While most Americans still wait for their chance to receive a vaccine for COVID-19, a new study may offer some consolation in the meantime — if you got a seasonal flu shot, you may be somewhat protected against COVID-19, according to a new study published in the American Journal of Infection Control.
Researchers at the University of Michigan conducted the study by looking at the medical records of patients within the Michigan Medicine health system. Their starting point was looking at all of the 27,201 people who had received a lab test for COVID-19. Within this large group, participants who had received a flu shot, as noted in their medical record, had a number of different COVID-19-related outcomes compared with those who hadn’t received a flu shot. The group was almost evenly divided in their flu shot status, with 47.8% receiving a flu shot during the previous flu season and 52.2% not receiving one.
Effects of receiving a flu shot
The first major difference that researchers observed was that patients who had gotten a flu shot were 24% less likely to test positive for COVID-19. What’s more, among those who did test positive, patients who had gotten a flu shot were 42% less likely to require hospitalization, and 55% less likely to require mechanical ventilation (breathing assistance) at the hospital if they were admitted. They were also likely to have a shorter hospital stay, by an average of 24%.
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Of course, a person’s flu shot status wasn’t the only factor that helped predict whether they tested positive for COVID-19. Those who tested positive were also more likely to have chronic health conditions like pulmonary (lung) disease (19.8% versus 14.6% of patients who tested negative), congestive heart failure (10.3% versus 7.8%), diabetes (21.5% versus 9.8%) or hypertension (high blood pressure) (36.0% versus 22.5%). There were also age and racial differences between those who tested positive versus negative for COVID-19. The average age of those who tested positive was 50.7, while it was 47.1 among those who tested negative; African Americans made up 35.5% of those who tested positive and just 11.4% of those who tested negative.
But even after controlling for other factors linked to COVID-19 test status — including race and ethnicity, gender, age, smoking status and presence of the health conditions listed above — whether someone got a flu shot was still a significant predictor of whether they tested positive for COVID-19.
These results, the researchers wrote, point to “a possible association between the [flu] vaccine and decreased risk of COVID-19 and improved clinical outcomes,” although the exact way or ways in which a flu shot may help protect against COVID-19 aren’t entirely clear. One possibility, though, is an immune system response known as “trained immunity,” meaning that when you activate certain immune cells against one pathogen — like the flu virus — it also improves their response against other pathogens, like the virus that causes COVID-19. The researchers note that this type of beneficial response has been seen or suspected with other vaccines, such as the measles vaccine in children.
But it’s also possible, the researchers note, that some or all of the benefit linked to the flu shot could be from what’s known as the “healthy user effect.” Under this theory, people who get the flu shot are more likely to take other health precautions in their lives, including those meant to reduce the risk of COVID-19.
Regardless of exactly how it may offer protection against COVID-19, one thing about the flu shot is clear from this study — it can’t hurt your chances of escaping harm from COVID-19. “Until the COVID-19 vaccine becomes widely available, the influenza vaccine should be promoted to reduce the burden of disease during this pandemic,” the researchers concluded.
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