Flu Vaccine Linked to Lower Risk for Stroke: INTERSTROKE Flu Vaccine Linked to Lower Risk for Stroke: INTERSTROKE

Having had a recent acute febrile illness was associated with an increased risk for ischemic stroke, whereas having received an influenza vaccination was associated with a reduced risk for stroke in a large new case–control study.

“While influenza vaccination is a cost-effective method to prevent influenza, it is also an effective way to reduce the burden of stroke,” study author Christopher Schwarzbach, MD, Ludwigshafen Hospital, Germany, said.  

“Our results therefore encourage the wider use of influenza vaccination,” he concluded.

Schwarzbach presented these data from the INTERSTROKE study on May 5 at the European Stroke Organisation Conference 2022, held in Lyon, France and online.

He explained that acute inflammatory disease is thought to increase the risk for cerebrovascular events, and the seasonality of influenza-like illness appears to be associated with the seasonality of cardiovascular and cerebrovascular events. Previous observational studies have also shown a link between influenza vaccination and a reduced risk for stroke.

The current INTERSTROKE study was a large international case–control study conducted between 2007 and 2015 that involved 13,447 cases (patients within 5 days of their first stroke) and a similar number of age- and gender-matched people from 32 countries across the world.

All cases and control subjects were systematically asked whether they had had acute febrile illness in the previous 4 weeks and whether they had received an influenza vaccination within the previous year.

Conditional logistical regression was used to quantify the results, with adjustment for 13 different possible confounding factors, including hypertension, activity, smoking, cardiovascular risk factors, and socioeconomic factors.

Results showed that having had an acute febrile illness in the previous 4 weeks was more commonly reported in the patients with an acute ischemic stroke (8.7%) than in control patients (5.6%). After adjustment for confounding factors, this gives an adjusted risk ratio of 1.18, which was of borderline statistical significance (95% confidence limits, 1.01 – 1.39), Schwarzbach reported.

The association between recent febrile illness and acute ischemic stroke was stronger when compared with community control subjects (adjusted odds ratio [aOR], 2.0), but it was absent when compared with hospital control subjects.

The association was also only apparent in Australia, China, North America, and Western Europe; it was not seen in other parts of the world.

There was no association between acute febrile illness and acute cerebral hemorrhage.

Flu Vaccine Linked to Halving of Stroke Risk

Having received a flu vaccine in the previous year was strongly associated with a lower risk for any type of stroke (aOR, 0.53), ischemic stroke (aOR, 0.57), and hemorrhagic stroke (aOR, 0.34).

Schwarzbach noted that these results were also consistent in an extended statistical model that included variables that might reflect a willingness to be vaccinated and when compared with both community and hospital-based control subjects.

The strength of the association between influenza vaccination and reduced risk for stroke was similar when compared with either community or hospital control subjects, and was only moderately stronger during than outside the influenza season.

The association was also seen in all regions of the world apart from Africa and South Asia, Schwarzbach reported, but he noted that vaccination rates in these two regions were extremely low.  

The researchers also found that the magnitude of the associations between flu vaccination and lower risk for stroke were stronger in individuals who had multiple annual vaccinations, with an odds ratio of 0.54 in those who had had a vaccine every year for the previous 5 years, and of 0.79 in those who had received one to four vaccinations in the previous 5 years.

Mechanism: Immune Stimulation?

Discussing possible mechanisms behind these results, Schwarzbach noted that the finding that the association with influenza vaccination and reduced stroke risk was independent of seasonality was surprising. “We had expected the protective effect of vaccination to be bigger during the influenza season, but this wasn’t the case.”

He suggested that one explanation might be the inclusion of regions of the world where this seasonality doesn’t exist.

But he pointed out that the finding of a stronger association between flu vaccination and lower stroke risk in those who had received more vaccinations has given rise to another theory: that it is the stimulation of the immune system rather than the protection of infection against influenza that is the key factor.

In an interview with Schwarzbach, Guillaume Turk, MD, professor of neurology at GHU Paris, pointed out that causal inferences are always difficult in case–control studies and in clinical epidemiology in general.

“What makes you think that this association between influenza vaccination and decreased risk is causal rather than due to unmeasured confounders? For example, patients who received vaccination may have received more medical attention and may have been more aware of the risk factors for stroke,” he asked.  

Schwarzbach replied: “Yes, this is the issue of healthy user bias, which is always a problem in this type of research and is hard to address.”

“What we tried to do here is to adjust for variables that might influence the willingness of people to get vaccinated,” he added. “These were mainly socioeconomic factors. But, of course, this is something that we can’t rule out.”

Schwarzbach noted that for more reliable information on this association, prospective studies are needed.

“A Plausible Effect”

Discussing the study after the presentation, William Whiteley, BM, a clinical epidemiologist at the University of Edinburgh, United Kingdom, and a consultant neurologist in NHS Lothian, said vaccination was a potentially important way to reduce stroke.

“In this study, there was a plausible effect on reducing stroke incidence from vaccination against influenza, and also a plausible increase in the risk of stroke from having a recent febrile illness, which we have seen in other studies,” he commented.

Whiteley noted that this observation was particularly relevant now because of the COVID-19 pandemic.

“We’ve all been worried about the risk of heart attack and stroke after COVID, where we’ve seen quite early high risks, and we are also optimistic about the effect of vaccination on reducing those incidences. We’ve seen data from the UK that there may be around a 20% reduction in risk of stroke from vaccination. So, it’s all quite plausible, but at the moment it’s all based on observational evidence and we really need some randomized evidence,” he said.  

“Vaccination and infections have all sorts of odd confounders,” he added. “People who get vaccines tend to be more healthy than those who don’t get vaccines, so you can start to see quite implausible effects of vaccination on overall mortality, which probably aren’t real, and you probably can’t get rid of that totally with statistical methods.”

Alastair Webb, MD, University of Oxford, United Kingdom, asked how reliable the current findings were, given that the occurrence of febrile illnesses and receipt of vaccines were all self-reported, and although there was an association for ischemic stroke and febrile illness, this seemed to go in the opposite direction for hemorrhagic stroke. He also noted that the 50% reduction in stroke risk with vaccination in this study seemed “quite a large magnitude of effect.”

Whiteley replied: “Yes, it is large, but it is promising.” He cited a previous meta-analysis of randomized studies that showed a roughly 25% to 35% reduction in vascular events after flu vaccination, but noted that there was a lot of heterogeneity between studies.

“I’m not sure we’re going to see much more randomized evidence, but I think we can probably all agree that having a vaccine against flu or COVID is a good thing for all of us,” Whiteley concluded.  

The INTERSTROKE study was funded by the Canadian Institutes of Health Research, Heart and Stroke Foundation of Canada, Canadian Stroke Network, Health Research Board Ireland, Swedish Research Council, Swedish Heart and Lung Foundation, The Health & Medical Care Committee of the Regional Executive Board, Region Vastra Gotaland (Sweden), AstraZeneca, Boehringer Ingelheim (Canada), Pfizer (Canada), MSD, Chest, Heart and Stroke Scotland, and The Stroke Association, with support from The UK Stroke Research Network. The authors report no relevant disclosures.

European Stroke Organisation Conference (ESOC) 2022. Presented May 5, 2022.