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Electronic “Nudges” Linking Flu Vaccination to Potential CV Benefits Boosted Uptake in Older Adults in Denmark



Electronic nudges and reminders highlighting the potential cardiovascular (CV) benefits of influenza vaccination led to a modest uptake in vaccination rates among older adults in Denmark, according to results of the NUDGE-FLU trial.

Findings were presented at the 2023 American College of Cardiology’s (ACC) Annual Scientific Session and published simultaneously in The Lancet.

Despite the effectiveness of influenza vaccination in preventing infection and related complications, and widespread recommendation of its use by public health authorities and clinical practice guidelines, global influenza vaccination rates remain suboptimal. In the US specifically, influenza vaccination uptake was lower from mid-October 2021 to the end of January 2022 compared with the previous season, despite significant public health campaigns surrounding both influenza and COVID-19 vaccination.

“Although countries worldwide implement various influenza vaccination campaigns, these communication strategies have been largely untested in rigorous nationwide implementation trials,” wrote researchers.

Earlier studies have analyzed different “nudge” strategies—messages designed specifically to educate, remind, and push positive health behaviors— to increase influenza vaccination in various populations, but the NUDGE-FLU trial is the largest of its kind and involved all adults aged ≥65 years in Denmark.

Investigators randomized 964 870 Danish citizens (mean age, 74 years; 51% women) across 691 820 households to receive usual care or to receive an electronic letter containing 1 of 9 specific messages about the upcoming influenza season and the need for vaccination to determine which “behavioral nudge,” if any, would increase vaccine uptake among older adults. The letters were delivered via Denmark’s electronic letter system on September 16, 2022. The primary endpoint was the receipt of influenza vaccination on or before January 1, 2023.

The messages delivered in the electronic letters ranged from a basic letter without name personalization to one highlighting the potential CV benefits of the influenza vaccine such as protection against myocardial infarction (MI), heart failure (HF), and other cardiovascular diseases (CVDs), one offering a recommendation from a leading health authority, and one considered a loss-framing note emphasizing that low uptake of flu vaccination places individuals and their loved ones at risk.


At baseline, 27.4% of participants had any chronic CVD, 12.9% had diabetes of any type, and 57.6% had hypertension. During the previous influenza season (2021-2022), 76.9% of participants had been vaccinated against influenza, according to the study results.

Subsequent influenza vaccine uptake was higher in 2 groups compared with the usual care group: those who received the electronic letter highlighting the potential CV benefits (81% vs 80.12%; P<.001) and those who received repeated letters, ie, the first at randomization and a reminder at day 14 (80.85% vs 80.12%; P=.0006).

“The only two nudge strategies that significantly increased flu vaccine uptake were the simple reminder and explaining that flu vaccination may also prevent cardiovascular events,” said chief investigator Tor Biering-Sørensen, MD, MSc, MPH, PhD, professor, Center for Translational Cardiology and Pragmatic Randomized Trials, Copenhagen University Hospital, Hellerup, Denmark, in an ACC press release.

Biering-Sørensen added that these 2 strategies improved influenza vaccination rates across major subgroups, including adults with and without established CVD. The CV benefits message was particularly effective among participants who had not received an influenza vaccination the season before (Pinteraction=.0002).

In the release, Biering-Sørensen said he anticipates these increases in influenza vaccine uptake may have been more pronounced in countries where vaccination rates are low. In Denmark, influenza vaccination rates are high, increasing from 75% in 2020-2021 to 78% in 2021-2022 in adults aged ≥65 years, according to investigators. In the US, approximately 66% of adults in the same age group were vaccinated against influenza as of January 29, 2022, according to the US Centers for Disease Control and Prevention.

Additional research is needed to evaluate the messaging around influenza and related CV outcomes given that the most effective message to boost vaccination was the CV-benefits letter.

“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,” said Biering-Sørensen. “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 and colleagues stated that although the magnitude of effectiveness was modest with the behavioral electronic letters, “the low-touch, inexpensive, and highly scalable nature of these electronic letters might be informative for future public health campaigns.”

The research team noted that because the study was limited to the Danish population, which has free access to influenza vaccination, the increase in vaccine uptake may be different in countries where cost could be a barrier for individuals.

Reference: Johansen ND, Vaduganathan M, Bhatt A, et al. Electronic nudges to increase influenza vaccination uptake in Denmark: a nationwide, pragmatic, registry-based, randomised implementation trial. The Lancet. Published online March 5, 2023. doi:10.1016/ S0140-6736(23)00349-5

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