13 March 2023

Risk of heart problems makes Danes get flu vaccination


More Danes got vaccinated for the seasonal flu if they received a nudging letter from the health authorities, which linked the flu and the risk of subsequent heart problems, study concludes. The study is published in The Lancet and is the largest of its kind.


Each year, seasonal influenza accounts for more than 500,000 deaths globally and disproportionately affects certain high-risk groups, including older adults and those with diabetes, heart disease and other chronic conditions, compared to people not in high-risk groups. An annual flu shot is widely recommended to prevent influenza infection or lessen its severity and complications should someone get sick.

New study from the University of Copenhagen and Herlev og Gentofte Hospital shows that flu vaccination rates increased moderately among older Danish adults in September 2022 by making a connection between the flu and the risk of subsequent heart problems.

The study is published in The Lancet and is the largest of its kind and involved all adults 65 years or older in Denmark.

“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,” says Tor Biering-Sørensen, MD, MSc, MPH, PhD, professor, Center for Translational Cardiology and Pragmatic Randomized Trials, Copenhagen University Hospital, Hellerup, Denmark, and the study’s chief investigator.

The researchers expect these increases in flu vaccine uptake to be even more pronounced in countries where vaccination rates are low. Denmark has among the highest flu vaccine rates to start with more than 80% this season.

In comparision only 49 percent of U.S. adults received a flu vaccine during the 2012-2022 influenza season, despite widespread recommendations for annual flu vaccination..

Many countries lag behind

Vaccination rates aren’t nearly what they should be in many countries, and that’s especially true for younger people with chronic diseases like heart disease.

“Figuring out ways to increase the percentage of people who get the flu shot and other vaccines is important. We are looking into other risk groups such as those with diabetes in a separate analysis,” says university Professor and Physician at Rigshospitalet Tor Biering-Sørensen.

“Vaccines help prevent infectious diseases and for flu, specifically, many countries lag well behind the WHO recommendation for over 75% of the population to be vaccinated.”

The NUDGE-FLU trial randomized 964,870 Danish citizens age 65 years and older (across 691,820 households) to receive one of nine different electronic letters that included a specific message about the upcoming flu season and the need for vaccination to see which, if any, would increase influenza vaccination uptake among older adults.

Compared with usual care, influenza vaccination rates were significantly higher in the group receiving a letter highlighting potential cardiovascular benefits of vaccination (81.00% vs. 80.12%) and the group receiving repeat letters about the importance of flu vaccination generally and to let them know about available flu vaccines at the start of the study and 14 days later (80.85% vs. 80.12%).

Waiting for hospitalisation data

Tor Biering-Sørensen explains that they need more research to test the messaging around the flu and related cardiovascular outcomes. Specifically, they are waiting for data to see e.g. a decrease in number of patients in hospital due to seasonal flu or co-morbidities.

“The flu vaccine may have broader benefits that we don’t yet know. 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,” says Tor Biering-Sørensen.

He added that these improvements were accomplished through a simple email that could be replicated, making it a very low-cost intervention that could help prevent flu-related deaths and associated complications.

The study is limited to the Danish population that has free access to influenza vaccination, so the bump in vaccine uptake may be different if cost is a barrier.

The study was funded by Sanofi. This study was simultaneously published online in the Lancet at the time of presentation. http://doi.org/10.1016/S0140-6736(23)00349-5


Professor and Physician Tor Biering-Sørensen
+45 28 93 35 90