Flu Shots: Are They 'Live Virus' Vaccines or Not?

In the United States, the flu returns on a yearly basis. If you're putting off your annual flu shot because you're concerned about getting the illness from the virus in the vaccine, that's actually not possible.

"Flu vaccine has often had a bad rap," said Michael Knight, MD, a primary care physician and assistant professor of medicine at George Washington University Medical Faculty Associates in Washington, DC. "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."

The confusion may arise, in part, from the different ways these vaccines are developed—so we asked experts to explain more about flu vaccines, including the types that are available and how they are made.

What Types of Vaccines Are There?

In general, according to the U.S. Department of Health & Human Services (HHS), there are a number of general vaccine types available:

• Live, attenuated: Contains a live but weakened version of a virus or bacteria
• Inactivated: Contains an inactivated version of a virus or bacteria
• Toxoid: Consists of a weakened toxin that may cause an illness
• Subunit, recombinant, polysaccharide, and conjugate: Include parts of the virus or bacteria rather than either in its entirety
• Messenger RNA (mRNA): Produces proteins that are designed to initiate an immune response to a virus
• Viral vector: Contains a changed version of another virus (used as a vector) as a method of protection against a virus

In particular, live, attenuated vaccines contain a live but weakened version of a virus or bacteria, while inactivated vaccines contain an inactivated version of a virus or bacteria. "Many years ago, when we first started getting flu shots, they were live attenuated virus vaccines," Dr. Knight told Health. "So if the patient was pregnant or had a weakened immune system, we couldn't give that to them," Dr. Knight said.

Consequently, later flu vaccines contained inactivated viruses, which eliminated the chance of getting sick after a flu vaccination. However, though most flu vaccines contain inactivated viruses, there is one flu vaccine that does include a live, attenuated virus per the Centers for Disease Control and Prevention (CDC).

How Are Flu Vaccines Made?

Inactivated Vaccines

The egg-based process to produce the majority of flu vaccines was first developed more than 80 years ago, the CDC explains. The process starts with the CDC or another lab in the World Health Organization Global Influenza Surveillance and Response System. These public health entities provide private manufacturers with the viruses that have been chosen for the season's vaccine, the CDC says. 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, told Health.

"The technology is very elaborate. The eggs have to be especially pure, bred [specifically] on special farms. Those chickens are under cover so that wild birds can't get at them and infect them," Dr. Schaffner said. "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—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," Dr. Schaffner said. "You take the select pieces of the virus that are important in stimulating the immune response, purify that, and that's the vaccine." Dr. Schaffner explained 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 aged 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 amount of the inactivated virus as the regular shot. Another version has the same amount of antigen but also has an ingredient called an adjuvant which helps spur the immune system to mount a stronger response.

Live, Attenuated Flu Vaccines

There is one kind of flu vaccine that does contain a live, weakened virus. That's the nasal mist, which is available for people aged 2 to 49 who are not pregnant and with no serious underlying health conditions according to the CDC.

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," Dr. Schaffner said. "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."

What Else To Know About Flu Vaccines

All the different flu vaccines available during a specific flu season will protect against the four viruses chosen as most likely to cause problems for that season. Of note, the development period for antibodies to protect you from influenza viruses is about two weeks. Thus, you should aim to get vaccinated by the end of October, according to the CDC, though you may be allowed to get your vaccination as early as July or August if appropriate for your situation.

Individuals aged six months or older should be vaccinated, with the exception of specific populations (e.g., children under six months) per the CDC. Additionally, those who have egg allergies can receive flu vaccinations; however, there are two egg-free flu vaccines as of August 2022. Furthermore, people with severe egg allergies (where reactions include more than just hives) would need to be vaccinated under the supervision of a healthcare provider, the CDC says.

Ultimately, if you still have any other concerns about getting your flu vaccine, talk to your healthcare provider for more information and guidance.