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Flu Shots and Ischemic Stroke Risk in Older Adults

Recently published research reveals an important benefit for middle-aged and older adults: a reduction in ischemic stroke risk.

Sara Rodríguez-Martín, PharmD, PhD, of the department of biomedical sciences pharmacology sector in the school of medicine at the University of Alcalá (IRYCIS) and the clinical pharmacology unit at University Hospital Príncipe de Asturias, both in Alcalá de Henares, Madrid, Spain and colleagues conducted the research on the flu shot and ischemic stroke (IS) risk.

Between 2001-2015, researchers used a Spanish primary care database to gather information about adults who had been in the registry for at least a year and were between the ages of 40-99 years. Study participants had no history of cancer or stroke. A total 3.8 million people met the criteria for the study; of these, researchers found 14,322 cases of IS, in addition to selecting 71,610 controls.

Researchers classified IS cases as non-cardioembolic (9542/41.4% vaccinated) or cardioembolic (4780/40.5% vaccinated). “Five controls per case were randomly selected, individually matched with cases for exact age, sex and date of stroke diagnosis (index date). A patient was considered vaccinated when he/she had a recorded influenza vaccination at least 14 days before the index date within the same season,” researchers wrote.1 They used pneumococcal vaccination as a negative control.

More vascular risk factors, vascular disease, and co-medication were found in vaccinated study participants, the researchers said. “After full adjustment, the association of influenza vaccination with IS yielded an AOR of 0.88 (95% CI: 0.84-0.92) was found, appearing early (AOR 15-30 days= 0.79;9 5% CI: 0.69-0.92) and slightly declining over time (AOR >150 days= 0.92; 95% CI: 0.87-0.98),” the researchers wrote.1 They added that the risk was similar for both types of IS cases in all the subgroups: subjects with medium or high vascular risk, men, women, and those over the age of 65 years during pre-epidemic flu, epidemic flu, and post-epidemic flu periods. Researchers reported no associated reduction of IS risk with the pneumococcal vaccine.

Researchers said the results, “are consistent with a moderate protective effect of influenza vaccine on IS in the general population, which is consistent with other published studies. The fact that a reduced risk was also observed during pre-epidemic periods is, however, intriguing and suggests that other mechanisms different from the prevention of influenza infection may play a role,” They emphasized that the rate of flu vaccination is still too low and should be encouraged.

William Schaffner, MD, professor of preventive medicine in the department of health policy and professor of medicine in the division of infectious diseases at the Vanderbilt University School of Medicine in Nashville, Tennessee and who is unaffiliated with the study spoke with Drug Topics® about the study’s results and its impact.

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Schaffner said over the past decade information has accumulated that influenza infection invokes an inflammatory response. “After patients recover from acute infection this response lingers, which affects the small blood vessels that go to the heart and brain and put older patients at increased risk for heart attacks and strokes. This is new information – physicians were not taught this in medical school,” he said.

Schaffner added that it has been postulated that vaccines can prevent cardiovascular events. “This is a study that shows the statistical significance that influenza vaccinations indeed prevent post-influenza stroke in middle-aged and older people. We are just beginning to understand these post-infectious complications. The inflammation response takes longer to wind down post-infection than previously thought,” he explained.

“This adds to the imperative for patients to get vaccinated for influenza each year. Pharmacists can share this information with older patients as another way to encourage them to get the shot,’ Schaffner said.

Reference

1. Rodríguez-Martín S, Barreira-Hernández D, Gil M, García-Lledó A, Izquierdo-Esteban L, De Abajo FJ. Influenza vaccination and risk of ischemic stroke: a population-based case-control study [published online ahead of print, 2022 Sep 7]. Neurology. 2022;10.1212/WNL.0000000000201123. doi:10.1212/WNL.0000000000201123.


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