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

A pharmacist prepares a syringe with the Pfizer-BioNTech vaccine at Virginia Hospital Center in Arlington. (John McDonnell/The Washington Post)

This story has been updated.

After months of waiting, three highly effective coronavirus vaccines have received 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.

Your questions about coronavirus vaccines, answered

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, the latest shot authorized by the FDA, 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 other 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.

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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.

Spike protein: These are the telltale proteins found on the surface of the coronavirus that give the virus its family name and help enable it to enter and infect human cells. The spikes, which circle the virus’s core like a crown (or “corona” in Latin), interact with receptors in human cells similarly to the way keys and locks work together, allowing the virus to gain access.

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Efficacy vs. effectiveness: There’s a difference, as The Washington Post’s Ben Guarino explained in a recent edition of our Coronavirus Updates newsletter. Efficacy reflects how well the vaccine works in a controlled setting like a clinical trial, while effectiveness is what happens in the real world. For instance, the Pfizer-BioNTech vaccine’s 95 percent efficacy means that it was 95 percent effective at preventing symptoms of covid-19 in trials, which didn’t quite reflect real-world conditions.

Although it is possible the shots will be as effective in the real world as they were in the trials, “the efficacy numbers usually dip” when vaccines or other medical interventions are released into the general population, says William Schaffner, medical director of the National Foundation for Infectious Diseases.

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Additionally, neither efficacy nor effectiveness is an accurate way to gauge your personal risk of getting sick, Guarino wrote. That risk can depend on a variety of factors including your exposure to the virus, your immune system and any underlying health conditions.

At this point in the pandemic, experts are urging people to focus less on efficacy numbers and instead prioritize getting vaccinated with whichever authorized vaccine is being offered to them. The two mRNA vaccines and the Johnson & Johnson single-shot vaccine offer comparably robust protection against severe covid disease, Schaffner says.

“They’re all designed principally to avert your becoming seriously ill, and that’s the major point,” he says.

Infection vs. disease: Again, there is a difference. According to the Mayo Clinic, infection occurs when viruses, bacteria or other disease-causing pathogens enter your body and start replicating. An infection progresses to disease when the cells in the body become damaged by the invading germs, which can lead to symptoms of an illness appearing. But it is also possible for you to be infected and show no symptoms, also known as being asymptomatic.

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Herd immunity: The term refers to when enough people become immune to a disease that the virus is unable to transmit to new hosts, The Post’s William Wan reported. For the United States to achieve herd immunity amid the pandemic, experts estimate that 70 to 85 percent of Americans need to be protected either through vaccinations or previously having had covid. But with just 2 percent of the country’s population fully vaccinated as of Feb. 3 and the emergence of more transmissible virus variants that existing vaccines may be less effective against, experts say the battle is probably far from over.

Coronavirus: What you need to know

Vaccines: The CDC recommends that everyone age 5 and older get an updated covid booster shot designed to target both the original virus and the omicron variant. Here’s some guidance on when you should get the omicron booster and how vaccine efficacy could be affected by your prior infections.

Variants: Instead of a single new Greek letter variant, a group of immune-evading omicron spinoffs are popping up all over the world. Any dominant variant will likely knock out monoclonal antibodies, targeted drugs that can be used as a treatment or to protect immunocompromised people.

Tripledemic: Hospitals are overwhelmed by a combination of respiratory illnesses, staffing shortages and nursing home closures. And experts believe the problem will deteriorate further in coming months. Here’s how to tell the difference between RSV, the flu and covid-19.

Guidance: CDC guidelines have been confusing — if you get covid, here’s how to tell when you’re no longer contagious. We’ve also created a guide to help you decide when to keep wearing face coverings.

Where do things stand? See the latest coronavirus numbers in the U.S. and across the world. In the U.S., pandemic trends have shifted and now White people are more likely to die from covid than Black people. Nearly nine out of 10 covid deaths are people over the age 65.

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