Vaccine side effects signal their effectiveness

A child and man receive vaccines.

Adults hesitant about getting a COVID-19 vaccine or flu shot due to common side effects should be reassured that mild to moderate symptoms are normal and signal the body is learning how to respond to each virus, according to two NHLBI-supported studies.  
Through a paper published in the European Journal of Heart Failure, researchers assessed the impact of how 5,210 adults with heart failure responded to a regular or high-dose flu vaccine, which is provided to adults ages 65 and older. More than one-third of adults, almost 38%, experienced side effects, like having a sore arm or feeling tired. These minor symptoms foreshadowed vaccine effectiveness. Compared to adults with heart failure who didn’t report side effects, those who did were less likely to be hospitalized for any cause or die prematurely. One exception was adults who experienced side effects that were severe, which affected 2.9% of study participants and were associated with an increased risk for hospitalization or death.  
In a letter published in JAMA Network Open, researchers echoed similar findings about COVID-19 vaccines. After partnering with 928 middle-aged to older adults from the Framingham Heart Study who received a Moderna or Pfizer vaccine, researchers found six in 10 study participants experienced mild to moderate symptoms. Common examples included having a fever, chills, muscle soreness, or feeling tired. However, regardless of symptom onset, nearly all adults had increased antibody production – a sign the vaccines were working. Some adults, like women and people with prior COVID-19 infections, were more likely to report side effects. While more research is needed to understand these differences, the authors underscored that common side effects signal the immune system is responding. They also conclude the findings support ongoing recommendations for COVID-19 booster shots in older adults.