins that can use their tips to recognize foreign agents very specifically. Once they recognize a particular invader, antibodies that match the invader replicate in vast numbers and then stay in the blood, ready to pounce if they ever come across the specific shape again.

Sometimes, as with a particularly dangerous germ like COVID-19, the fight between the immune system and the invading germ can get perilous, with no guarantee that the immune system will win before a person gets very sick.

Vaccines take advantage of our body’s immune system by imitating an infection without actually making people sick. Vaccines can be made from dead or weakened versions of germs or from portions of germs, such as a bit of the protein capsule that makes up the outside of a virus. If the real germ comes along later, a large number of antibodies will be ready to surround and kill the germ before the germ can get very far.

Sometimes people need to take more than one dose of a vaccine so the immune system reaction becomes strong enough to protect against an infection.

Why do vaccines sometimes make my arm sore?*

Soreness a day or two after you get a shot of vaccine is a sign that your immune system is hard at work, making antibodies. All the white blood cells milling around the vaccine can cause inflammation. Making sure to move your arm around soon after you get a dose of vaccine can help spread it around and make your arm less sore later.

How do mRNA vaccines work?

The candidate vaccine that made the news this week is made out of mRNA. No vaccine has been made from mRNA before. mRNA is a molecule normally found in almost all cells and carries code for making protein. A vaccine made of mRNA would use the body’s own protein-making machinery to make some viral proteins, which the immune system would then recognize as foreign and mount an immune response against.

An advantage of mRNA vaccines is that they are relatively fast and easy to make, so it should be possible to make large amounts of the vaccine quickly if it is proven to work well.


The image above shows a human white blood cell surrounding bacteria to fight an infection. Bacteria are colored green in this picture from an electron microscope. Source: National Institute of Allergy and Infectious Diseases

* My 9-year-old daughter—and editor—asked me this question. Do you have other questions about vaccines? Leave them for me in the comments, and I will try to answer them in a future post.


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