Oct. 22, 2021 -- The COVID and flu vaccines are important, and both are quite effective at preventing serious illness or death. But that’s where much of their similarities end. Here’s the science behind both.

The methodology

Two of the three COVID vaccines (Pfizer and Moderna) adopted in the United States are mRNA, or messenger RNA. The shots work by delivering molecules of antigen-encoding mRNA into immune cells, triggering an immune response. They represent nearly 20 years of research and are relatively easy to produce.

For the flu vaccine, scientists harvest the virus in eggs, inactivate it, and then purify the antigen before making it on a wide scale.

The strains they attack

One of the biggest differences between the COVID vaccine and the flu vaccine is that the COVID vaccine is effective against all the currently circulating strains of the virus. The flu shot, on the other hand, is designed to handle the strains of flu scientists determine will likely circulate each year.

But given that last year’s flu season was very mild because of masking and distancing, there’s concern the 2021-22 season could be severe.

“This year, the shot is quadrivalent, which means it is designed to protect against four strains,” says Rachael Lee, MD, an infectious disease specialist and assistant professor in the University of Alabama’s Division of Infectious Diseases. “Other years, it will be trivalent.”

Side effects

Both the flu and COVID vaccines can produce side effects, but the immune response to the COVID vaccine tends to be a bit harder on the body. In either case, side effects include symptoms of the diseases, like achy muscles, soreness where you get the shot, mild fevers, headaches, and sometimes a very mild cough. Symptomatic effects from both shots generally don’t last longer than24 hours.

How long they protect you

While researchers are still learning about how long the COVID vaccines are effective, the general thinking is that they offer protection longer than the flu shot, according to Jill Ferdinands, PhD, an epidemiologist in the Influenza Division of the National Center for Immunization and Respiratory Diseases at the CDC. Research suggests that within a few months of receiving the flu shot, immunity begins to wane.

But in the near future, scientists may create flu shots using mRNA technology, bringing them on par with the COVID shots’ effectiveness. In the meantime, the most important thing to know is that you should get them both.

“The data shows that your symptoms will be much milder if you get these vaccines,” says Lee. “If you get vaccinated, it will help with public health efforts.”

This is particularly true this year, when hospitals are overwhelmed with COVID patients and experts predict the flu might make a strong return.

“Our hospitals have now learned how to manage pandemic surges,” says Lee, “but we want to prevent that going forward. The vaccines are the tools to do that.”

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Rachael Lee, MD, infectious disease specialist, University of Alabama Medicine, Birmingham; assistant professor of infectious diseases, University of Alabama at Birmingham.

Jill Ferdinands, PhD, epidemiologist, Influenza Division, National Center for Immunization and Respiratory Diseases, CDC.

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