How the RNA vaccines from Moderna and Pfizer work

Both rely on a new vaccine strategy using mRNA
Posted at 4:21 PM, Nov 16, 2020
and last updated 2020-11-16 21:56:02-05

SAN DIEGO (KGTV) — The back-to-back announcements of promising clinical trial results from Pfizer and Moderna may usher in a new era for vaccine technology.

Both companies have candidates that rely on a new kind of vaccine strategy: RNA. Preliminary data show both candidates are more than 90 percent effective.

On the surface, the vaccine candidates look like any other you’ve taken. They work by training your body to build up defensive weapons against a virus, like antibodies and T-cells.

But instead of training your body by introducing a killed virus or a fragment of a virus, the vaccines from Moderna and Pfizer introduce a set of instructions called messenger RNA enclosed in a little blob of fat.

“The key concept of RNA is that they’re messages, and they’re meant as short-term messages,” said Dr. Shane Crotty of the La Jolla Institute for Immunology.

Crotty said there are 5,000 to 10,000 messages within a cell at any given time.

Once administered, the mRNA in the vaccine instructs some of your cells to make the coronavirus’ signature spike protein. The spike protein on the surface of the coronavirus allows it to infiltrate and hijack cells.

Even though the spike protein is just one of about 25 genes the virus has, preliminary results from the clinical trials suggest it is enough material to train the immune system without making the recipient sick.

Crotty likened the process of training the immune system to spotting a sedan. If the coronavirus is a car, the spike protein might be the door.

“There's no way that car door could turn into a whole car, but it's enough of a piece of a car for your immune system to recognize that car if it saw it again,” he said.

Scientists started by sequencing the virus’ DNA from a sick patient, then encoding that genetic sequence into an mRNA instruction molecule that can be read by the manufacturing part of cells.

From there, Dr. Crotty said it’s a bit like the messenger app Snapchat.

The mRNA gets injected into the body, sending temporary instructions to your cells that last for a while, then disappear. mRNA does not genetically modify cells, he stressed.

“They’re around as messages for some period of time and then they go away, and the cells get back to their normal job of reading their own messages instead of reading the messages you’re injecting in the vaccine,” he said.

The concept has been around since the 1990s, but there are currently no RNA vaccines on the market for any pathogen, so Dr. Crotty said it’s hard to estimate how long their protective effects will last.
Some vaccines offer a lifetime of protection, like the measles vaccine. Others offer decades of protection. The flu vaccine only lasts about six months.

Dr. Crotty said the length of protection depends on how fast the virus mutates and how long the immune cells survive in the body.

But even if the COVID-19 vaccine turns out to need an annual update, he’s optimistic. The best feature of RNA vaccines is that they can be quickly reprogrammed.

Both Moderna and Pfizer are still in their Phase 3 trials, but they expect to finish them by December. Together, they estimate they’ll have enough doses for about 35 million people by the end of this year, primarily for healthcare workers and high-risk individuals.