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What Is “Natural Immunity”? And Why Should You Get the Vaccine Even if You Already Had COVID?

How immunity works and why you should get vaccinated—even if you had COVID.
August 10, 2021
What Is “Natural Immunity”? And Why Should You Get the Vaccine Even if You Already Had COVID?
Rendering of white blood cells of attacking a virus (Shutterstock)

A theme has bounced around the internet of late in praise of so-called “natural immunity” against COVID-19 produced by previous infection with the virus—opposed in theory to the immunity generated by the vaccines. It’s worth unpacking why both the infection and the vaccines provide “natural” immunity against COVID-19, and why it’s so critical to get the vaccine even if you’ve been previously infected with COVID.

First, the most important part: The CDC reported last week that people with immunity from prior infections, but no vaccine, are more than twice as likely to get reinfected with COVID-19 than people who had a prior infection and got the vaccine.

There is enormous benefit to getting vaccinated, no matter your infection history.

So that’s the start. The next step is understanding that a call to rely on “natural immunity” is, first and foremost, a call to get infected with COVID.

What many are calling “natural immunity” is just infection-mediated immunity as opposed to vaccine-mediated immunity. In addition to the risks of the acute infection—which include death—we still know little about “long COVID,” those lingering feelings of difficulty breathing, fatigue, brain fog, or a host of other symptoms that some people experience for months after infection.

Getting the vaccine uses our natural immune systems while side-stepping the risks that come from infection. It also turns out that the vaccines seem to provide better protection against mutating variants. To explore why this is, let’s look at how the immune system works.

Our immune systems are made up of a complex cast of cellular characters with different functions that interact with each other to kill anything that tries to infect us. The same cells do the same tasks whether induced by infection or by vaccination, with the goal of eradicating the invader and protecting as much of our bodies’ tissues as possible.

When something foreign (an “antigen”) enters the body, no matter what it is, the immune system checks it out and responds. Both viruses and vaccines (carefully designed to look like parts or all of those viruses) are antigens that cause a commotion. The mRNA vaccines made by Pfizer and Moderna are designed as little antigen instruction manuals: They instruct cells to manufacture small, harmless versions of part of the COVID virus called the spike protein. And then the same immune systems that have protected us our whole lives then react as though we were being attacked. This is why some people experience side effects the day after receiving a vaccine: Because the body is fighting a mock battle against what it believes is a genuine COVID infection. We feel these immune battles across our bodies as symptoms such as fever, headache, and muscle aches.

Throughout the body, we have cells ready to attack any invader onsite for an imprecise but instant immune response. We also have guard cells that wait and watch for antigens (virus or vaccine) that they can snatch up, process, and transport to special immune system hubs called lymph nodes for a slower but more effective response. When one of these guard cells shows up waving an antigen around, many different kinds of immune cells, including T cells and B cells, receive the message and start their tasks, including going to kill infected cells and passing the alarm message to more and more specialized cells. The cells involved in the defense multiply rapidly in those small spaces. That’s why when you get sick you sometimes have painful and swollen lymph nodes in your neck or jaw.

B cells, the antibody factories, start making just-okay antibodies right away, but they also undergo a flurry of self-improvement where they get really specific for the antigen that was presented. It takes a few days, but then these B cells start pumping out super-advanced antibodies. Often, all this is enough to kill the infection, although as we all know, not always. A few B cells then stick around as “memory cells” for months or years, ready to spring into action with the advanced antibodies right away if the antigen shows up again. That’s why childhood vaccines offer such long-term immunity.

Scientists have recently found that the immune response to the mRNA COVID vaccines is better prepared for the threat we face now—not just the threat from the original variant.

There’s one tiny but critical piece of the COVID spike protein (called the receptor binding domain) that is especially useful to have antibodies against in order to defeat variants like Delta because it works like a key to enter our cells and cause havoc. By just showing the immune system the spike protein, the vaccine gives it the chance to learn that receptor binding domain really, really well.

In the case of a full-blown infection, scientists found that the body had fewer antibodies against that critical piece. It’s like giving someone a study guide of the most important class material versus telling them to reread everything from the whole year for the final exam.

Let’s put the immune system another way: Think of the body as an office building and the immune system as the building’s security team. If a courier arrives at the building with a package, it gets handed off to the same person at the front door no matter what’s in the package. That person carries the package to the same destination in the building whether it’s a bomb or a picture of a bomb. The same cellular security teams are alerted in both cases. The difference is whether they have to deal with an active, live munition, or just a harmless photo.

We’re dealing with an ever-mutating virus that has changed and become more contagious. The vaccine is far safer than the infection, and highly effective at generating our natural immune response. Fear is a profound emotion and it is critical to hear and respect fears about the vaccine. But we have enormous evidence that the vaccine has far less risk in pursuit of natural immunity than infection does. We also know that almost everyone will either get infected or get the vaccine. We all need to choose wisely.

Erika Lynn-Green and Howard Forman

Erika Lynn-Green, MSc, MSc, is an MD candidate at Harvard Medical School. She holds degrees in global health and social policy from the London School of Economics and Queen Mary University of London as a Marshall Scholar. Howard P. Forman, MD, MBA, (@thehowie) is a Professor of Radiology and Biomedical Imaging, Public Health (Health Policy), Management, and Economics at Yale University.