Some COVID-19 variants can break through vaccine defenses, and even vaccinated people can get sick.
But there's a second line of defense, called killer T cells, and they're keeping vaccinated people from getting seriously ill, going to the hospital or dying.
The new kinds of vaccines — the ones that use mRNA, like Moderna and Pfizer-BioNTech, or viral vectors, like Oxford-AstraZeneca — they're really good at this.
To understand why, let's look at how most vaccines work.
The Sinopharm and Sinovac vaccines are typical of the old kind. They work by injecting a dead virus.
One part of your immune system responds to it by making antibodies to lots of areas of that dead virus.
The best ones are those that stick to one particular part of the virus — the part it uses to get inside your cells.
They're called neutralizing antibodies. When the real virus comes along, they will block it from starting an infection.
But omicron is heavily mutated. Those neutralizing antibodies don't stick to it as well.
The virus can get into your cells and start an infection.
Infection turns your cells into virus factories. They churn out more viruses, which infect more cells, which make more viruses, and so on.
Here's where killer T cells come in. These cells patrol the body, searching for and destroying infected cells.
Killer T cells knock out the virus factories.
So, when antibodies don't stop the virus from causing an infection, killer T cells keep it from getting out of control.
The new vaccines are good at recruiting killer T cells because they work differently from old vaccines. Here's how.
Remember how the old vaccines inject a dead virus? Well, the new ones inject genetic instructions to make just a piece of that virus.
Your cells take up the instructions and make the virus piece themselves.
It's a little like what the virus does — turning your cells into virus factories. But these virus pieces don't do any harm.
What they do is train T cells to recognize infected cells.
Meanwhile, the part of the immune system that makes antibodies also goes to work on those virus pieces.
So, while the old vaccines mostly made antibodies, the new ones put two branches of the immune system to work for you: antibodies and T cells.
So, if a mutated virus gets past the antibodies and starts an infection, killer T cells usually keep it from turning severe.
The new mRNA vaccines are good at recruiting T cells. That's the Moderna and Pfizer-BioNTech shots.
The viral vector vaccines from Oxford-AstraZeneca, Johnson & Johnson and Sputnik V do this, too.
These are holding up against variants. People who got these vaccines are getting breakthrough infections, but for the most part, they're not getting seriously ill or dying. These new kinds of vaccines are really helping.