It turns out there’s only one maternity parka in the state of Maryland, which I learned when my friend Caitlin offered it to me. She’d inherited it from her friend Mollie, who inherited it from another friend, and back and back and back. At some point this coat must have been purchased, but that could have been in 2004 or 1977. The style was basic (black, puffy, hooded) and designed for a particular, temporary set of circumstances: You are pregnant, and it is cold.

A week after that conversation my friend Kristy arrived with a box of spandex-waisted jeans. My sister-in-law mailed roomy pullovers; a neighbor brought by two sundresses along with a bag of greens from the farmers market.

My friend Danielle was due the same month I was but still went to the post office to send her favorite sweater from earlier pregnancies, saying Texas weather wasn’t cold enough to warrant her keeping it. That’s what she said, but Texas this year had freak snow and ice storms that left much of the state frozen without power for weeks, including Danielle’s house, so I think she did need the sweater but decided that I needed it more. She knew that while I hoped this would be my first child, it wasn’t my first pregnancy; the others ended in miscarriage, and so I didn’t have the optimism to buy my own maternity clothes.

Have you ever heard of that alleged Ernest Hemingway story — the one written to prove tremendous pathos could be packed into just six words? “For sale. Baby shoes. Never worn.” One way to guarantee that you won’t end up selling unused baby shoes was to refuse to buy them in the first place.

These borrowed clothes, though? They had been worn, and worn well. They were the charmed garments of women with successful outcomes. Maybe they would protect me.

These were absolutely ludicrous things that I told myself, mantras that I realized were ludicrous even while repeating them.

In the middle of my second trimester, trucks began hauling vaccine all around the country, to hospitals and clinics and repurposed stadiums. Grandchildren helped their grandparents book appointments, churches organized vans, Krispy Kreme organized giveaways.

Five percent of the population got the shot, then 15, then 30; meanwhile my pregnancy app said I was carrying a pear, then a pomegranate, and then a pineapple in my borrowed maternity clothes. And I realized that science had given me a vocabulary for the superstition I had developed about my lucky wardrobe: I was trying to achieve herd immunity.

Aren’t we all? Dr. Fauci gave a magic number, 70 percent, and suddenly there was a way to quantify how much of America needs antibodies before everyone gets to be safe. Suddenly every person you meet — the cashier, the mechanic, the chiropractor, the soccer dad — is someone who is either working toward 70 percent or is not, someone who can either protect you or can’t, or who isn’t even inclined to try.

“You go ahead and get one,” a vaccine-hesitant acquaintance told me back in April. “But if it really works, then I shouldn’t have to get one, too.”

But that wasn’t how it worked, I sputtered. Public health doesn’t work if we consider only our own immune systems. Some people won’t generate antibodies in response to the vaccine. Some aren’t eligible to receive it. Every new infection gives the virus an opportunity to change in a way that will make other people’s vaccinations less effective. The goal isn’t for me to do what’s best for me; the goal is to do what’s best for everyone, including a bunch of people I’ve never met. That’s how we open schools and eliminate travel restrictions and allow hospital visitors. That’s how we get to the other side of this — by being part of a herd.

And that became the question of the back half of the pandemic: Do you believe you are part of a herd? Do you believe our fates are connected? Do you acknowledge that if you have a driver’s license, the government already has more private information about you than it would if you join a vaccine registry? Can you understand that if you say you’re waiting for more data on vaccine safety, you are also making other people less safe?

Herd immunity isn’t just a goal, it’s also a mind-set. It’s a mind-set that says we are in this together because the alternative, that we are alone in our giant maternity parkas, is almost too horrible to bear.

I have never spent more time thinking about vulnerability than I have the past eight months. I thought about it when vaccine was released but not yet recommended for pregnant people, which meant relying on others to make appointments that I couldn't make myself. And yet, I was still pregnant.

I thought about it when, with still-muddled data, I signed up for Pfizer anyway, and then signed up for the CDC’s self-reported study of pregnancy and the vaccine, and then checked the box saying that my arm still hurt and I still had body aches and chills but, yes, I was still pregnant.

I thought about it when mask recommendations were lifted for the vaccinated, wondering how many of the bare faces I was now seeing belonged to truly vaccinated people and how many belonged to those who never planned on getting the shot and took the news of relaxed masking rules as a free pass. It didn’t matter so much for me right now, but it did for the millions of children and immuno-compromised adults. It would for me, later, when toting an unvaccinated baby, because for now I was still pregnant.

Here is where my metaphor of spandex pants and puffy coats and Pfizer-induced night sweats completely falls apart. Because borrowed maternity clothes are tangible, but their magic is symbolic. Vaccine immunity is intangible, but the magic (which is to say, the science) is real.

The only thing they have in common is that both represent the best kind of herd mentality: A desire to belong to something bigger than yourself. To be grateful for what others have been through and passed along. To want to pass it along yourself — or, in the case of the virus, prevent it from being passed.

Vulnerability can arrive all at once, after all. One day you are a protector. The next, you need protection. Pregnancies end, masks slip, outbreaks break out. What happens to your body often has nothing to do with you but with a series of events or decisions made months ago, back and back and back. You can look at a sudden onslaught of vulnerability as a sign that you are weak, or you can look at it as a sign that you recognize this only works — immunity, society, living, life — when we all see weak spots and immediately rush to surround them with strong ones.

You can look at that protection as charity, or you can look at it as a patriotic duty, or you can look at it, as I came to, as love.

In my ninth month of pregnancy, here is what I think about the borrowed outfit I am wearing, assembled from the hand-me-downs of three different friends. I think they are not magical clothes. Of course they’re not. There is nothing I could wear or not wear, buy or not buy that would change any outcome.

But damned if I didn’t love putting on Caitlin’s pants. It’s not the pants that are lucky, but the messaging behind them — that if you have a herd, they will keep you immune, in body and soul, however they possibly can.