Electronic reminders to encourage older adults in Denmark to get vaccinated against flu may have unintentionally lowered uptake of the COVID-19 vaccine slightly, but not enough to hamper clinical outcomes, according to a research letter published in JAMA Network Open.
Researchers in Denmark and the United States and vaccine manufacturer Sanofi evaluated rates of COVID-19 vaccination, confirmed SARS-CoV-2 infection, and infection-related hospitalization among more than 964,000 participants in the 2022-23 Northern Hemisphere respiratory virus season.
Letter slightly boosted flu vaccine uptake
The study was a secondary analysis of the NUDGE-FLU (Nationwide Utilization of Danish Government Electronic Letter System for Increasing Influenza Vaccine Uptake) trial, which found that electronic letters emphasizing the potential cardiovascular benefits of flu vaccination and repeated messaging increased flu vaccination rates by about 1 percentage point.
That study randomly assigned all eligible adults aged 65 years or older in Denmark to usual care or one of nine different electronic letters designed according to behavioral science principles and delivered through the government's electronic letter system. The average age among the 964,870 participants was 73.8 years, and 51.5% were women.
"Seasonal vaccination against influenza and COVID-19 is widely recommended by public health authorities and major professional societies, but vaccination uptake remains suboptimal among high-risk individuals," the authors of the new study wrote. "Vaccination implementation strategies have traditionally focused on promoting single vaccines, and potential off-target effects on uptake of other vaccines have not been well elucidated."
No differences in clinical COVID outcomes
In total, 87.7% of participants were vaccinated against COVID-19 in the 2022-23 flu season. Of the 691,820 adults included in the analysis set, 86.3% received COVID-19 vaccination.
Receiving any intervention letter encouraging flu vaccination was tied to slightly lower odds of being vaccinated against COVID-19 than receiving usual care (86.16% vs 86.52%; difference, −0.36 percentage points). Similar results were seen for most intervention letters, but the two letters that increased flu vaccination rates (cardiovascular-gain letter and repeated letter) didn't significantly lower COVID-19 vaccination rates.
Our findings serve as an important reminder that close coordination with other public health efforts and contemplating potential off-target consequences should be considered integral when designing specific public health messaging campaigns.
There were no differences in COVID-related clinical outcomes between participants receiving an intervention letter and those assigned to usual care.
A total of 9,619 of 345,493 (2.78%) letter recipients and 9,577 of 346,327 in the usual-care group (2.77%) contracted COVID-19 (hazard ratio [HR], 1.01). COVID-19 hospitalization occurred in the same proportion (0.30%) of letter recipients and those receiving usual care (HR, 0.98).
"To our knowledge, this is the first evidence in the context of a randomized clinical trial suggesting that implementation strategies promoting one form of vaccination may have unintended consequences on the uptake of other preventative interventions," the researchers wrote.
"Even though the observed absolute decrease in COVID-19 vaccination was only modest and the statistical significance of our findings may have been primarily due to a large sample size, our findings serve as an important reminder that close coordination with other public health efforts and contemplating potential off-target consequences should be considered integral when designing specific public health messaging campaigns," they added.
