AJIC: Impact of the Influenza Vaccine on COVID-19 Infection Rates and Severity

Photo Credit - CDC PHIL 

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Although the flu vaccine is not designed to protect against SARS-CoV-2 (aka `COVID-19') we've seen mounting evidence over the past decade that getting the flu shot can reduce other medical risks, including heart attacks, strokes, and even the risk of dementia.

AHA: Study Shows Flu Shots Reduce Deaths From Heart Failure

(AAIC®) 2020 Presentation : Flu & Pneumonia Shots Appear To Reduce Dementia Risk In Elderly

Flu Vaccine May Lower Stroke Risk in Elderly ICU Patients

Most of these benefits appear to be linked to reduced incidence and severity of influenza infection in at-risk patients (see CDC: Another Study Linking Severe Influenza To Heart Damage), but it has also been suggested there may be other - as yet, undefined - factors at work as well. 

While the mechanisms behind any potential benefit are unknown, we've seen a few studies suggesting that - at least statistically - receipt of the flu vaccine has been linked to lower SARS-CoV-2 infection rates, and/or reduced severity.   

Yesterday, in Study: Human Rhinovirus Infection Blocks SARS-CoV-2 Replication Within the Respiratory Epithelium, we looked at how infection with one virus can impede infection with another (aka `viral interference'), making it at least plausible that influenza vaccinations might temporarily ramp up the recipient's innate immune system enough to convey some protection against other infections. 

A few recent studies on SARS-CoV-2 and flu vaccinations include:

• Last year, in the preprint The effect of influenza vaccination on trained immunity: impact on COVID-19, researchers from the Netherlands hypothesized that receipt of the quadrivalent flu vaccine could boost innate immunity after finding SARS-CoV-2 infection `. . .  was less common among Dutch hospital employees who had received influenza vaccination during the 2019/2020 winter season (RR = 0,61 (95% CI, 0.4585 - 0.8195, P = 0.001).'

• Similarly, last January researchers from the University of Missouri School of Medicine published The Flu Vaccination May Have a Protective Effect on the Course of COVID-19 in the Pediatric Population, finding that children vaccinated against flu were less likely to be symptomatically infected with SARS-CoV-2 (p=0.028, adj. OR=0.714, 95% CI [0.529, 0.964]) than those not vaccinated

• And yet another study, Inactivated trivalent influenza vaccination is associated with lower mortality among patients with COVID-19 in Brazil, looking at over 53,000 hospitalized COVID patients during the first half of 2020, found mortality was consistently lower among patients who received a flu vaccine after March 1st, 2020. 

While interesting, none of this proves causality.  There may be other factors at work, including that those who routinely get flu vaccines may live healthier lifestyles, or are more inclined to wear facemasks, or social distance during this pandemic. 

But the studies keep coming, suggesting there may be more to this theory. 

Today we've got excerpts from an AJIC study, and a press release from the University of Michigan, on yet another finding that receipt of the flu vaccine appears to be linked to reduced infection, and severe illness, with COVID-19. 

Follow the link to read the study in its entirety.  When you return I'll have a brief postscript.

Impact of the influenza vaccine on COVID-19 infection rates and severity 

Anna Conlon, PhD 1, Carmel Ashur, MD, MS 1, Laraine Washer, MD, Kim A. Eagle, MD
Marion A. Hofmann Bowman, MD

Open Access Published:February 22, 2021 

DOI:https://doi.org/10.1016/j.ajic.2021.02.012

Highlights

• With vaccines against COVID-19 not yet broadly available, there is interest in assessing the role of the influenza vaccine in COVID-19 susceptibility and severity.
• The odds of testing positive for COVID-19 was reduced in patients who received an influenza vaccine compared to those who did not by 24%.
• Vaccinated patients testing positive for COVID-19 were less likely to require hospitalization or mechanical ventilation and had a shorter hospital length of stay.
• The influenza vaccine should be promoted to reduce the burden of COVID-19.

 

Background

With a unique influenza season occurring in the midst of a pandemic, there is interest in assessing the role of the influenza vaccine in COVID-19 susceptibility and severity.

Methods

In this retrospective cohort study, patients receiving a laboratory test for COVID-19 were identified. The primary outcome was comparison of positive COVID-19 testing in those who received the influenza vaccine versus those who did not. Secondary end points in patients testing positive for COVID-19 included mortality, need for hospitalization, length of stay, need for intensive care, and mechanical ventilation.

