The flu shots most people get each fall do not use mRNA. They rely on older methods built around the flu virus itself or one of its proteins. But mRNA flu vaccines are no longer just an idea: one has been reported as the first FDA-approved mRNA seasonal flu shot in the US.1Harvard Health. FDA approves first mRNA-based flu vaccine Whether your shot is mRNA now depends on which product your pharmacy or clinic stocks, not on flu vaccines as a group.
What the Usual Flu Shot Is Made Of
Most flu shots start in chicken eggs. The egg method has been used to make flu vaccines for more than 70 years.2Families Fighting Flu. Types of Flu Vaccines | Flu Vaccine Information Health agencies give manufacturers starter flu viruses, which are injected into fertilized chicken eggs to multiply.3Families Fighting Flu. Types of Flu Vaccines | Flu Vaccine Information The virus-filled fluid is then collected, the virus is killed, and the part that trains the immune system is purified.4Families Fighting Flu. Types of Flu Vaccines | Flu Vaccine Information Because the virus is no longer alive, a standard flu shot cannot give you the flu. “Purified” here means the killed virus is broken apart so mainly the surface proteins your immune system learns from remain. These are called inactivated vaccines, and your record or the product information may use that word instead of “killed.”
Besides egg-grown shots, the only egg-free flu vaccines licensed in the US have been the cell-grown vaccine and the recombinant vaccine.5Centers for Disease Control and Prevention. Recombinant Influenza (Flu) Vaccine The recombinant vaccine, Flublok, is made by having insect cells produce the flu’s surface protein, with no flu virus grown at all.6PubMed Central. Current and Emerging Cell Culture Manufacturing Technologies for Influenza Vaccines Since it contains only that protein, your body learns to recognize flu without ever meeting the virus.
The nasal spray is different again. It uses live flu viruses weakened so they cause mild or no symptoms.7Morbidity and Mortality Weekly Report. Using Live, Attenuated Influenza Vaccine for Prevention and Control of Influenza Supplemental Recommendations of the Advisory Committee on Immunization Practices (ACIP) They are designed to multiply only in the cooler nose, not the warmer lungs.8Centers for Disease Control and Prevention. Live Attenuated Influenza Vaccine [LAIV] (The Nasal Spray Flu Vaccine) The spray is licensed only for certain ages and for people without certain health conditions, so it isn’t an option for every adult.
To find out which type you got, check your vaccination record or pharmacy receipt for the brand name. The product’s patient information sheet states how it was made. If the type matters to you, ask the pharmacist before the shot.
The mix-up is common. Many people got mRNA COVID vaccines and assume all newer vaccines work the same way, and flu and COVID shots are often given at the same visit, which blurs things further.
Why Egg-Grown Vaccines Can Miss the Mark
Flu virus grown in eggs has to adapt to survive there, and the adaptations can alter the part of the virus your immune system learns to recognize. When one common flu strain was grown repeatedly in eggs, its surface protein picked up changes. Grown in cells, it did not.9PubMed Central. Comparison of antigenic mutation during egg and cell passage cultivation of H3N2 influenza virus The vaccine can then end up training your body against an egg-adapted version that looks a bit different from the virus spreading among people. Experts estimate avoiding these egg changes could make flu vaccines about 9% more effective on average, and up to 16% for that strain in people under 65.10PubMed Central. Estimation of Reduction in Influenza Vaccine Effectiveness Due to Egg-Adaptation Changes—Systematic Literature Review and Expert Consensus
A partial mismatch lowers protection but does not make the shot useless. Egg-free shots like the cell-grown and recombinant vaccines avoid the egg changes.
Time is the other problem. Conventional flu vaccine production takes roughly six months.11Social Science Research Network. Shorter mRNA Lead Times and the Option to Wait in Influenza Vaccine Design Because manufacturing can’t start until experts pick which strains go into the vaccine, that choice has to be made long before flu season. The virus can drift in the meantime,12Harvard Health. FDA approves first mRNA-based flu vaccine so the dominant strain in winter may differ somewhat from the one chosen. Some seasons the guess is close and some it is looser, which is why vaccine benefit varies from year to year.
