Even when a flu shot doesn’t stop infection, it can still provide significant protection against severe influenza-related outcomes, including hospitalization, intensive care admission and death, new research from McMaster University suggests.
The systematic review and meta-analysis, published in the journal Clinical Microbiology and Infection, examined data from hundreds of studies at sites all over the world, which collectively involved more than 670,000 patients.
A single seasonal influenza vaccination could reduce the risk of severe outcomes by as much as 55 per cent, large-scale study showed.
“This research shows in absolute terms that there is immense benefit to getting the seasonal influenza vaccine,” says Mark Loeb, a professor in McMaster’s Department of Pathology and Molecular Medicine, and one of the lead authors on the new paper.
“This was a very, very comprehensive study. Previous work like this has looked at relatively few outcomes, but ours looked at several hard outcomes beyond just hospitalization and did so across a vast body of literature. Doing so revealed a strong bottom line: The seasonal flu shot reduces very severe influenza-related outcomes.”
The global investigation was inspired by revelations from the COVID-19 pandemic, says McMaster Professor Matthew Miller, who co-led the study with Loeb.
“During the pandemic, we saw that the COVID-19 vaccines remained very effective at keeping people out of hospitals, even after new variants of the virus emerged and breakthrough infections became more and more widespread,” Miller says.
“That experience really changed the global mindset around what we should look for in vaccines — it helped us recognize that keeping people alive and out of the hospital is a critically important property of an effective vaccine.”
Because influenza viruses mutate quickly, like coronaviruses do, the research team hypothesized that the flu shot may likewise confer unappreciated protection upon those who experience breakthrough flu infections.
“We know it’s not perfect at preventing infection, but we wanted to know if the flu shot helped reduce severe outcomes,” says Miller, who is scientific director of the Michael G. DeGroote Institute for Infectious Disease Research (IIDR) and executive director of McMaster’s Global Nexus.
Analyzing massive global datasets, the researchers showed that it does, in fact, do that.
Specifically, the study revealed that seasonal influenza vaccination reduced the risk of hospitalization by 42 per cent and the risk of death by 36 per cent. The study also showed that the flu shot reduced the risk of pneumonia, ICU admission, and ventilatory support by more than 50 per cent each.
The researchers observed that flu shots protected best against these severe outcomes in years when the vaccine’s composition closely matched the dominant flu strain in circulation.
Protection was also observed to be strongest in children, and against the flu subtype H1N1; however, the vaccines also offered meaningful protection from severe outcomes associated with H3N2 and influenza B viruses.
These findings reinforce the importance of keeping up to date on your seasonal flu shot, Loeb says.
This study was supported by the Canadian Institutes of Health Research and conducted in large part by former McMaster postdoctoral fellow Sergey Yegorov, who is now an assistant professor at Nazarbayev University in Kazakhstan.