In-Depth: Can the flu vaccine boost protection against COVID?

Flu vaccine, even when just 20% effective, saves tens of thousands of lives
Posted at 7:45 PM, Jan 13, 2022
and last updated 2022-01-13 23:43:25-05

SAN DIEGO (KGTV) – The Centers for Disease Control and Prevention says a flu shot will not protect you against COVID-19, but a growing body of research suggests it might add a small but meaningful amount of defense.

Several studies during the pandemic have found links between flu vaccination and better outcomes with COVID-19, although the authors note the protective effect appears modest.

Still, researchers at Cornell University published a model this week showing that even 5 percent of added protection from the flu vaccine would dramatically reduce COVID-19 caseloads and the burden on hospitals.

“We know that unrelated vaccines have these heterologous effects, and a reasonable person could tell you that if you used them during a pandemic, it would benefit,” said Dr. Nathaniel Hupert, an associate professor at Weill Cornell Medicine and the paper’s lead author.

Decades of experiments suggest vaccines often provide a generalized immune boost that can protect against multiple pathogens, the paper argues.

Last year, researchers at the University of Miami analyzed medical records on nearly 75,000 people who had tested positive for COVID. They found people who had gotten a flu vaccine beforehand had a 20 percent lower risk of winding up in the ICU.

Their risk of developing certain complications dropped even more: the risk of blood clots dropped 40 percent, sepsis was reduced by 45 percent, stroke by 58 percent.

Another study in the journal Nature analyzed adults over 65 in the U.S. and found similar results. People who had gotten a flu vaccine in the months before the pandemic had a 24 percent lower chance of getting infected with COVID. They also had a 28 percent lower chance of developing severe disease from COVID.

Neither study pinpointed the cause of the protection. Were the benefits caused by the flu vaccine itself or some other variable?

“It’s possible that when you rev the immune system in any way, there is some generalized protection against other things,” said Dr. Omid Bakhtar, the laboratory medical director at Sharp Healthcare.

For nearly 100 years, researchers have found examples of one vaccine offering protection against a different virus or bacteria. In general, they think it’s because vaccines activate the body’s most primitive defenses that guard against all infections: what’s called the innate immune system. This branch includes things like natural killer cells. It is distinct from the antibodies, B cells and T cells in the adaptive immune system that gets the most public attention.

“The theory has been around for a long time. And I think that with the right stimulus, you might have a beneficial effect, but it’s not going to be the same kind of effect you get from a COVID vaccine or for something that’s more directed. And it won’t be as long-lasting,” said UC San Diego infectious disease expert Dr. Robert Schooley.

Dr. Schooley is skeptical that the flu vaccine offers cross-protection against COVID. He says the association studies might be skewed by what’s called “healthy user bias.” People that get an annual flu vaccine might be more health-conscious overall or more likely to wear masks.

The flu vaccine doesn’t jumpstart the innate immune system for very long, he said.

“It’s a very trivial tweak to the innate immune system and probably for a couple of days keeps you tuned in, but it’s not going to do this over the weeks and months of a COVID season,” he said.

Even so, Dr. Hupert argues that any added protection, however small, is a good thing.

“Each and every additional protective measure that we can muster across populations at risk – even small ones like those we modeled – will lead to fewer infections, which means fewer new variants, which may mean a quicker end to the pandemic,” he said.