Vaccine terms explained: Efficacy vs. effectiveness, herd immunity and others

Vaccine terms explained: Efficacy vs. effectiveness, herd immunity and others

After months of waiting, two highly effective vaccines against the novel coronavirus are being distributed in the United States, while the company behind a third recently released promising clinical trial data and has applied for emergency use authorization from the Food and Drug Administration. But amid the rollout, there have been many questions about the vaccines. Here are the key vaccine-related terms you need to know.

Vaccine: The Centers for Disease Control and Prevention defines a vaccine as “a product that stimulates a person’s immune system to produce immunity to a specific disease, protecting the person from that disease.”

Traditional vaccines usually use disease-causing pathogens (viruses or bacteria) that have been weakened or killed or a laboratory-generated protein. For instance, the Johnson & Johnson vaccine that was submitted to the FDA on Feb. 4 for review uses an adenovirus, the one that causes the common cold, which has been engineered to be harmless. The adenovirus carries a gene from the coronavirus into human cells, which then produce the spike protein that primes the immune system to fight off a subsequent infection by the virus.

But the two authorized coronavirus vaccines, by Pfizer-BioNTech and Moderna, use a new technology involving mRNA that gives the body’s cells instructions to make a protein that triggers an immune response.

A pharmacist prepares a syringe with the Pfizer-BioNTech vaccine at Virginia Hospital Center in Arlington.

Vaccine terms explained: Efficacy vs. effectiveness, herd immunity and others

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Antibodies: These are the proteins created by the immune system soon after you get infected or vaccinated. They help you fight off infection and can protect you from getting a disease again, but the length of protection varies for different diseases and from person to person.

Antibody protection typically wanes over time, says Joshua Barocas, an infectious-disease physician at Boston Medical Center. “It doesn’t mean that you’ve lost your immunity. It just means that different parts of your immune system take over.”

Antigens: The term is another way to refer to foreign substances, such as bacteria or viruses, in the body that can cause disease. Antigens trigger the immune response that can produce antibodies.

mRNA: Also known as messenger RNA, mRNA is genetic material that contains instructions for making proteins.

This technology, which has been studied for other diseases including flu, is a critical component of the Pfizer-BioNTech and Moderna vaccines. The vaccines use a synthesized scrap of genetic information wrapped in a protective fat layer to prevent disintegration. When the vaccine goes into the muscle cells of the upper arm, the molecular instructions it contains tell your cells to create a protein that looks like the spike protein found on the surface of the coronavirus. The immune system then registers the protein as a foreign body and begins building an immune response and making antibodies, the same way it would if you were infected with the coronavirus.