How an mRNA Flu Vaccine Works
An mRNA vaccine carries a small piece of genetic code, called mRNA, that tells your cells to make a harmless flu protein.13Harvard Health. FDA approves first mRNA-based flu vaccine The code is wrapped in tiny fat bubbles that carry it into cells.14PubMed Central. Lipid nanoparticles for mRNA delivery Inside the cell, the code is read and the protein is made, as it would be during an infection.15PubMed Central. Lipid nanoparticle-based mRNA vaccines in cancers: Current advances and future prospects Your immune system responds to the protein without any flu virus being present.
The shot goes into the upper arm muscle, and cells nearby make the flu’s surface protein for a short time. Immune cells spot the protein as foreign, and your body builds antibodies and memory cells against it. If you later meet real flu virus, those memory cells can respond more quickly.
The genetic instructions stay outside the cell’s nucleus, where DNA is kept, so they cannot change your DNA. Your body breaks them down after the protein is made. And no live or killed flu virus is in the shot.
The recombinant shot aims at the same target, the flu’s surface protein. The difference is who makes it: recombinant shots deliver the finished protein, made in insect cells, while mRNA shots have your cells make it themselves. That has a further effect. Because the protein is made inside cells, mRNA vaccines tend to activate both antibody-helping and infected-cell-killing immune cells, while protein vaccines mainly activate the first.16PubMed Central. Comparison of Immune Responses Elicited by SARS-CoV-2 mRNA and Recombinant Protein Vaccine Candidates
How Well mRNA Flu Vaccines Work
In a trial of about 40,000 adults 50 and older, flu illness occurred in 2.0% of those given the mRNA vaccine mRNA-1010 versus 2.8% given a standard shot.17PubMed. Efficacy and Safety of an mRNA Seasonal Influenza Vaccine in Adults That means the mRNA shot was about a quarter more effective at preventing flu than the standard vaccine.18PubMed. Efficacy and Safety of an mRNA Seasonal Influenza Vaccine in Adults It also produced higher antibody levels than the standard shot against all three strains it covered.19PubMed Central. Humoral Immunogenicity of mRNA-1010, an mRNA-based Seasonal Influenza Vaccine, in Adults ≥50 Years A combined mRNA flu-and-COVID vaccine produced immune responses at least as good as separate flu and COVID shots in adults 50 and older.20JAMA. Immunogenicity and Safety of Influenza and COVID-19 Multicomponent Vaccine in Adults ≥50 Years: A Randomized Clinical Trial One trial in one flu season is encouraging but not the final word on how the shot performs every year.
Influenza B has been the weak spot. Earlier trials found mRNA flu vaccines matched or beat standard shots against influenza A but not influenza B.21PubMed. Efficacy, Immunogenicity, and Safety of Modified mRNA Influenza Vaccine In one smaller earlier trial, protection against B looked worse than the standard shot, though it was not designed to measure this.22PubMed Central. An mRNA-based seasonal influenza vaccine in adults: Results of two phase 3 randomized clinical trials and correlate of protection analysis of hemagglutination inhibition titers Newer versions have improved, but results vary by strain and product. What matters for you is how a specific current product performed, not mRNA flu vaccines as a single group.
“Better” has limits. The comparisons are against standard-dose shots, not the high-dose ones made for seniors, and most results so far are in adults 50 and older. Any flu vaccine still beats none, and protection against severe illness matters more than avoiding a mild case.
