Study: Linking Flu Vaccination, Potential Cardiovascular Benefits Increases Vaccination Rates


Vaccination rates were significantly higher in study groups who received a letter highlighting the potential cardiovascular benefits of vaccination and who received repeat letters about the importance of flu vaccination in general.

Connecting the flu and risks of subsequent heart problems significantly increased flu vaccination rates among older Danish adults, according to findings presented at the American College of Cardiology 2023 Scientific Session.

Researchers also found that a reminder message the day before the launch of a national vaccine program also helped encourage people to get vaccinated.

Seasonal influenza accounts for more than 500,000 deaths globally each year and some high-risk groups are disproportionately affected, including older adults and those with diabetes, heart disease, and other chronic conditions. An annual flu vaccine is widely recommended to prevent influenza or lessen its severity and complications. However, vaccination rates are still not nearly what they should be in many countries, including the United States, and the investigators noted that this is especially true for younger people with chronic diseases.

“Figuring out ways to increase the percentage of people who get the flu [shot] and other vaccines is important,” said chief investigator Tor Biering-Sørensen, MD, MSc, MPH, PhD, a professor at the Center for Translational Cardiology and Pragmatic Randomized Trials at the Copenhagen University Hospital, in a press release.

Nudge strategies are messages designed to educate, remind, and push positive health behaviors. Earlier research has tested different strategies to increase flu vaccination in select populations, but this new study is the largest of its kind and involved all adults 65 years of age or older in Denmark.

The NUDGE-FLU trial randomized 964,870 Danish citizens across 691,820 households to receive 1 of 9 different electronic letters featuring a specific message about the upcoming flu season and the need for vaccination. The study aimed to examine which message, if any, would increase influenza vaccination uptake among older adults.

The communications were delivered via the country’s electronic letter system on September 16, 2022. Citizens were either in usual care, meaning they received no letter, or they received 1 of the 9 nudge strategies. In addition to the letter linking the flu vaccine with a reduction of cardiovascular events, the approach sent the standard flu pamphlet 2 weeks before the flu season started and a reminder on the day the vaccine become widely available. The other messages included the following:

  • A clear recommendation from a leading health authority to get vaccinated.
  • A nudge for individuals not to be in the minority and to get vaccinated.
  • A nudge to join the majority to prevent the flu from spreading (a collective goal strategy).
  • The role flu vaccination can play in helping in a pandemic to protect the recipient and their loved ones (a gain framing strategy).
  • The danger of too few people getting vaccinated placing the recipient and their loved ones at risk (a loss framing strategy).
  • A prompt to make a plan to get vaccinated and record their appointment in a space provided.
  • A basic letter without name personalization.

The primary endpoint was the receipt of influenza vaccination on or before January 1, 2023. Although flu vaccination can continue throughout the influenza season, the investigators noted that fall is the ideal time to assure protection.

Compared with usual care, the study found that influenza vaccination rates were significantly higher in the group receiving a letter highlighting the potential cardiovascular benefits of vaccination (81% vs 80.12%) and the group receiving repeat letters about the importance of flu vaccination generally, to let them know about available flu vaccines, and 14 days later (80.85% vs 80.12).

“The only 2 nudge strategies that significantly increased flu vaccine uptake were the simple reminder and explaining that flu vaccination may also prevent cardiovascular events,” Biering-Sørensen said in the press release.

These strategies improved vaccination rates across major subgroups, including participants with cardiovascular disease. The cardiovascular benefits message resulted in a greater increase in vaccine uptake among the participants who had not been vaccinated for influenza in the prior season. This message stated that “in addition to its protection against influenza infection, influenza vaccination also seems to protect against cardiovascular disease, such as heart attacks and heart failure.”

Biering-Sørensen said he expects these increases in flu vaccine uptake may have been even more pronounced in countries where vaccination rates are low. Denmark has among the highest flu vaccine rates to start with, at more than 80% in the 2022-2023 season. In comparison, during the 2021-2022 season, only 49% of the US adults received a flu vaccine despite widespread recommendations for annual flu vaccination.

More research is necessary to test the messaging around the flu and related cardiovascular outcomes given that was the most effective message to boost flu vaccination.

“As cardiologists, it’s very interesting that just telling people that we can also prevent other downstream issues like cardiovascular outcomes was what worked the best of all the nudge strategies—even better than the reminder, which we expected would be positive,” Biering-Sørensen said in the press release. “A lot of studies have shown that people who get the flu vaccine have a lower risk of cardiovascular outcomes, and there may be protective effects [for the heart] that are not specific to flu infection. The flu vaccine may have broader benefits that we don’t yet know.”

Biering-Sørensen said these improvements were accomplished through a simple email, making it a replicable and very low-cost intervention that could help prevent flu-related deaths and associated complications. The investigators will be further analyzing the data to see whether certain nudge messages worked better in those with diabetes and other chronic complications.


Linking Flu Vaccination to Potential Cardiovascular Benefits Gets More People to Roll Up Their Sleeves. News release. American College of Cardiology 2023 Scientific Session. March 5, 2023. Accessed March 8, 2023.

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