The Science Behind Flu Shots: How Do Vaccinations Work?

At one time or another, everyone has come down with an illness or felt under the weather during the colder months. Winter is an infamously common time for illnesses like colds or the flu. But why? Respiratory viruses tend to spread more efficiently during the winter months, as people spend more time indoors and in close proximity to one another.

To combat the spread of influenza viruses, flu vaccines have become a cornerstone of preventive healthcare and play a critical role in safeguarding public health.* That said, how do vaccinations work exactly? Now, more than ever, it's important to learn how flu vaccinations work and why herd immunity is a matter of public health.

How Do Vaccinations Work?

Flu shots contain a weakened or inactivated form of influenza, the virus that causes the flu. The weakened or inactivated influenza virus cannot cause infection, but it stimulates the immune system to recognize the disease, preparing the body to fight the actual virus if you are exposed.

FluMist Quadrivalent, an intranasal spray, is an alternative to injectable flu vaccines. It works by stimulating a similar response to traditional flu vaccines; however, it's made with a combination of live influenza virus strains. The flu virus enters the body through the mucous membranes of the nasal cavity, an effective administration route for a flu vaccine.

When you receive a flu shot or nasal spray, your immune system recognizes foreign elements from the virus (antigens) present in the vaccine. Your body activates specialized immune cells, including B cells and T cells, which produce antibodies and memory cells tailored to the specific flu virus strain. B cells generate antibodies that bind to the viral antigens present in the vaccine. These antibodies neutralize the virus and prevent its entry into host cells, thwarting infection.

Memory B cells "remember" the virus's antigens and remain in the body after the initial immune response. In case of future exposure to the actual virus, memory cells facilitate a rapid and robust immune reaction, preventing severe illness.

Supporting Herd Immunity and Community Protection

The Centers for Disease Control and Prevention (CDC) recommends that everyone 6 months and older receive a flu vaccine yearly. Not only do vaccinated individuals have greater protection should they contract the flu, but they are also less likely to transmit the virus to others. Widespread vaccination contributes to herd immunity, or community protection. When a significant portion of the population has flu immunity through vaccination, the transmission of the virus slows down, protecting those most vulnerable to the illness.

There are a few groups of individuals who cannot receive the flu shot or should proceed with caution if they do:

• Infants younger than 6 months old cannot receive a flu vaccine.

• Individuals with a prior severe or life-threatening allergy to a previous flu vaccine ingredient (other than egg protein) should also not have the flu shot.

• Individuals with an egg allergy should speak with their provider about which flu shot may suit them. Some formulations of flu vaccines do not contain egg products.

• Some individuals with a history of Guillain-Barré syndrome (a neurological condition that causes paralysis) should not get a flu shot. Speak with your doctor first.

Flu vaccines also protect individuals with weakened immune systems, who may have more difficulty recovering from the flu if they become infected, including the elderly, long-term care facility residents, patients with chronic lung conditions, cancer patients and individuals who take immune-suppressing drugs.

The U.S. Food and Drug Administration has approved FluMist for individuals between the ages of 2 and 49 who have a low risk of contracting severe flu. While it may seem like a no-brainer to get a nasal spray instead of an injection, the CDC advises that the following populations not receive a nasal flu vaccine:

• Children between 2 and 17 years old who are receiving salicylate-containing medications, such as aspirin.

• Children 2 to 4 years old with asthma or who have experienced wheezing in the past year.

• People with compromised immune systems for any reason, including a nonfunctioning or missing spleen.

• Pregnant people.

• People with an active cerebrospinal fluid leak.

• People with cochlear implants.

• People who have recently taken certain antiviral drugs for the flu.

• People who care for severely immunocompromised persons should avoid contact for at least seven days after receiving a nasal flu vaccine.

Can the Flu Vaccine Cause Me To Get the Flu?

Despite any misconceptions, the facts are clear: The flu vaccine cannot cause the flu. According to the CDC, "The vaccines either contain inactivated virus, meaning the viruses are no longer infectious, or a particle designed to look like a flu virus to your immune system."

The nasal spray flu vaccine contains weakened (attenuated) viruses that are not potent enough to cause flu illness. The weakened viruses are cold-adapted, which means they're designed to multiply only at the cooler temperatures found within the nose and not the lungs or other areas where warmer temperatures exist.

If you develop flu-like symptoms immediately after receiving a flu shot, such as headache, low-grade fever, nausea, muscle aches and fatigue, you are most likely experiencing the side effects that result from the activation of the immune system, rather than the flu itself.

Decreasing Flu Severity, Death and Burden

Getting a flu vaccine does not guarantee you will not get the flu, but it greatly reduces your likelihood of severe illness if you do become sick. The CDC estimates that during the 2019-2020 flu season, flu vaccines prevented an estimated 7.1 million illnesses, 3.4 million medical visits for the flu, 100,000 flu-related hospitalizations and 7,100 flu-associated deaths.

The flu places a tremendous burden on the healthcare system and economy. In the U.S., direct healthcare costs related to flu have been estimated at $3.2 billion per year. Indirect costs, including lost wages due to worker absenteeism, could be around $8 billion annually.

Adapting To Changing Viral Strains

It is best practice to get a flu shot yearly. This is necessary because the flu virus mutates over time, leading to different strains circulating each flu season.

Scientists continuously monitor and predict prevalent strains, updating flu vaccine formulations to match these evolving viruses. So, keep in mind that flu vaccines are updated from one season to the next based on that continual research.

When Should I Schedule My Flu Shot?

The flu season begins around October and lasts through April but typically peaks between December and March. For most people who need only one dose of flu vaccine for the season, September and October are generally good times to be vaccinated against flu. Ideally, everyone should be vaccinated by the end of October.

Our pharmacists are here to help you stay healthy this flu season and to answer any vaccine-related questions you might have. You can schedule a flu shot by using our Rite Aid scheduler or by visiting one of our pharmacies.*

Clinically reviewed on Jan. 30, 2024.