Heart disease is one of the most common conditions found in adults hospitalized with influenza in recent flu seasons. Still, individuals with heart disease can be hesitant to receive the yearly vaccine due to adverse reaction concerns.

However, a new study led by Brigham and Women’s Hospital researchers suggests that those with high-risk heart disease who experience mild adverse reactions to the flu vaccine are less likely to experience cardiopulmonary hospitalization or death. The study, published in The European Journal of Heart Failure, was led by Alexander Peikert, MD, a research fellow in the Brigham’s Division of Cardiovascular Medicine. Dr. Peikert presented the findings at the American Heart Association 2022 Scientific Sessions and was featured in a U.S. News article highlighting the study.

Study Findings

The researchers analyzed data on 5,260 patients recently hospitalized for heart failure or heart attack across 157 medical centers in the U.S. and Canada, available from the INVESTED trial. The patients were an average of 65 years old and received either a standard or high-dose flu vaccine. Data for 7,154 vaccinations given over three flu seasons were also analyzed, and the participants were followed until the end of each flu season (up to 10 months). Deaths were recorded up to three years after vaccination.

37.8% of participants experienced a mild, moderate, or severe reaction to the flu vaccine, but 76.4% of those reactions were considered mild. Common side effects included:

• Injection site pain—60.3%
• Muscle aches and pains—34.5%
• Overall discomfort—22%

Reactions involving the injection site were associated with reduced cardiopulmonary or death risks. Those who experienced a mild to moderate injection site reaction had a 19% lower risk of dying or being hospitalized for cardiopulmonary events. There was no difference in hospitalization or death rates for those who received the higher-dose vaccine versus the standard dose.

Interpreting the Results

These findings suggest mild to moderate adverse reactions to the flu vaccine may be a marker of immune response. Individuals with high-risk heart disease who experienced a mild to moderate adverse vaccine reaction had a reduced risk of cardiopulmonary hospitalization or death. The small percentage of participants who reported a severe reaction to the vaccine had a 68% higher chance of hospitalization or morbidity, suggesting those individuals were predisposed to a malfunctioning immune system.

These results only apply to individuals with heart disease and likely do not apply to the general public. Mild to moderate flu vaccine reactions should not deter individuals with high-risk heart disease from receiving the vaccine, as this may be a marker of immune response and indicate a reduced risk of cardiopulmonary hospitalization or death.