Why Flu Shot Reactions Might Not Be a Bad Thing

Having a mild or moderate reaction to an influenza vaccine might not be a bad thing if you have heart disease and are over the age of 65.

In a recent study of more than 5,000 participants, the people who had reactions to an annual flu shot—such as pain at the injection site—appeared to be 19% less likely to be hospitalized for heart or lung problems or to die from any cause.

Alexander Peikert, MD, a postdoctoral research fellow at the Brigham and Women’s Hospital in Boston and Harvard Medical School and the lead author of the recent study, told Verywell that catching the flu can increase health problems and risk of death in people with underlying heart or lung issues, but that “the risk for morbidity and mortality is substantially reduced by [the] flu vaccine.”

In the United States, the flu vaccine is recommended for everyone over the age of 6 months, but not everyone who is eligible gets one. According to Centers for Disease Control and Prevention (CDC) data from last year’s flu season, only about half of people aged 6 months and older got a flu shot.

People often cite reactions as a reason to avoid being vaccinated, but according to Peikert, those reactions shouldn’t be feared.

“It’s part of the physiological response from your body to the vaccine, so people shouldn’t be afraid,” he said.

Flu Vaccine Reactions In Heart Disease Patients

To conduct the study, researchers looked at patients who had recently been hospitalized for heart failure or heart attack, and also had additional risk factors for cardiovascular problems such as diabetes, tobacco use, or kidney disease.

The patients randomly received either a high-dose trivalent inactivated flu vaccine or a standard-dose quadrivalent inactivated flu vaccine for up to three flu seasons between September 2016 and January 2019. Over the course of the study, research monitored adverse reactions within seven days of a flu shot, including pain, redness, or swelling at the injection site, muscle pain, fever, overall discomfort, and headache.

The patients who experienced mild to moderate vaccine reactions had a 19% lower risk of dying for any reason. They were also less likely to be hospitalized for heart or lung problems.

The higher-dose flu shot was more strongly tied to adverse reactions.

Why Flu Shots Help People With Heart Disease

Peikert said that the beneficial nature of flu vaccination in patients with heart disease might be related to the immune response that the vaccine triggers.

“It’s very plausible if you look at the biology,” Peikert said. “Because the immune response after receiving a vaccine is an inflammatory response—it’s called reactogenicity.”

“Influenza vaccine in somebody who has had a first heart attack is just as important as giving them aspirin or a beta blocker or an antihypertensive drug,” Greg Poland, MD, the director of the Vaccine Research Group at the Mayo Clinic and the editor-in-chief of the journal Vaccine, told Verywell. “Would I hesitate to give anybody with an [heart attack] or heart failure the flu vaccine? No, in fact, that’s an indication to give it to them—I would give flu vaccine at the time of discharge from the hospital.”

Why a Flu Shot Matters If You’re Older

If you’re older than 65, a case of the flu can be more dangerous than it would have been when you were younger.

According to Poland, the people who are most seriously affected by influenza are “predominantly people over the age of 65 and who are compromised for any of a long list of reasons, like heart disease, lung disease, endocrine diseases—the list goes on.”

Poland also noted that a severe case of the flu in older people is associated with a loss of independence—meaning they aren’t able to continue living on their own.