Researchers from Brigham and Women’s Hospital, in collaboration with Danish researchers, have identified two types of electronic letters that resulted in modest but “statistically significant” increases in influenza vaccination rates, especially among patients who had not received the flu vaccine the previous year, in a study to evaluate best strategies for increasing such rates.

The study, published in The Lancet, included nearly 1 million Danish adults over age 65 and tested nine different electronic messaging tactics. In particular, the trial found that emphasizing the potential cardiovascular benefits of vaccination and a repeat-letter strategy with a 14-day follow-up letter resulted in 0.89% and 0.73% increases in vaccinations, respectively, compared with a usual-care group. 

“This was a first-of-its-kind nationwide implementation trial to help address the very topical issue of overcoming vaccine hesitancy at a national level,” says co-author Muthiah Vaduganathan, MD, of the Division of Cardiovascular Medicine, in a press release. “Electronic letters are a low-cost intervention that can reach across a population and have meaningful implications. Our efforts show that mass communication systems are feasible and can be rigorously tested, and we believe similar models should be tested in the US and other countries as well.”

The usual-care group received a letter typically sent by the Danish Health Authority under Denmark’s universal health system, which provides free influenza vaccines. Notably, those without influenza vaccination in the prior season had a 2.46% absolute increase in vaccine uptake with the cardiovascular-themed letter, highlighting that this messaging strategy could be an effective intervention in a vaccine-hesitant group.

Vaduganathan, co-director of the Brigham’s Center for Cardiometabolic Implementation Science, worked with other Brigham researchers, investigators at the University of Copenhagen, and others to design the study’s electronic messages according to behavioral science principles. 

During the 2022-2023 influenza season, the study team worked directly with the Danish Health Data Authority and Statens Serum Institute, the Danish authority responsible for infectious disease preparedness, to implement the trial, called the Nationwide Utilization of Danish Government Electronic Letter System for Increasing InFLUenza Vaccine Uptake trial.

“Even small increases in influenza vaccination rates may have important public health implications when applied at a population level, including prevention of numerous deaths, hospitalizations, and illnesses,” says first author and coordinating investigator Niklas Dyrby Johansen, MD, of the Center for Translational Cardiology and Pragmatic Randomized Trials at the University of Copenhagen, in a press release.

While the observed improvement in vaccine uptake was slight, its potential impact is high while being extremely low-cost. According to US Centers for Disease Control and Prevention data, a 0.89% absolute increase in vaccination rate could prevent 7,849 illnesses, 4,395 medical visits, 714 hospitalizations, and 66 deaths each year in the US. If 112 electronic letters are needed to encourage one additional vaccine, the cost of encouraging each additional vaccine is less than $5, according to the researchers.

Of note, important differences between Denmark’s universal health system and the US health system may limit the translation of these results. At baseline, Denmark’s vaccination rates in adults over 65 are approximately 75%, higher than in the US, and attitudes toward vaccination may differ. 

Additionally, the US does not have a centralized communication platform analogous to the one used by the Danish government for priority notifications. However, the authors note that a large proportion of the US population could be reached with text-message platforms typically used to alert populations of local emergencies, as well as messages deployed by large insurers to their covered populations.