Contrary to findings from some previous studies, influenza vaccine does not increase the risk for Guillain-Barré syndrome (GBS), a large new study suggests.
Investigators in Taiwan analyzed 14-year data on more than 13 million older adults who received the influenza vaccine. Results showed that the incidence of GBS did not increase during the analyzed time frames of 7 days, 14 days, and 42 days after vaccination.
The risk also did not increase on the basis of different age groups, sex, and comorbidities.
“For people who are older than 65 years old, influenza vaccinations do not increase incidence of GBS, regardless of an individual’s immune status or underlying health conditions,” lead author Yu-Tung Anton Huang, PhD, a research fellow at the Center for Big Data Analytics and Statistics, Chang Gung Memorial Hospital, Linkou, Taiwan, and co-investigators told Medscape Medical News via email.
The findings were published online September 21 in JAMA Network Open.
Novel Research Method
GBS is a “rare and severe disorder,” typically caused by an “antecedent infection that results in an aberrant autoimmune response targeting peripheral nerves and their spinal roots,” the researchers write. Onset of GBS commonly occurs 1 to 4 weeks following the infection.
“Despite its rarity, GBS is an important topic of discussion when vaccinating against influenza,” owing to a “landmark study” that suggested that the vaccine increased GBS risk up to eightfold, with the highest risk 2 to 3 weeks after vaccination and elevated risk lasting for 42 days, the investigators note.
Findings of other studies have “continued the debate” about whether vaccination increases GBS risk, with mixed findings, they write.
“Given the general administration of the influenza vaccine among older adults and the higher prevalence of GBS among this age group, the safety of influenza vaccination in this population merits attention,” they add.
The researchers used a self-controlled case study (SCCS) design because it is based on a “case-only approach that automatically controls for individual-level, time-invariant confounders” and has “gained recognition as a reliable method for examining the association between vaccination and adverse effects.”
Using Taiwan’s National Health Insurance research database (NHIRD) “may provide a unique advantage in examining the association between influenza vaccination and GBS,” given that more than 99% of the population is covered by this universal healthcare program, they write.
“The original intention of the research was to reduce the public’s fear of vaccine side effects,” the investigators told Medscape Medical News.
“It has long been suspected that GBS may be associated with influenza vaccination, and it is listed as a possible side effect of influenza vaccination,” they said. By using a different research method, they hoped to conduct “more appropriate research” to clarify the issue.
The researchers conducted a population-based, retrospective cross-sectional study using the SCCS approach, with data collected between January 1, 2003, and December 31, 2017, from Taiwan’s NHIRD.
Adults who had received the influenza vaccine and who had been hospitalized for GBS within 6 months of their vaccination were enrolled (n = 374; mean age, 75 years; 57.5% men). Patients who died within 6 months of vaccination were excluded.
The investigators controlled for sex, age, Charlson Comorbidity Index (CCI) severity, and other comorbidities ― notably cancer and autoimmune diseases.
Benefits Outweigh Risks?
Results showed no statistically significant increases in incident rate ratios (IRRs) for GBS during the first 7, 14, and 42 days following vaccination.
| Period (Days) | IRR (95% CI) | P value |
|---|---|---|
| 1 – 7 | 0.95 (0.55 – 1.61) | .84 |
| 1 – 14 | 0.87 (0.59 – 1.29) | .48 |
| 1 – 42 | 0.92 (0.72 – 1.17) | .49 |
Similarly, there were no significant differences in the IRRs for overall risk during days 1 to 42 in subgroup analyses, when stratified according to different covariates.
| Covariate | IRR (95% CI) | P value |
|---|---|---|
| Age: 65 – 74 years 75 – 84 years ≥85 years | 0.93 (0.66 – 1.31) 0.85 (0.58 – 1.26) 1.10 (0.57 – 2.11) | .68 .43 .79 |
| Sex: Men Women | 0.97 (0.71 – 1.33) 0.85 (0.58 – 1.23) | .87 .39 |
| CCI severity | 1.03 (0.77 – 1.38) | .84 |
| Comorbidities: Cancer Autoimmune disease | 0.68 (0.28 – 1.64) 1.10 (0.11 – 10.53) | .39 .94 |
“Older adults have a higher risk of complications from influenza infection and therefore stand to benefit the most from influenza vaccination,” the investigators write. The findings “suggest that the benefit of the influenza vaccination may outweigh the potential concern of GBS risk in this population.”
However, they noted in their email that because the study examined a nationwide, general population in Taiwan, “the effects of different races could not be seen.” More data are needed before the findings can “apply to all locations around the world,” they added.
“Reassuring” Findings
Commenting for Medscape Medical News, Steven Hawken, PhD, senior scientist, Clinical Epidemiology Program, Ottawa Hospital Research Institute, and associate professor, School of Epidemiology and Public Health, University of Ottawa, Canada, said that although the study is “reassuring in showing that influenza vaccination is safe in a large population of older adults, there are some limitations in the approach the authors used compared to other studies of this question.”
For example, the investigators included the first 7 days following vaccination in the risk period, “when it is very unlikely that any GBS hospitalization was caused by the vaccination because it is too soon after the vaccine,” said Hawken, who was not involved with the study.
He added that a “potentially larger issue” is that the researchers excluded individuals who died within 6 months of receiving their influenza vaccination, whether or not they developed GBS following vaccination. Even though this would “represent a very small number of individuals, it would potentially be enough to bias the study,” he said.
Other studies, including those conducted by Hawken’s group, “have shown that although the absolute risk increase in GBS risk due to influenza vaccination is very small ― on the order of 1 in a million, it is real,” he said.
On the other hand, “more importantly, it’s been shown that influenza illness increases the risk of GBS more than the vaccine does, so under many circumstances, receiving the vaccination reduces the net risk of GBS by reducing the rate and severity of influenza infection,” Hawken said.
Because the “absolute risk is extremely small,” the risk is “arguably far outweighed by the benefits of influenza vaccination,” he added.
No source of funding for the study was listed. The investigators and Hawken have reported no relevant financial relationships.
JAMA Netw Open. Published online September 21, 2022. Full article
Batya Swift Yasgur, MA, LSW, is a freelance writer with a counseling practice in Teaneck, NJ. She is a regular contributor to numerous medical publications, including Medscape and WebMD, and is the author of several consumer-oriented health books as well as Behind the Burqa: Our Lives in Afghanistan and How We Escaped to Freedom (the memoir of two brave Afghan sisters who told her their story).
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