[Originally published: May 20, 2021. Updated: October 22, 2021.]
If you are fully vaccinated, you may be excited about gathering with family and friends again. You might even be planning a winter vacation. But there are still nagging questions about how long protection from the coronavirus vaccines will last. For instance, will your shot wear off gradually or suddenly? Will you need a booster?
“We can only say that a vaccine is protective as long as we are measuring it,” says Yale Medicine infectious diseases specialist Jaimie Meyer, MD, MS.
Vaccine longevity became a hot topic in August, when some studies began to suggest vaccine effectiveness was waning, although data also showed the vaccines were still highly effective against hospitalization. In one study, data from the state of New York showed vaccine effectiveness dropping from 91.8 to 75% against infection.
In September, the CDC approved a Food and Drug Administration (FDA) authorization for a booster dose of the Pfizer-BioNTech vaccine for select groups after also considering data from such sources as the Israeli Ministry of Health. In October, the CDC approved authorizations for a Moderna booster for the same groups, and a Johnson & Johnson booster for anyone who got J&J as their primary vaccine. A “mix-and-match” approach now allows people to choose their booster, even if it’s different than the one they got originally.
How are we monitoring the coronavirus vaccines?
Pfizer and Moderna have been monitoring immunity in people who were given their vaccines in the initial clinical trials—both companies had reported strong overall efficacy at the six-month mark. (Pfizer reported on its efficacy in a preprint that has not been reviewed by outside scientists; Moderna released a company statement.)
One thing researchers are monitoring in vaccine recipients is levels of antibodies, which are proteins produced by the body’s immune system when it detects harmful substances, and that are easily measured from blood samples. “Antibodies are a really good marker for protection against infection, so we will be monitoring those levels for as long as we can measure them,” says Akiko Iwasaki, PhD, a professor of immunobiology at Yale School of Medicine.
A report in The New England Journal of Medicine (NEJM) in April showed that 33 participants who had received the Moderna vaccine during the Phase I trial had a gradual decline in antibody protection—and, based on the slope, Iwasaki says, that is hopeful news. “If antibodies are going down very quickly, you would expect that to last for a short time.” The slow decline raises hopes that the mRNA vaccines will be protective for at least a year, if not longer, she says. (It should be noted that the trial occurred prior to Delta becoming the predominant virus variant.)
Another measure is T cells, which scientists are still studying for their ability to kill virus-infected cells in the context of COVID-19, and which may also provide important protection. T cells are more difficult to measure, Iwasaki says. But they may be important—last year, a study in Nature showed that people who were infected with severe acute respiratory syndrome (SARS), a different coronavirus outbreak that killed almost 800 people in 2003, maintained T-cell immunity 17 years after they recovered.
Still another way to predict how long protection might last is by looking at natural immunity, says Dr. Meyer. This means studying immunity people developed after infection with COVID-19. “We know for at least the first few months after symptomatic disease—and even longer—that people are unlikely to become reinfected,” she says.
But it’s important to know that immunity induced by the mRNA vaccines is stronger and more reliable than natural immunity, says Iwasaki. That’s because levels of natural immunity tend to differ from person to person. “Vaccines normalize the response to a very high level, where it uniformly uplifts everybody,” she says. “If you are starting with the high level, even if you start to decline from that level, it will take much longer before you need a booster.”
This is a reason why the CDC recommends vaccinations for people who have had a COVID-19 infection as well as for those who have not.
Could one type of vaccine last longer than another?
No one knows for sure whether one vaccine will last longer than another. Instead, one question to ask might be whether Pfizer and Moderna’s mRNA vaccines, which had an especially robust response, also have potential to be the longest lasting, Dr. Meyer says.
The two mRNA vaccines use a relatively new technology that delivers a tiny piece of genetic code from the SARS CoV-2 virus into the body to provide instructions for making copies of spike proteins that will stimulate an immune response. The Johnson & Johnson vaccine takes a more traditional approach that involves an inactive adenovirus (a common virus that can cause colds and other illnesses when it’s active).
“The mRNA vaccines are a novel tool that hasn’t been widely rolled out with any other virus, and so far in clinical trials they have had a much more robust immune response,” Dr. Meyer says. Whatever the answer to the question of which will last the longest, the Pfizer and Moderna mRNA vaccines work similarly, so it seems likely that they will have a similar impact on immunity, she says.
However, while both vaccines still are considered highly effective, some recent studies showed Moderna to be more protective. A study published in The New England Journal of Medicine found the Moderna vaccine to be 96.3% effective in preventing symptomatic illness in health care workers compared to 88.8% for Pfizer. Another study, from the CDC, found Moderna’s effectiveness against hospitalization held steady over a four-month period, while Pfizer’s fell from 91% to 77%. But scientists say more data is needed to fully understand the differences between the two vaccines.
“It’s also possible that the length of immunity is somewhat dependent on the patient,” Dr. Meyer adds. While more research is needed, there could be variations in immune responses from person to person based on such factors as age, medical conditions, and medications they may be taking. Overall, though, the mRNA vaccines appear to be so effective that they level the playing field in terms of achieving protection from infection, says Dr. Meyer.
What can we do in the meantime?
It will be important to follow the CDC’s recommendations on booster shots. “The good news is that Pfizer and Moderna made their mRNA vaccines easy to update,” Dr. Meyer says. “It just has to be tweaked a little bit, like having a computer code that needs a couple of minor edits. It’s relatively easy to build.”
It’s also critical that as many people as possible get their first two vaccination shots—or their one-shot if they are choosing Johnson & Johnson, Dr. Meyer says. “The hope is that the case rate will go down and more people will be less likely to be exposed.” That advice is especially important with the Delta virus, which has proven to be more contagious than previous variants, prompting the CDC to issue stricter guidelines calling for everyone—vaccinated or not—to wear masks indoors in areas of high transmission.
Even if Delta goes away, “I think those preventive measures will become even more important as the year passes, because potentially your immunity is going to wane over time,” Dr. Meyer says.
Meanwhile, people need consider the amount of virus activity in their area, and what they need to do to protect the immuno-compromised, and children and others who can’t get the vaccine. “I tell my family, 'It's great that you’re vaccinated. That’s wonderful, and you will have a lot more freedom and flexibility. But even the vaccines don’t have 100% guarantees, so whatever you do, you want to keep weighing the risks.'" she says.