Three big questions loom as the COVID-19 vaccine rolls out. Next, will the disease still be spread by someone who has been vaccinated? Second, as the virus itself develops, will the vaccine remain effective? And third, how long will the safety of a vaccine last?
In our immune systems, answers to these questions lie. And since our immune systems are both remarkably adept and remarkably difficult to anticipate, the responses are not straightforward.
Let’s begin with the first question about whether the disease can still be transmitted by people who are vaccinated. That’s not only an open question for this vaccine, but for vaccinations in general, says Marion Pepper, an immunologist at the University of Washington.
“I think it’s hard to tell because multiple pathogens are constantly bombarding us and we don’t know when the immune system is responding,” she says. Maybe we’re having diseases that don’t make us sick, so we never know about them. Yet we may be transmitting illnesses.
The immune system gears up to create antibodies that directly target the virus when a person is infected or inoculated with a vaccine. Those antibodies naturally wane over time. But there’s always a recollection of the virus in the immune system, and if it ever shows up again, cells will spring into action and start making a fresh batch of antibodies. The process can take three to five days, however.
In the meantime, a virus in the body will theoretically begin to replicate.
“It’s a bit of a race between the immune system and the virus,” says Dr. Michel Nussenzweig, a Rockefeller University researcher at the Howard Hughes Medical Institute.
If the immune response kicks in rapidly, it will contain very little virus. Your capacity to spread illness “is really a function of the amount of virus you generate,” says Nussenzweig.
It seems likely that the immune system of an individual would win the arms race, but scientists do not have the evidence yet to confidently claim that. That is why it is always important for people who have been vaccinated to wear a mask and take other precautions before it is sorted out.
Another wild card here is that a population of so-called T cells are found in your lungs and nasal passages, which are primed to recognise cells that have been infected with a virus. Since they remain inside tissues, this sort of T cell is much harder to research, so scientists testing blood samples don’t end up seeing them.
Since these T cells are prepared to respond immediately, they can also help bridge the difference between the time you get infected and the time your immune system is able to mount a full antibody response.
Stephen Jameson, an immunologist at the University of Minnesota School of Medicine, says, “In influenza, certain T cells that are embedded in the tissue may have a drastic effect of restricting the infection.” But if they succeed in COVID-19 as well, he says, “we don’t really know enough yet.”
It is more difficult to address the second question on whether the vaccine can remain successful even as the virus develops. So far, scientists are not too worried about the new strains of the virus that are circulating worldwide. Vaccines are clearly still working against them. But the virus, with unknown effects, will begin to morph.
“While everyone is obviously worried about the evolution of a virus, over time, your memory B cell responsiveness often develops,” Pepper says.
Memory B cells, since they recognize an infection, are an essential part of the immune system. These lurk in your bone marrow, and if the virus they “know” appears in your body, they are ready to morph into antibody-producing cells.
But they don’t just remember one particular antibody that has operated in the past against a virus. New antibodies that are identical and may be more effective against a strain of virus that your body has never seen can also be produced randomly.
“This is about the only time in the body that mutations are purposely inserted into the DNA by a mature cell,” Pepper says.
However remarkable as this method is, there are limitations to it. This mechanism can be outmaneuvered by viruses that experience major changes from one year to the next, including the flu. That’s why, every year, you need a new flu shot. Far slower than the flu, the coronavirus that causes COVID-19 mutates, but it is not yet clear if memory B cells would be sufficiently adaptable to keep the virus in check permanently.
Your immune system may have a very long memory in certain cases.
“You can get lifelong immunity from certain natural infections,” Jameson says. “You only get it once and for the rest of your life, you’re safe.”
To cause an immune response, vaccines imitate a natural infection. But in order to maintain the immunity high, vaccinations can need a boost. For instance, the memory B cells targeting the coronavirus that causes COVID-19 do not have the staying power of the cells that protect us from measles. So far, scientists have found that after the COVID-19 case, these memory B cells persisted for several months, but it is too early to say much about whether they would eventually fade.
“The positive thing is that if it turned out there was a drop in the immune response, there would be an opportunity,” Jameson says. “Then, like many other vaccinations, maybe… after a year or something, you’ll get another booster.”
These questions show how much scientists in recent years have come to learn about our immune system. COVID-19 also illuminates what we also don’t know about how we are shielded from infectious germs by the immune system.
“Seeing this happen in real time has been really fascinating,” Pepper says, “because we’re learning so much about this virus and the immune response to it, in a way we’ve never done before.”