Getting a flu shot is a smart choice. While safe for most people, the flu shot may cause side effects -- most of which are mild and don't interfere with your daily activities. It is relatively common after receiving the shot to experience soreness or redness at the injection site. Severe side effects are extremely rare. Flu shots come in different forms that are either injected into a muscle or into the skin. Neither type of shot contains live flu virus, and their side effects differ slightly.
Injection Site Reaction, Intramuscular Shot
The most common side effect of the intramuscular flu shot is soreness at the injection site, according to a September 2013 "Morbidity and Mortality Weekly Report" from the Centers for Disease Control and Prevention. The authors report that side effect occurs in 10 to 64 percent of people who receive an intramuscular flu shot. Soreness is typically mild, does not interfere with daily activities and lasts less than 2 days. Injection site reactions are more common with high-dose flu shots given to seniors. A July 2009 "Journal of Infectious Diseases" article reported that injection site soreness occurred in 36 percent of seniors receiving the high-dose shot compared to 24 percent in those who received the standard-dose shot. However, this soreness is still reported to be mild.
Injection Site Reaction, Intradermal Shot
The intradermal flu shot is injected into the skin and has slightly different side effects than the intramuscular flu shot. According a September 2012 article published in "Influenza and Other Respiratory Viruses," there is no difference in side effects between intradermal and intramuscular flu shots within the first 3 days after injection. However, the rate of redness and swelling at the injection site within 7 days is higher with the intradermal shot than with the intramuscular injection. Another study published in November 2015 in the journal "Vaccine" found that among people receiving an intradermal flu shot, 86 to 92 percent reported redness, 50 to 59 percent reported swelling and 33 to 45 percent reported itching at the injection site.
Generalized symptoms after a flu shot, such as fever or headache, are uncommon and usually classified as mild to moderate. The authors of the November 2015 "Vaccine" study found that in people receiving the intradermal vaccine, 9 to 15 percent reported feeling ill and 10 to 13 percent reported headache on at least one day in the week after the injection. However, it's not clear whether these symptoms had any relationship to getting the flu shot. According to CDC, people who receive the intramuscular flu shot are no more likely to experience generalized symptoms after the vaccine than are people who do not receive a flu shot.
Warnings and Precautions
CDC notes that serious reactions to the flu shot -- defined as those causing hospitalization, life-threatening illness, permanent disability or death -- are extremely rare and occur in roughly 1 to 2 people out of every million vaccinated. Possible symptoms of an allergic reaction include an itchy rash or hives, facial swelling and difficulty breathing. These symptoms typically develop within a few minutes to hours after the shot. The likelihood of developing Guillain-Barré syndrome -- a neurological condition resulting in temporary paralysis -- is roughly 1 in a million, according to CDC. If you believe you may be experiencing a serious reaction after a flu shot, seek immediate medical attention.