Mechanism: Immune stimulation?
Discussing possible mechanisms behind these results, Dr. 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 Dr. 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.
Dr. 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.”
Dr. 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, PhD, a clinical epidemiologist at the University of Edinburgh 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.
Dr. 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 U.K. 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 (England), 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.”
Dr. Whiteley replied: “Yes, it is large, but it is promising.” He cited a previous meta-analysis of randomized studies that showed a roughly 25%-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,” Dr. 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, Pfizer, MSD, Chest, Heart and Stroke Scotland, and The Stroke Association, with support from The UK Stroke Research Network. The authors reported no relevant disclosures.
A version of this article first appeared on Medscape.com.