Killed or weakened, the viruses in flu shots can't give you the flu.

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Putting off your annual flu shot? If you're dragging your feet because you're concerned about getting the flu from the flu vaccine, let's take that worry off the table right now: scientists say that's just not possible.

The confusion may arise, in part, from the different ways these vaccines are developed.

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Credit: Getty Images

The flu vaccine that most people get does not contain live virus. These shots are made with inactivated viruses, meaning that the viruses or virus parts in the shot have been killed. The nasal mist flu vaccine, however, is a bit different. It's made with "attenuated" virus, meaning the virus has been weakened but not killed.

We asked experts to explain how flu vaccines are made and what difference it makes whether you get one that delivers live-but-weakened viruses or one that contains inactivated viruses.

Does the flu shot contains live virus—or not?

"Many years ago, when we first started getting flu shots, they were live attenuated [aka weakened] virus vaccines," Michael Knight, MD, a primary care physician and assistant professor of medicine at George Washington University Medical Faculty Associates in Washington, DC, tells Health. "So if the patient was pregnant or had a weakened immune system, we couldn't give that to them," he says.

But that's no longer the case. Today's flu shots are inactivated, meaning the viruses—or viral parts--in this type of vaccine can't possibly infect anyone because they're not alive. These are the shots that the majority of adults receive each season.

"The flu shots you get do not have a live virus that could cause infection," says Dr. Knight.

There's one exception: a nasal mist flu vaccine is made with live, "attenuated" viruses, says the US Centers for Disease Control and Prevention (CDC). The viruses weakened but not killed. Still, they cannot cause influenza, says CDC. (More on that below.)

Here's how inactivated flu vaccines are made

The egg-based process to produce the majority of flu vaccines was first developed more than 70 years ago, explains the CDC. The process starts with the CDC or another lab in the World Health Organization Global Influenza Surveillance and Response System. These pubic health entities provide private manufacturers with the viruses that have been chosen for the season's vaccine, says the CDC. These viruses are then grown in eggs according to very strict regulations established by the US Food and Drug Administration.

"The technology was developed in the late 1930s and 40s, when scientists first discovered you could grow the influenza virus in chicken eggs," William Schaffner, MD, medical director of the National Foundation for Infectious Diseases and professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine, tells Health.

"The technology is very elaborate. The eggs have to be especially pure, bred specially on special farms. Those chickens are under cover so that wild birds can't get at them and infect them," says Dr. Schaffner. "And when you consider that we have to produce millions and millions of doses of vaccines, we need a lot of eggs."

There's also a more modern way of producing vaccines, by growing them in animal cells in very large test tubes, as the CDC points out.

But whichever way the virus is grown, it's produced in very large amounts and then extracted from the eggs or cells. "Then you kill the virus, break it up into pieces," says Dr. Schaffner. "You take the select pieces of the virus that are important in stimulating the immune response, purify that, and that's the vaccine." He explains that it's impossible to reconstitute the virus from those pieces, and that's why it's impossible to get the flu from the vaccine.

There are special versions of the flu vaccine for people age 65 and up. These contain the same inactivated virus as the standard dose vaccine. One version of the vaccine for seniors is high-dose, and it contains four times the (dead) virus as the regular shot. Another version has the same amount of antigen, but also has an ingredient called an adjuvant which help spur the immune system to mount a stronger response. Neither of these vaccines is capable of causing an influenza infection.

What's an 'attenuated' or weakened flu vaccine?

There is one kind of flu vaccine that does contain a live—but weakened—virus. That's the nasal mist, which is available for non-pregnant people age 2 to 49 with no serious underlying health conditions.

For this vaccine, the CDC explains that the viruses are first grown in eggs just as they are for the injection. But then instead of being killed, the viruses go through a different production process that weakens them. According to the CDC, "The weakened-viruses are cold-adapted, which means they are designed to only multiply at the cooler temperatures found within the nose, and not the lungs or other areas where warmer temperatures exist."

"It's a miracle of modern science," says Dr. Schaffner. "The viruses are engineered on a molecular level, so they can't multiply at the higher temperature that's just a tiny bit higher in the lung compared to the nose. The viruses are incapable of multiplying at that temperature, so they can't produce influenza at all."

Dead or alive?

All the different flu vaccines available this season will protect against the same four viruses chosen as most likely to cause problems this winter. But none of them can give you the flu.

"Flu vaccine has often had a bad rap," says Dr. Knight. "People say, 'My friend got the flu vaccine and then they got the flu the next day.' But the flu vaccines we use can't cause the flu."

What they can all do, however, is reduce your chance of getting influenza, or of becoming seriously ill with it.

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