As COVID-19 vaccinations open up for kids and booster-seeking adults, some are hesitating not because of the vaccine itself but because of the tool that delivers it. A study earlier this year in the United Kingdom found that adults who reported a fear of needles or blood were twice as likely as others to be hesitant about getting a COVID-19 vaccine.
They may be most fearful of their own reaction. Fear of blood and needles is the only phobia that commonly causes fainting, and nobody wants to pass out in the back of a CVS. But researchers say a woozy response to needles may have evolved to protect us from dangerous situations. For being alive in the era of drugstore vaccine clinics, you might have a fainting ancestor to thank.
Fear of blood or needles is common, affecting between 20 and 30 percent of younger adults. It’s more prevalent in women than men, and less common the older you are. A true phobia affects fewer people — only 3.5 percent of Americans in one study. “People very often conflate fear and fainting,” says Dr. Amy Baxter, an emergency medicine physician at Augusta University in Georgia who studies vaccination pain. Although fainting is a symptom of what’s called blood-injection-injury phobia, people who report no fear may still faint around needles.
This is a type of fainting called vasovagal syncope. It runs in families, and it involves the vagus nerve, which travels from the brain all the way to the colon. Usually, “its responsibility is to lower your blood pressure and chill you out,” Baxter says.
Seeing gore or medical procedures triggers some people’s vagus nerves, and other parts of the nervous system, to jump into overdrive. The blood vessels widen and the blood pressure and heart rate plummet. You may feel nauseated and clammy, and your vision may blur. As blood pools in the lower body instead of returning to the brain, you may even find yourself flat on the floor.
This is, obviously, inconvenient. But it’s not dangerous, unless you hurt yourself on the way down. And the consistency of the reaction has made some scientists wonder whether it’s more a feature than a bug in our wiring. Could fainting at danger be an evolved trait?
Other animals aren’t known to faint from fear or from seeing blood or needles. There are mammals with responses that look similar, though, at least superficially. Many animals freeze in place when they’re in danger. Some, like the opossum, vividly feign death.
When animals freeze in response to danger, their heart rates slow down, the way some humans’ do in reaction to a needle. But animals that are freezing or playing dead generally don’t have a drop in blood pressure, which is the key characteristic of a human’s vasovagal reaction, Dr. Giancarlo Carli, a retired professor of human physiology at the University of Siena in Italy, wrote in a paper this year. That suggests a fainting human is not very similar to a freezing animal, he says.
Still, fainting is safe and may represent a “protective heart and brain response,” Carli says. “It is not a disease.”
Blaine Ditto, a professor of psychology at McGill University, agrees that fainting may have been a helpful adaptation somewhere in our evolutionary history. “There are several things that from a survival point of view are better if you faint,” he says.
For one thing, if a person is wounded, a drop in blood pressure means they won’t lose blood as quickly. “You aren’t going to be squirting,” Ditto says.
Lying on the ground also helps blood get back to the brain. In a 2015 paper, scientists pointed out that the human brain demands a huge amount of blood but unfortunately is at the very top of the body. Maybe, they argued, fainting — or taking on a “gravitationally neutral position” — is the brain’s tool for self-preservation in a dangerous moment.
The response is less helpful when the danger is not a lion or a bear but a nurse. Fear of fainting can keep people from seeking medical procedures and dental work and from donating blood, Ditto says. “Nobody likes the idea of losing control.” But, he adds, fainting “has nothing to do with any kind of physical or psychological weakness.”
True fainting in response to a vaccine is uncommon. And there are ways to make it even less likely. For people squeamish about donating blood, Ditto has studied a technique called applied muscle tension, in which people repeatedly squeeze the muscles of their legs, arms, and abs before or during the donation.
Similarly, Baxter recommends clenching your stomach muscles while you’re getting a shot, then blowing out through pursed lips. If you do feel faint, lifting one knee and hugging it can help push blood back up to your head, she says.
Distraction is especially helpful for kids, Baxter says. But reacting to needles isn’t childish.
“There’s a stigma about weakness in this country,” Baxter says. “Armed with the knowledge that this is your body’s response, not a sign of weakness, people may be more willing to ask for tips and help that exist.”
Elizabeth Preston is a science journalist in the Boston area.