How Do You Know if Your COVID-19 Vaccine Is Still Effective?

Some people use antibody tests to determine whether they have protection from COVID-19. Experts say antibody or serology tests shouldn't be used for this purpose.

"A vaccinated person is very likely to get a negative result from a serology test, even if the [COVID] vaccine was successful and protective. That's because different serology tests detect antibodies to different parts of the virus," says the MD Anderson Cancer Center. "Some tests detect antibodies to the spike protein of the virus, which are produced in response to viral infection or the vaccine. Others detect antibodies to a different part of the virus called the nucleocapsid protein, which are produced in response to infection, but not by the current vaccines." So, a negative test doesn't necessarily mean that a vaccine has faded, nor a positive test mean that you have full immunity.

Further, a person could have lots of antibodies to the delta variant or the original strain, but be completely unprepared for omicron or future variants. COVID strains are "like a group of cousins," says Dr. William Schaffner, professor of infectious diseases at Vanderbilt University Medical Center (VUMC). "They're all part of the same family, they all have the same last name, but we know that each has their own individual characteristics and personality."

Several studies have shown that the effectiveness of the three most common vaccines in the U.S. (Pfizer, Moderna and Johnson & Johnson) declines over time, though their effectiveness is still currently over 50 percent, the level to which COVID vaccines had to get in order to secure approval from the Food and Drug Administration back in 2020, according to The New York Times. The general decline in effectiveness is why boosters are recommended by the CDC and others.

What if you contracted COVID-19? Do you still need to be immunized against it?

"One of the things that's been pretty remarkable with COVID is that the immunity that's generated from a natural infection is not always the same," says Dr. Kathryn Edwards, professor of pediatrics at VUMC. "We've really been shown over and over that natural immunity from an infection often doesn't provide long lasting protection.

"We don't know exactly how much antibody is needed to prevent an infection," she adds. With that said, it's important to get a COVID-19 booster as soon as you're eligible, even if you've been vaccinated earlier or contracted COVID-19.

Some vaccines offer lifelong protection against a disease while other vaccines (like the flu vaccine) have to be regularly boosted. The difference in protection depends on the nature of the virus behind the disease.

"The measles virus is basically the same one that was around in 1936," Schaffner says, calling it a "very stable virus." Because of this, the measles vaccine offers lifelong protection after the second dose. There is a reliable antibody test for measles, but the vaccine's efficacy is so proven and solid that you probably don't need it, as long as you were fully vaccinated as a child. Other live viruses like mumps and rubella are also remarkably stable, and thus don't need to be repeated.

The influenza virus, however, is similar to COVID-19, in that it's very "plastic," says Schaffner. "They put on a new sport coat once in a while that changes the way we measure protection against them." That's why the experts recommend an annual flu shot to protect against the strain that's expected to dominate that season. Like COVID-19, the various influenza strains are related, so even if a different flu virus than expected takes over, you're still going to enjoy some protection and likely milder disease.

The Tdap vaccine is another one that doesn't last forever. This immunization protects against a number of diseases (tetanus, diphtheria and pertussis). A booster is recommended every 10 years, or with every pregnancy. This is because pertussis, also known as whooping cough, has surged in the last couple of decades. Moms and babies are particularly susceptible to this respiratory disease. It's also helpful to get boosted on schedule because if you step on a nail or other sharp metal object, tetanus won't be nearly as much of a concern if you're pre-protected. Decades' worth of research have set this booster schedule.

If you're concerned about whether or not you're still protected by a particular vaccine, the best course is to talk to your doctor. An antibody test might be recommended, but more than likely if you're past due they'll recommend getting up to date.

The CDC publishes an ongoing vaccine schedule for adults. It's exhaustively vetted and updated according to the most recent data, so it will let you know which vaccines are due when.