Scientists are increasingly confident that vaccines provide long-lasting protection against the coronavirus and that boosters will not be necessary for at least a year, perhaps much longer.
The vaccines are holding up well against all coronavirus variants so far. That means boosters probably won’t be needed anytime soon to protect against variants. Even more promising, it suggests that unlike influenza, the coronavirus may not require seasonal shots to keep up with mutations.
There’s also growing evidence that the immune system retains a long memory for fighting off the virus after infection or vaccination. People who received the first shots in clinical trials more than a year ago are still showing signs of strong protection. Other signals of long-term effectiveness, such as the presence of certain types of immune cells found in the bone marrow, are promising as well.
Even so, many vaccine experts say, there’s still a lot of uncertainty about the durability of coronavirus vaccines. But losing vaccine protection is probably something most people won’t have to worry about for quite some time.
“I expect we will have to be revaccinated eventually, but I don’t think it’s going to be in a year-or-two time frame,” said Dr. Joel Ernst, an infectious disease expert at UCSF. “Obviously we don’t have a long period of observation, because we haven’t had the vaccines that long. But so far I’m reassured that the vaccine-induced immunity seems to be pretty durable.”
When the first vaccines were approved in the U.S. last fall, scientists and public health officials were astounded by how powerfully protective they were. The first two vaccines, made by Pfizer and Moderna, were more than 90% effective at preventing mild illness in Phase III clinical trials, and those results have held up in the real world.
But the studies were only a few months old when the vaccines were approved, so there was no way to know how long the protection would last. And in December and January, new variants of the virus began to emerge that seemed to be at least somewhat resistant to the vaccines. That raised further concerns that the vaccines would need to be boosted, possibly as early as this fall.
Neither of those concerns has turned into a problem, though. The Phase III trial participants still seem to be well-protected, now nearly a year from their first shots. Metrics to measure immunity have hardly waned over time, infectious disease experts say, and more important, those early participants haven’t been getting sick at rates that would suggest vaccines are failing.
A few variants do indeed lower the effectiveness of the vaccines. But the vaccines are so strong that it doesn’t matter — they still work just fine.
“So far all the variants are susceptible to vaccine-induced immune responses. Even though some are less susceptible, they’re still within the range we expect for very good protection,” said Dr. Catherine Blish, a Stanford infectious disease expert. “It is possible that new variants would emerge that would be worse, in which case that would accelerate the booster schedule. But I think at this point, we can all feel comfortable that we have at least a year and perhaps longer before we have to worry.
“Which is great,” she added. “I’d love to be able to get through this fall and winter knowing that we are going in with pretty good protection.”
There are several markers scientists consider for vaccine durability. The one most frequently referred to for coronavirus vaccines is antibody protection — the amount and type of neutralizing antibodies, the proteins that kill the virus before it can cause infection, that are produced by the immune system in response to vaccination.
Antibodies are a useful marker of vaccine strength partly because they’re easy to detect and analyze from blood samples. Studies have found that the coronavirus vaccines produce an impressive antibody response that drops sharply soon after vaccination, then holds steady over time. That’s to be expected — it wouldn’t be efficient, or even safe, for the blood to retain high doses of antibodies.
“You’d have to worry about clogging. We’d be taking up a lot of space if everything you’ve encountered was floating in your blood,” said Dr. Jay Levy, a UCSF infectious disease expert and longtime HIV researcher.
But vaccines don’t only trigger antibody production. They also prompt the body to form types of immune cells that can fight off infection themselves or quickly manufacture more antibodies if a person comes into contact with a virus again. Those cells tend to lodge quietly in the bone marrow and the lymph nodes, making them harder for scientists to study to get a more complete picture of the immune response from vaccines.
Those studies are starting to come out, though. One small study, results of which were published last week in Nature, found protective levels of those memory cells in the bone marrow of most participants many months after vaccination.
Another hopeful sign of long-lasting immunity is the presence of similar memory cells from people exposed to other viruses years or even decades after infection or vaccination. People who were infected with the coronavirus that caused the 2003 severe acute respiratory syndrome pandemic, known as SARS, still had those memory cells nearly 20 years later, according to one study.
Scientists even found evidence of lingering immune response to the 1918 influenza in survivors nearly a century later.
But many infectious disease experts are wary of comparing this coronavirus to immune durability with other viruses — even other coronaviruses. Infection with the coronavirus that causes SARS may confer lasting protection, but other coronaviruses that cause only mild symptoms associated with the common cold are able to reinfect people over and over again. “It’s a little dangerous to make those analogies,” said Dr. Robert Siegel, a Stanford virologist.
He agrees, though, that it appears the vaccines will prove effective for at least a year or two — long enough that people who are vaccinated can take a breather from the pandemic before worrying about when they might be vulnerable again.
Siegel said boosters may end up being targeted to certain groups rather than as a blanket approach. People who are immune-compromised and didn’t have a strong reaction to initial vaccination may need another shot sooner than others, for example. Or public health officials may decide that people who got the somewhat less effective one-dose Johnson & Johnson vaccine should get a booster before anyone else.
But it’s also important to consider the global landscape, Siegel said. With such powerful vaccines widely available in the U.S. and already more than half the population at least partially vaccinated, it might not be ethical to deliver boosters until other parts of the world have started to catch up.
“Maybe if vaccines were unlimited, people would want a booster,” Siegel said. “But right now, immunity is good enough that from an ethical standpoint, we should be sending vaccines to people who don’t have them.”