Flu Shot Facts, Part 3 of 5: Is the Flu Shot Safe? Side Effects and Common Myths

This series breaks down "Flu Shots for Older Adults: What to Know & Do for 2026" by geriatrician Dr. Leslie Kernisan, MD MPH, of Better Health While Aging. The original article and its insights are hers; you can read the full piece here.

Worries about safety are one of the most common reasons older adults put off a flu shot. In this part, we look at what Dr. Kernisan says about side effects and the myths that keep people from getting protected.

Myth: "The flu shot gave me the flu."

This is probably the most common flu shot myth, and Dr. Kernisan addresses it directly: the flu shot cannot cause influenza. Inactivated flu vaccines are made with killed virus, so there is no live virus that could cause an infection.

What side effects are normal?

According to the article, the most common side effects are "arm soreness and sometimes redness" where the shot was given. These usually pass quickly.

What about the new mRNA flu vaccine?

Dr. Kernisan notes that mFLUSIVA "does tend to cause more fatigue and body aches after administration than other flu vaccines." That is useful to know ahead of time, so a tired day afterward is not a surprise. Across all the flu vaccines, she emphasizes that serious side effects are rare. (Part 2 covers the four vaccines recommended for older adults.)

Myth: "It is not 100% effective, so why bother?"

It is true that no flu vaccine is perfect. But the takeaway of Dr. Kernisan's article is that for older adults, the benefits of vaccination far outweigh the mostly minor side effects. As we covered in Part 1, even imperfect protection prevents hundreds of thousands of illnesses and tens of thousands of hospitalizations among older adults.

Quick answers

Can the flu shot give you the flu? No. Inactivated flu vaccines contain killed virus and cannot cause influenza.

What are common flu shot side effects in seniors? Arm soreness and sometimes redness at the injection site.

Does the mRNA flu vaccine have more side effects? Per Dr. Kernisan, mFLUSIVA tends to cause more fatigue and body aches than other flu vaccines. Serious reactions are rare.

From the Indigo team

We agree with planning around the shot. If your loved one is getting vaccinated, consider scheduling it on a day without other big appointments, keep them well hydrated, and let their caregiver know so they can keep an eye on how they are feeling.

Coming up in Part 4 of 5: Protection beyond the flu shot, and how the flu and COVID are alike (and different).

Source: Kernisan, Leslie, MD MPH. "Flu Shots for Older Adults: What to Know & Do for 2026." Better Health While Aging, September 18, 2026. https://betterhealthwhileaging.net/flu-shots-in-aging-what-to-know/

This post summarizes an article by Leslie Kernisan, MD MPH, for general education. It isn't medical advice. Talk with your or your loved one's doctor about which vaccine is right for you.