Flu Vaccine: How Safe Is It and How Well Does It Work?

Why does getting a flu vaccine matter each year?

In an October 13 university interview, public health professor Denise Hernandez encouraged eligible people to receive their seasonal flu vaccine. Her message emphasized two benefits: lowering the chance of influenza illness and reducing the likelihood of serious complications if infection occurs. For most people in the United States, the autumn vaccination campaign provides an opportunity to build protection before winter activity increases. The university recommends vaccination by the end of October. [University of Texas at Arlington interview](https://www.uta.edu/news/news-releases/2026/10/13/flu-season-is-here-is-it-time-for-your-shot)

The medical reason for annual vaccination is that immune protection fades and influenza viruses continually change. Vaccine composition is reviewed regularly to account for those changes. The CDC recommends seasonal vaccination for everyone aged six months and older in the United States, with rare exceptions. WHO particularly emphasizes vaccination for pregnant people, young children, older adults, people with chronic conditions and health workers. Eligibility and campaign timing should follow local guidance outside the United States. [CDC vaccination basics](https://www.cdc.gov/flu/vaccines/) and [WHO seasonal influenza guidance](https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal))

How effective is the flu vaccine if vaccinated people can still get sick?

The CDC reports that, in seasons when vaccine viruses closely resemble circulating influenza viruses, vaccination has reduced the risk of a flu-related medical visit by approximately 40% to 60%. This describes a relative reduction in a particular outcome; it does not mean that the same percentage of vaccinated people become completely immune. It is also a historical range rather than a prediction for the season beginning this autumn. Studies summarized by the CDC show additional benefits against hospitalization and severe illness. [CDC evidence on vaccination benefits](https://www.cdc.gov/flu-vaccines-work/benefits/)

Protection varies because circulating viruses can change after vaccine strains are selected, and immune responses differ with age and health. Exposure shortly before vaccination, or during the roughly two weeks needed to develop antibodies, can also lead to illness. A breakthrough infection therefore does not establish that vaccination provided no benefit. Researchers assess several outcomes separately, including medically attended illness, hospitalization and intensive care. Studies of hospitalized patients have associated prior vaccination with less severe disease, although observational comparisons can be influenced by differences between vaccinated and unvaccinated patients. [CDC explanation of breakthrough infections](https://www.cdc.gov/flu/vaccines/keyfacts.html) and [CDC evidence review](https://www.cdc.gov/flu-vaccines-work/benefits/)

What are the common and rare risks of flu vaccination?

Common reactions to an injected flu vaccine include arm soreness, fatigue, headache and sometimes fever. They generally resolve within a few days. Injectable vaccines contain inactivated viruses or viral proteins, so they cannot cause influenza. Illness beginning after vaccination may instead reflect an infection acquired before protection developed. The nasal spray uses weakened viruses and has different eligibility restrictions; it is not recommended during pregnancy or for people with weakened immune systems. [CDC vaccine safety information](https://www.cdc.gov/vaccine-safety/vaccines/flu.html) and [CDC vaccine overview](https://www.cdc.gov/flu/vaccines/keyfacts.html)

Serious risks deserve proportionate discussion. CDC monitoring indicates that, in seasons when an increased risk of Guillain-Barré syndrome has been detected, it has amounted to approximately one or two additional cases per million flu vaccine doses. Influenza illness itself is more strongly associated with this neurological condition. Severe allergic reactions are also rare. People with a previous serious vaccine reaction or a history of Guillain-Barré syndrome should discuss vaccine selection with their clinician. Difficulty breathing or swelling of the face or throat after vaccination requires emergency care. [CDC safety findings](https://www.cdc.gov/vaccine-safety/vaccines/flu.html)