Results

A total of 27,201 patients received laboratory testing for COVID-19. The odds of testing positive for COVID-19 was reduced in patients who received an influenza vaccine compared to those who did not (odds ratio 0.76, 95% CI 0.68-0.86; P < .001). Vaccinated patients testing positive for COVID-19 were less likely to require hospitalization (odds ratio, 0.58, 95% CI 0.46-0.73; P < .001), or mechanical ventilation (odds ratio, 0.45, 95% CI 0.27-0.78; P  = .004) and had a shorter hospital length of stay (risk ratio, 0.76, 95% CI 0.65-0.89; P < .001).

Conclusion

Influenza vaccination is associated with decreased positive COVID-19 testing and improved clinical outcomes and should be promoted to reduce the burden of COVID-19.

          (Continue . . . )

For more background on this study, we have the following press release from the University of Michigan. 

NEWS RELEASE 23-MAR-2021
Flu shot associated with fewer, less severe COVID cases

MICHIGAN MEDICINE - UNIVERSITY OF MICHIGAN 
 

People who received a flu shot last flu season were significantly less likely to test positive for a COVID-19 infection when the pandemic hit, according to a new study. And those who did test positive for COVID-19 had fewer complications if they received their flu shot.

These new findings mean senior author Marion Hofmann Bowman, M.D., is continuing to recommend the flu shot to her patients even as the flu season may be winding down.

"It's particularly relevant for vaccine hesitance, and maybe taking the flu shot this year can ease some angst about the new COVID-19 vaccine," says Hofmann, an associate professor of internal medicine and a cardiologist at the Michigan Medicine Frankel Cardiovascular Center. Michigan Medicine is the academic medical center of the University of Michigan.

Researchers reviewed medical charts for more than 27,000 patients who were tested for a COVID-19 infection at Michigan Medicine between March and mid-July of 2020. Of the nearly 13,000 who got a flu shot in the previous year, 4% tested positive for COVID-19. Of the 14,000 who hadn't gotten a flu shot, nearly 5% tested positive for COVID-19. The association remained significant after controlling for other variables including ethnicity, race, gender, age, BMI, smoking status and many comorbid conditions, Hofmann says.

People who received their flu shot were also significantly less likely to require hospitalization, although the researchers didn't find a significant difference in mortality between the two groups. No one in the study tested positive for both infections at the same time.

The underlying mechanism behind the association isn't yet clear, Hofmann says.

"It is possible that patients who receive their flu vaccine are also people who are practicing more social distancing and following CDC guidelines. However, it is also plausible that there could be a direct biological effect of the flu vaccine on the immune system relevant for the fight against SARS-CoV-2 virus," she says.

Prospective longitudinal studies to examine the effect of the flu vaccine on respiratory illness are ongoing, including the Household Influenza Vaccine Evaluation (HIVE) study through the University of Michigan's School of Public Health.

"It's powerful to give providers another tool to encourage their patients to take advantage of available, effective, safe immunizations," says co-first author Carmel Ashur, M.D., M.S., an assistant professor of internal medicine and a hospitalist at Michigan Medicine.

Months ago, Hofmann was concerned about misinformation she kept seeing online that connected the flu vaccine with a COVID-19 infection. Prominent media outlets like Reuters debunked this theory, and she knew her team's data could also help address vaccine hesitancy.

"Instead of a concerning connection between COVID-19 and the flu shot, our publication provides more confidence that getting your flu shot is associated with staying out of the hospital for COVID-19," she says.

Before the pandemic hit, Hofmann and co-first author Anna Conlon, Ph.D., a U-M Medical School student, educated Frankel CVC patients about another encouraging association with the flu vaccine: cardiovascular protective effects.

"There's robust data that the flu shot prevents heart attack and hospitalizations for heart failure, which is an additional reason to get your vaccine every flu season," Conlon says.

         (Continue . . . )

Influenza claims tens of thousands of American lives every year, even though many of those deaths are attributed to heart attacks and strokes.   

Despite its often lackluster effectiveness  - particularly in my age group (65+) - I get flu vaccine every year because even a little protection can be lifesaving. The downsides of doing so are miniscule and it not only helps to protect me, it helps to protect others. 

The evidence that it may help prevent, or attenuate, COVID-19 may not be robust, but there appears to be enough of a pattern in the data to warrant further research. 

In the meantime - the benefits of the flu vaccine are already well established - and if it helps against COVID-19, so much the better.