Side Effects of mRNA Flu Shots
In one large trial, 70% of mRNA recipients had arm reactions versus 43% with a standard shot.23PubMed. Efficacy, Immunogenicity, and Safety of Modified mRNA Influenza Vaccine Fever occurred in about 6% of mRNA recipients versus about 2% with the standard shot.24PubMed. Efficacy, Immunogenicity, and Safety of Modified mRNA Influenza Vaccine The most common reactions were injection-site pain and tiredness.25PubMed Central. Immunogenicity and safety of mRNA-based seasonal influenza vaccines encoding hemagglutinin and neuraminidase Still, reactions were mostly mild to moderate, and serious side effects were similar between groups.26PubMed. Efficacy, Immunogenicity, and Safety of Modified mRNA Influenza Vaccine “Arm reaction” covers pain, redness, and swelling at the injection site, not only severe soreness.
Dose and age matter. Higher doses caused more reactions, and older adults reacted less than younger adults at the same dose.27PubMed Central. Safety and immunogenicity of an investigational mRNA-lipid nanoparticle-based monovalent influenza vaccine: Results from a phase 1, randomized, dose-escalation study Early trials reported no treatment-related serious side effects.28PubMed Central. Safety and immunogenicity of a phase 1/2 randomized clinical trial of a quadrivalent, mRNA-based seasonal influenza vaccine (mRNA-1010) in healthy adults: interim analysis A sore arm and fatigue usually peak within a day or two, and any fever is short-lived. A brief fever after a vaccine also differs from one caused by flu, which tends to last longer and come with body aches and cough.
Why Researchers Want mRNA for Flu
mRNA flu vaccines may be made in two to three months versus about six for conventional vaccines.29Social Science Research Network. Shorter mRNA Lead Times and the Option to Wait in Influenza Vaccine Design Strain picks can then be made closer to flu season, improving the match.30Harvard Health. FDA approves first mRNA-based flu vaccine Like recombinant vaccines, mRNA vaccines skip egg growth, avoiding the egg changes described earlier.
The bigger hope is broader protection. The flu’s surface protein has a head that mutates quickly and a stalk that stays more constant between strains. Antibodies against the head are what usual shots mostly aim for, which is why they need regular updating. In mice, mRNA vaccines aimed at the stalk protected against different flu types, including a bird flu strain.31PubMed Central. Protective efficacy of a universal influenza mRNA vaccine against the challenge of H1 and H5 influenza A viruses in mice If that held up in people, it could reduce the need for yearly updates, though human trials would take years.
Flu A and B viruses come in many subtypes, and today’s yearly shots cover only a few. In mice and ferrets, an mRNA vaccine covering all 20 known flu A and B subtypes protected against matched and mismatched strains.32PubMed Central. A multivalent nucleoside-modified mRNA vaccine against all known influenza virus subtypes But these are animal results, not available for people, and animals often respond differently from humans.
Cost, Storage and Trust
mRNA vaccines currently cost more per dose than traditional inactivated flu vaccines.33Merck (MilliporeSigma). Cost Modeling Vaccine Manufacturing: Estimate Production Costs for mRNA and other Vaccine Modalities Costs are expected to fall as manufacturing scales up,34PubMed Central. mRNA and Next-Generation Vaccine Platforms for Pandemic Influenza Preparedness which would make wider availability more likely. What you pay depends on your insurance and which products your clinic carries, so the price on paper may not be your price.
Storage matters too. Vaccines that need freezing are harder for small clinics and pharmacies to handle than ones kept in a regular refrigerator. In lab tests, freeze-dried versions stayed effective for 12 weeks at room temperature, which could ease cold-storage needs.35PubMed Central. Lyophilization provides long-term stability for a lipid nanoparticle-formulated, nucleoside-modified mRNA vaccine That could widen access, including in rural areas, though it is a promising lab result rather than a product feature.
Trust is the other hurdle. Polarized attitudes toward COVID mRNA vaccines may affect how people view mRNA flu vaccines.36PubMed Central. Message in a bottle: mRNA vaccination for influenza Flu and COVID vaccines are different products with different trial results, so judging one by the other can mislead.
If you are wary of mRNA, you can still choose a non-mRNA flu shot. Egg-based, cell-grown, recombinant, and nasal spray options remain available.