[Originally published: May 20, 2021. Updated: July 9, 2021.]
If you are fully vaccinated, you may be excited about gathering with family and friends this summer. 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 it wear off gradually or suddenly? Will you need a booster shot? Unfortunately, we can’t answer these questions with certainty yet. Though researchers know the vaccines have been effective against COVID-19 thus far, there is no track record to provide data for the future, which is the only way to know for sure.
“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.
In the meantime, there are a few things we do know.
One is that the three coronavirus vaccines authorized for use in the United States provide a high degree of protection for at least three months, based on clinical trials that began as early as last July. But more data on the mRNA vaccines is emerging. A follow-up monitoring of those who got vaccines (and not placebos) in the mRNA vaccine trials provided new information. In April, a report in The New England Journal of Medicine (NEJM) said that in all 33 participants who had received the Moderna vaccine during the Phase I trial, protection remained high for six months after the second shot.
That same month, Pfizer reported that its vaccine was still highly effective at six months. The Johnson & Johnson vaccine was introduced at a later date and researchers are still gathering post-vaccination data.
“The current data on the [Pfizer and Moderna] mRNA vaccines is very promising,” says Akiko Iwasaki, PhD, a professor of immunobiology at Yale School of Medicine and an investigator of the Howard Hughes Medical Institute. Iwasaki notes that the NEJM report on the Moderna vaccine showed a gradual decline in antibody protection in its subjects—and, based on slope, she 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. “In addition to circulating antibodies, memory B cells and T cells can respond quickly to control infection. It could be many more years, unless there are variants that are able to evade this kind of immunity.”
How are we monitoring the coronavirus vaccines?
Pfizer and Moderna will continue to monitor immunity in clinical trial participants over time. They are also studying potential booster shots to extend vaccine durability and protect against viral variants. (While the vaccines have provided strong protection against the main variants we already know about, such as Alpha—first identified in the United Kingdom—we don’t know how well they will work against variants that might emerge.)
Antibody levels in people have been a key focus, partly because these proteins— produced by the body’s immune system when it detects harmful substances—are easily measured with 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,” Iwasaki says.
T cells, which scientists are still studying for their ability to kill virus-infected cells in the context of COVID-19, may also provide important protection, although that is more difficult to measure. 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.
“The other way that we can approximate, from a clinical level, how long protection lasts is by looking at natural immunity,” says Dr. Meyer. This means studying people who developed immunity after being infected 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 while natural immunity can provide useful information, it’s important to know that immunity induced by the mRNA vaccines is stronger and more reliable than from infection, 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 Centers for Disease Control and Prevention (CDC) recommends vaccinations for people who have had a COVID-19 infection as well as for those who have not, emphasizing that it is not yet known how long you are protected from getting sick again after having the disease.
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 (as much as 95% efficacy in clinical trials), also have potential to be the longest lasting, Dr. Meyer says. The two 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 different and more traditional approach that involves an inactive adenovirus (a common virus that can cause colds and other illnesses when it’s active).
“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 that question, the Pfizer and Moderna mRNA vaccines work similarly, so it seems likely that they will have a similar impact on immunity, she says.
“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.
Will you need a booster shot?
To some degree, we will have to watch and wait while researchers monitor the continued efficacy of the vaccines over the coming months and years. If there is a change in the population’s immunity, people can expect the CDC to make new recommendations.
Meanwhile, if a new virus variant continues to circulate or becomes predominant, a component of the vaccine might need to be changed to more closely reflect that variant; this would mean booster shots, Dr. Meyer says. “The good news is that Pfizer and Moderna made their mRNA vaccines easy to update,” she adds. “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.”
In fact, Pfizer announced in July that it had plans to start clinical trials to test a booster shot against the Delta variant. (The company is also seeking FDA authorization for a third dose of its original vaccine, although it’s unclear if or when that booster might be administered. Both the CDC and FDA have said booster doses aren’t necessary for fully vaccinated Americans at this time.)
What can we do in the meantime?
It’s critical that as many people as possible get vaccinated to help with the immediate crisis, Dr. Meyer says. “The hope is that the case rate will go down and more people will be less likely to be exposed.” (This will also help prevent new variants from emerging.)
While the CDC has relaxed some of its recommendations for vaccinated people—allowing them to stop wearing masks and social distancing in most settings—there are still many situations such as train stations and doctor’s offices where it advises keeping up with precautions.
“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. “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, especially when you live in areas that still have virus circulating in the community.
But it’s also OK to feel some hope, Iwasaki says. “We know that these vaccines are protecting from severe and lethal disease, even from the variants.” While scientists await data on Johnson & Johnson, and continue to monitor the mRNA vaccine recipients, data on the gradual decline of the antibodies in recipients of the mRNA vaccines suggest that they are likely to induce strong protection for a long time, she says.