How long does immunity last?
It all depends on the virus. And for new viruses, there's much we can only know with time.
Welcome to Not a Doctor, the only newsletter about health and science that compares Covid-19 to chardonnay.
I’m Melody Schreiber, a journalist and the editor of What We Didn’t Expect: Personal Stories About Premature Birth (out in November!). I’m not a doctor, or a scientist, or really an expert of any kind. I just like to ask questions and try to find the answers to them.
Today, we’re talking about immunity: how long it lasts, how it’s different from disease to disease, and why that matters.
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Like a fine wine, viruses get better with age (up to a point)
Earlier this year, my family had a nasty bout with RSV. It landed one of us in the hospital, and it took weeks for us to get back to normal.
RSV is a respiratory virus that can knock you out the first time you get it — lots of snot, coughing, high fever, the works. It’s especially dangerous for kids under 2. There’s no vaccine or cure for RSV, although there is a medicine given to very vulnerable children to reduce their chances of getting sick.
For most people, after they’ve already encountered RSV once, subsequent infections are really mild; it just feels like a cold, although sometimes it’s a nasty cold. That’s part of why the virus continues to spread every year; most people who have it don’t realize what it is, and how dangerous it can be for those are vulnerable to it. The elderly are included in these vulnerable populations; although they have usually been exposed to many different viruses in their lifetimes, other immune difficulties can change the game.
A few months after our tangle with it, RSV circulated through our community again. But this time, all we had were some minor sniffles. What made the difference?
When we get sick, our bodies produce antibodies to fight off infection. As long as you have those antibodies, you either don’t get sick again, or you get less sick.
But eventually, after a few months or weeks, our bodies start to chill out. We’re no longer infected with an active case, so why spend our precious time making all these antibodies?
However, that doesn’t mean we’ve lost our immunity. Our bodies have a wild card: T-cells. Also called “memory cells,” these cells are now trained to recognize the virus and spur the production of antibodies way faster and more efficiently than before.
Sometimes, if you fight off a virus once, you have immunity to it for years and years. Other times, as with dengue fever, you actually get sicker when you get it the next time, because of your body’s stronger reaction to a recognized threat.
In the case of RSV, we’re not protected entirely after our first brush with it, because this is a virus that continues making us sick even if we encounter the exact same strain again. But usually you don’t get as sick the next go-round. If you were hospitalized before, now you might just need some cold medicine.
Photo: Indi Samarajiva
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What does that mean for immunity to a new virus?
As you might have guessed, this process has implications for SARS-CoV-2, the virus that causes Covid-19.
Because SARS-2, as I like to call it, is a new virus, we don’t know exactly how it works yet. Scientists have done some truly impressive work over the past few months that will have implications for other respiratory illness, too; for instance, RSV and the flu may also be spread asymptomatically and through the air.
But there are some things you can only tell with time — and how long immunity lasts in one of them. Right now, the oldest cases of Covid-19 are in China, so some of the leading research on immunity is also being done there.
According to a recent study from Wuhan published as a preprint (which means it hasn’t been peer-reviewed or published in a reputable journal, so grain of salt), those who have confirmed, symptomatic Covid-19 cases have immunity for at least six months.
There’s nothing particularly magical about that six months except that’s how long researchers have had to study it and then publish their results. Later, that time might be extended to eight or ten or twelve months, as more time passes. It’s possible immunity to SARS-2 will last even longer, though immunity to other coronaviruses usually fades after a year or so.
On Friday, the U.S. CDC made some waves when it said people who had confirmed Covid-19 cases were immune for “up to” three months. But that’s simply because in the U.S., we only have research on immunity for the past three months or so. The CDC should have said “at least” three months. If the research from Wuhan holds up, that will likely be revised to six months or more.
One worry about SARS-2 has been the rapid disappearance of antibodies, especially in people who had milder infections. Does that mean they’re not immune? According to the Wuhan study, no.
“Given the debate concerning the duration of antibody responses in asymptomatic patients, we wondered if non-severe and severe COVID-19 cases might differ concerning their humoral immune responses,” the researchers wrote. “Interestingly, their overall responses were very similar.”
It’s not clear from this study, which only looked at symptomatic patients, if those who show no symptoms at all have immunity.
But another study, published last week in the journal Cell, found that T-cells specific to SARS-CoV-2 were present even in family members who had mild or no symptoms and were negative for antibodies in a blood test. That’s extremely encouraging news!
It’s important to note that those T-cells were specific to SARS-2, though. While some studies have shown that T-cells from other coronavirus infections — like the common cold — recognize SARS-2, that doesn’t mean they’ll protect us from infection. And it definitely doesn’t mean we shouldn’t take actions that are proven to limit the spread — from wearing masks to keeping a distance.
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In my next update, I want to talk about what all this means for you. If you had Covid-19 symptoms but no positive test, should you get an antibody test? What are the limits of those tests? And what does all of this mean for vaccines against everything from SARS-CoV-2 to RSV?
As always, please leave a comment or reply to this email if there’s something I’ve overlooked or something you’d like clarification on. Immunologists spend their whole lives examining these issues, so inevitably I’ve messed something up in translating their work into simple terms — please let me know if you spot any errors!
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