Midlife influenza vaccine not associated with increased risk of Parkinson’s disease, study shows

Receiving an influenza vaccine between the ages of 40 and 50 does not increase the risk of Parkinson’s disease, according to the results of a recent study published in JAMA Network Open.

While there was no increased risk of Parkinson’s disease, researchers noted that there was a decrease in the estimated prevalence of the disease eight years after vaccination, which may suggest that the vaccine has a protective effect against Parkinson’s disease. However, further research is needed, according to the authors, including Paul Brassard, M.D., from the Centre for Clinical Epidemiology at the Lady Davis Institute in Montreal.

Brassard and his colleagues studied the electronic medical records from the UK’s Clinical Practice Research Datalink Aurum. They identified 1,191,209 individuals, of whom 612,974 received a flu vaccine between the ages of 40 and 50 from 1995 to 2017 and 578,235 who did not receive the immunization during that time. Participants were followed up with for approximately 30 years. Specifically, Parkinson’s incident rates were 0.16 cases per 1,000 person-years in vaccinated adults, compared with 0.10 cases per 1,000 person-years in unvaccinated adults.

Parkinson’s disease affects more than 11 million people worldwide, making it the second most common neurodegenerative disease worldwide. In the United States, it affects 1 million people alone. The disease causes inflammation, which kills dopamine-producing neurons in the brain, specifically in the basal ganglia, the area of the brain that controls movement.

Motor symptoms such as shaking, stiffness and difficulty with balance appear when approximately 60% to 80% of dopamine-producing cells are lost. This may eventually lead to difficulty walking and talking or behavioral changes such as depression, fatigue or memory difficulties.

The cause of Parkinson’s is unclear, but previous research has suggested that infections from bacteria, viruses or fungi may be to blame. The influenza virus is one of the most prominent pathogens, and some studies have suggested it degrades dopaminergic neurons because of the inflammation the virus causes. Specifically, one study found that influenza infection was associated with up to a 73% increased risk of Parkinson’s 10 years after infection.

One of the earliest arguments in favor of this hypothetical connection is the rise in postencephalitic Parkinsonism cases after the 1918 influenza pandemic that lasted from 1918 to 1919. Data shows that people born during this time had a “two-to-three-fold increased risk of developing Parkinson’s, compared with those born before 1888 or after 1924.” Then, in 1969, a patient with Parkinson’s had improved motor symptoms after being given amantadine, which is used in influenza treatment.

“Future studies should ideally involve even larger sample sizes that would facilitate analyses stratified by year to account for the outcome heterogeneity potentially resulting from the continual evolutionary changes in influenza viruses and the varying effectiveness of influenza vaccines,” Brassard said in the study.

For example, Parkinson’s incidence rates have been shown to rise after other infections as well. The same study that found a 73% higher risk between influenza and Parkinson’s also found a 19% higher risk of Parkinson’s in patients with a urinary tract infection 10 or more years before their diagnosis.