11/18/2025
First, injectable influenza vaccines cannot cause influenza because they contain only parts of the virus. The live attenuated nasal spray vaccine is modified so that it cannot cause influenza because it cannot replicate at human body temperature. Fewer than 1% of vaccinated people might develop flu-like symptoms, such as mild fever and muscle aches, after vaccination. These vaccine side effects are not the same as having influenza, but people may confuse the side effects with illness.
Other reasons a person might have had influenza illness or an illness that they thought might be “the flu” after vaccination include:
• Protective immunity doesn’t develop until 1–2 weeks after vaccination. Some people who get vaccinated after influenza viruses are already circulating may be infected with influenza before the vaccination can stimulate protective immunity.
• For many people, “the flu” is any illness with fever, cold symptoms or gastrointestinal symptoms. If they get any viral illness, they may blame it on influenza vaccine or think they got “the flu” despite being vaccinated. Influenza vaccine only protects against specific influenza viruses, not all viruses.
• Vaccinated people can get influenza illness despite vaccination. Influenza vaccination is our best available protection against influenza disease; however, vaccination reduces the risk of infection, it does not prevent all infections.
Vaccine effectiveness (VE) varies by age and by season, depending upon the circulating viruses. In most recent seasons, influenza vaccination has reduced the risk of illness by between roughly 40% and 60% in the vaccinated population (including all ages) during seasons when most circulating influenza viruses are well-matched to the vaccine. VE is generally lower for adults age 65 years and older. Influenza vaccination has also been shown to reduce influenza disease severity even if someone does get sick after vaccination, and vaccination reduces the risk of influenza hospitalization and deaths in children and adults. Influenza vaccination also reduces the risk of stroke and acute cardiac events, like heart attack and heart failure, among people with heart disease.
For more information on this topic, go to: www.cdc.gov/flu-vaccines-work/index.html.
You’ll find this answer, as well as other influenza vaccine Q&As, here: www.immunize.org/ask-experts/topic/influenza.
